A computationally identified compound antagonizes excess FGF-23 signaling in renal tubules and a mouse model of hypophosphatemia

Production of biologically active human basic fibroblast growth factor (hFGFb) using Nicotiana tabacum transplastomic plants

MAIN CONCLUSION

We generated transplastomic tobacco lines that stably express a human Basic Fibroblast Growth Factor (hFGFb) in their chloroplasts stroma and purified a biologically active recombinant hFGFb. MAIN: The use of plants as biofactories presents as an attractive technology with the potential to efficiently produce high-value human recombinant proteins in a cost-effective manner. Plastid genome transformation stands out for its possibility to accumulate recombinant proteins at elevated levels. Of particular interest are recombinant growth factors, given their applications in animal cell culture and regenerative medicine. In this study, we produced recombinant human Fibroblast Growth Factor (rhFGFb), a crucial protein required for animal cell culture, in tobacco chloroplasts. We successfully generated two independent transplastomic lines that are homoplasmic and accumulate rhFGFb in their leaves. Furthermore, the produced rhFGFb demonstrated its biological activity by inducing proliferation in HEK293T cell lines. These results collectively underscore plastid genome transformation as a promising plant-based bioreactor for rhFGFb production.

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.

Computational characteristics of the structure-activity relationship of inhibitors targeting Pks13-TE domain

Multiple studies have established the Pks13-TE domain as a promising target for anti-tuberculosis drug development. However, recent findings have revealed that the lead compound currently in the pipeline for Pks13-TE has significant cardiotoxicity issues. Given the pressing need for new chemical structures for Pks13-TE inhibitors, this study aims to provide a detailed understanding of the Pks13-TE domain binding site through the application of computational chemical biology techniques. Our results highlight the size and shape of the Pks13-TE domain binding pocket, key residues including Asp1644, Asn1640, Phe1670, and Tyr1674 within the pocket, and inhibitor pharmacophore characteristics such as aromatic ring sites, positively charged sites, and hydrogen bond donors. To our knowledge, these simulation results are novel and contribute to the discovery of next-generation Pks13-TE inhibitors without similar prior studies.

High Plasma Levels of Fibroblast Growth Factor 23 Are Associated with Increased Risk of COVID-19 in End-Stage Renal Disease Patients on Hemodialysis: Results of a Prospective Cohort

End-stage renal disease (ESRD) patients are a population with high rates of COVID-19 and mortality. These patients present a low response to anti-SARS-CoV-2 immunization, which is associated with immune dysfunction. ESRD patients also present high plasma titers of Fibroblast Growth Factor 23 (FGF23), a protein hormone that reduces immune response in vivo and in vitro. Increased FGF23 levels associate with higher infection-related hospitalizations and adverse infectious outcomes. Thus, we evaluated whether ESRD patients with high FGF23 titers have an increased rate of SARS-CoV-2 infection.

METHODS

We performed a prospective cohort of ESRD patients in hemodialysis who had measurements of plasma intact FGF23 in 2019. We determined COVID-19 infections, hospitalizations, and mortality between January 2020 and December 2021.

RESULTS

We evaluated 243 patients. Age: 60.4 ± 10.8 years. Female: 120 (49.3%), diabetes: 110 (45.2%). During follow-up, 45 patients developed COVID-19 (18.5%), 35 patients were hospitalized, and 12 patients died (mortality rate: 26.6%). We found that patients with higher FGF23 levels (defined as equal or above median) had a higher rate of SARS-CoV-2 infection versus those with lower levels (18.8% versus 9.9%; Hazard ratio: 1.92 [1.03-3.56], p = 0.039). Multivariate analysis showed that increased plasma FGF23 was independently associated with SARS-CoV-2 infection and severe COVID-19.

DISCUSSION

Our results suggest that high plasma FGF23 levels are a risk factor for developing COVID-19 in ESRD patients. These data support the potential immunosuppressive effects of high circulating FGF23 as a factor implicated in the association with worse clinical outcomes. Further data are needed to confirm this hypothesis.

Identification of Small-Molecule Inhibitors of Fibroblast Growth Factor 23 Signaling via In Silico Hot Spot Prediction and Molecular Docking to α-Klotho

Fibroblast growth factor 23 (FGF23) is a therapeutic target for treating hereditary and acquired hypophosphatemic disorders, such as X-linked hypophosphatemic (XLH) rickets and tumor-induced osteomalacia (TIO), respectively. FGF23-induced hypophosphatemia is mediated by signaling through a ternary complex formed by FGF23, the FGF receptor (FGFR), and α-Klotho. Currently, disorders of excess FGF23 are treated with an FGF23-blocking antibody, burosumab. Small-molecule drugs that disrupt protein/protein interactions necessary for the ternary complex formation offer an alternative to disrupting FGF23 signaling. In this study, the FGF23:α-Klotho interface was targeted to identify small-molecule protein/protein interaction inhibitors since it was computationally predicted to have a large fraction of hot spots and two druggable residues on α-Klotho. We further identified Tyr433 on the KL1 domain of α-Klotho as a promising hot spot and α-Klotho as an appropriate drug-binding target at this interface. Subsequently, we performed in silico docking of ∼5.5 million compounds from the ZINC database to the interface region of α-Klotho from the ternary crystal structure. Following docking, 24 and 20 compounds were in the final list based on the lowest binding free energies to α-Klotho and the largest number of contacts with Tyr433, respectively. Five compounds were assessed experimentally by their FGF23-mediated extracellular signal-regulated kinase (ERK) activities in vitro, and two of these reduced activities significantly. Both these compounds were predicted to have favorable binding affinities to α-Klotho but not have a large number of contacts with the hot spot Tyr433. ZINC12409120 was found experimentally to disrupt FGF23:α-Klotho interaction to reduce FGF23-mediated ERK activities by 70% and have a half maximal inhibitory concentration (IC₅₀) of 5.0 ± 0.23 μM. Molecular dynamics (MD) simulations of the ZINC12409120:α-Klotho complex starting from in silico docking poses reveal that the ligand exhibits contacts with residues on the KL1 domain, the KL1-KL2 linker, and the KL2 domain of α-Klotho simultaneously, thereby possibly disrupting the regular function of α-Klotho and impeding FGF23:α-Klotho interaction. ZINC12409120 is a candidate for lead optimization.

Target-Based Small Molecule Drug Discovery for Colorectal Cancer: A Review of Molecular Pathways and In Silico Studies

Colorectal cancer is one of the most prevalent cancer types. Although there have been breakthroughs in its treatments, a better understanding of the molecular mechanisms and genetic involvement in colorectal cancer will have a substantial role in producing novel and targeted treatments with better safety profiles. In this review, the main molecular pathways and driver genes that are responsible for initiating and propagating the cascade of signaling molecules reaching carcinoma and the aggressive metastatic stages of colorectal cancer were presented. Protein kinases involved in colorectal cancer, as much as other cancers, have seen much focus and committed efforts due to their crucial role in subsidizing, inhibiting, or changing the disease course. Moreover, notable improvements in colorectal cancer treatments with in silico studies and the enhanced selectivity on specific macromolecular targets were discussed. Besides, the selective multi-target agents have been made easier by employing in silico methods in molecular de novo synthesis or target identification and drug repurposing.

Discovery of New Estrogen-Related Receptor α Agonists via a Combination Strategy Based on Shape Screening and Ensemble Docking

Estrogen-related receptor α (ERRα), a member of nuclear receptors (NRs), plays a role in the regulation of cellular energy metabolism and is reported to be a novel potential target for type 2 diabetes therapy. To date, only a few agonists of ERRα have been identified to improve insulin sensitivity and decrease blood glucose levels. Herein, the discovery of novel potent agonists of ERRα determined using a combined virtual screening approach is described. Molecular dynamics (MD) simulations were used to obtain structural ensembles that can consider receptor flexibility. Then, an efficient virtual screening strategy with a combination of similarity search and ensemble docking was performed against the Enamine, SPECS, and Drugbank databases to identify potent ERRα agonists. Finally, a total of 66 compounds were purchased for experimental testing. Biological investigation of promising candidates identified seven compounds that have activity against ERRα with EC50 values ranging from 1.11 to 21.70 μM, with novel scaffolds different from known ERRα agonists until now. Additionally, the molecule GX66 showed micromolar inverse activity against ERRα with an IC50 of 0.82 μM. The predicted binding modes showed that these compounds were anchored in ERRα-LBP via interactions with several residues of ERRα. Overall, this study not only identified the novel potent ERRα agonists or an inverse agonist that would be the promising starting point for further exploration but also demonstrated a successful molecular dynamics-guided approach applicable in virtual screening for ERRα agonists.

The Saga of Endocrine FGFs

Fibroblast growth factors (FGFs) are cell-signaling proteins with diverse functions in cell development, repair, and metabolism. The human FGF family consists of 22 structurally related members, which can be classified into three separate groups based on their action of mechanisms, namely: intracrine, paracrine/autocrine, and endocrine FGF subfamilies. FGF19, FGF21, and FGF23 belong to the hormone-like/endocrine FGF subfamily. These endocrine FGFs are mainly associated with the regulation of cell metabolic activities such as homeostasis of lipids, glucose, energy, bile acids, and minerals (phosphate/active vitamin D). Endocrine FGFs function through a unique protein family called klotho. Two members of this family, α-klotho, or β-klotho, act as main cofactors which can scaffold to tether FGF19/21/23 to their receptor(s) (FGFRs) to form an active complex. There are ongoing studies pertaining to the structure and mechanism of these individual ternary complexes. These studies aim to provide potential insights into the physiological and pathophysiological roles and therapeutic strategies for metabolic diseases. Herein, we provide a comprehensive review of the history, structure–function relationship(s), downstream signaling, physiological roles, and future perspectives on endocrine FGFs.

Novel Small Molecule Fibroblast Growth Factor 23 Inhibitors Increase Serum Phosphate and Improve Skeletal Abnormalities in Hyp Mice

Excess fibroblast growth factor (FGF) 23 causes hereditary hypophosphatemic rickets, such as X-linked hypophosphatemia (XLH) and tumor-induced osteomalacia (TIO). A small molecule that specifically binds to FGF23 to prevent activation of the fibroblast growth factor receptor/α-Klotho complex has potential advantages over the currently approved systemically administered FGF23 blocking antibody. Using structure-based drug design, we previously identified ZINC13407541 (N-[[2-(2-phenylethenyl)cyclopenten-1-yl]methylidene]hydroxylamine) as a small molecule antagonist for FGF23. Additional structure-activity studies developed a series of ZINC13407541 analogs with enhanced drug-like properties. In this study, we tested in a preclinical Hyp mouse homolog of XLH a direct connect analog [(E)-2-(4-(tert-butyl)phenyl)cyclopent-1-ene-1-carbaldehyde oxime] (8n), which exhibited the greatest stability in microsomal assays, and [(E)-2-((E)-4-methylstyryl)benzaldehyde oxime] (13a), which exhibited increased in vitro potency. Using cryo-electron microscopy structure and computational docking, we identified a key binding residue (Q156) of the FGF23 antagonists, ZINC13407541, and its analogs (8n and 13a) in the N-terminal domain of FGF23 protein. Site-directed mutagenesis and bimolecular fluorescence complementation-fluorescence resonance energy transfer assay confirmed the binding site of these three antagonists. We found that pharmacological inhibition of FGF23 with either of these compounds blocked FGF23 signaling and increased serum phosphate and 1,25-dihydroxyvitamin D [1,25(OH)2D] concentrations in Hyp mice. Long-term parenteral treatment with 8n or 13a also enhanced linear bone growth, increased mineralization of bone, and narrowed the growth plate in Hyp mice. The more potent 13a compound had greater therapeutic effects in Hyp mice. Further optimization of these FGF23 inhibitors may lead to versatile drugs to treat excess FGF23-mediated disorders. SIGNIFICANCE STATEMENT: This study used structure-based drug design and medicinal chemistry approaches to identify and optimize small molecules with different stability and potency, which antagonize excessive actions of fibroblast growth factor 23 (FGF23) in hereditary hypophosphatemic rickets. The findings confirmed that these antagonists bind to the N-terminus of FGF23 to inhibit its binding to and activation of the fibroblast growth factor receptors/α-Klotho signaling complex. Administration of these lead compounds improved phosphate homeostasis and abnormal skeletal phenotypes in a preclinical Hyp mouse model.

Supercomputer-Based Ensemble Docking Drug Discovery Pipeline with Application to Covid-19

We present a supercomputer-driven pipeline for in silico drug discovery using enhanced sampling molecular dynamics (MD) and ensemble docking. Ensemble docking makes use of MD results by docking compound databases into representative protein binding-site conformations, thus taking into account the dynamic properties of the binding sites. We also describe preliminary results obtained for 24 systems involving eight proteins of the proteome of SARS-CoV-2. The MD involves temperature replica exchange enhanced sampling, making use of massively parallel supercomputing to quickly sample the configurational space of protein drug targets. Using the Summit supercomputer at the Oak Ridge National Laboratory, more than 1 ms of enhanced sampling MD can be generated per day. We have ensemble docked repurposing databases to 10 configurations of each of the 24 SARS-CoV-2 systems using AutoDock Vina. Comparison to experiment demonstrates remarkably high hit rates for the top scoring tranches of compounds identified by our ensemble approach. We also demonstrate that, using Autodock-GPU on Summit, it is possible to perform exhaustive docking of one billion compounds in under 24 h. Finally, we discuss preliminary results and planned improvements to the pipeline, including the use of quantum mechanical (QM), machine learning, and artificial intelligence (AI) methods to cluster MD trajectories and rescore docking poses.

FGF23 and Bone and Mineral Metabolism

FGF23 is a phosphotropic hormone produced by the bone. FGF23 works by binding to the FGF receptor-Klotho complex. Klotho is expressed in several limited tissues including the kidney and parathyroid glands. This tissue-restricted expression of Klotho is believed to determine the target organs of FGF23. FGF23 reduces serum phosphate by suppressing the expression of type 2a and 2c sodium-phosphate cotransporters in renal proximal tubules. FGF23 also decreases 1,25-dihydroxyvitamin D levels by regulating the expression of vitamin D-metabolizing enzymes, which results in reduced intestinal phosphate absorption. Excessive actions of FGF23 cause several types of hypophosphatemic rickets/osteomalacia characterized by impaired mineralization of bone matrix. In contrast, deficient actions of FGF23 result in hyperphosphatemic tumoral calcinosis with high 1,25-dihydroxyvitamin D levels. These results indicate that FGF23 is a physiological regulator of phosphate and vitamin D metabolism and indispensable for the maintenance of serum phosphate levels.

Design and development of FGF-23 antagonists: Definition of the pharmacophore and initial structure-activity relationships probed by synthetic analogues

Hereditary hypophosphatemic disorders, TIO, and CKD conditions are believed to be influenced by an excess of Fibroblast Growth Factor-23 (FGF-23) which activates a binary renal FGFRs / α-Klotho complex to regulate homeostatic metabolism of phosphate and vitamin D. Adaptive FGF-23 responses from CKD patients with excess FGF-23 frequently lead to increased mortality from cardiovascular disease. A reversibly binding small molecule therapeutic has yet to emerge from research and development in this area. Current outcomes described in this work highlight efforts related to lead identification and modification using organic synthesis of strategic analogues to probe structure-activity relationships and preliminarily define the pharmacophore of a computationally derived hit obtained from virtual high-throughput screening. Synthetic strategies for the initial hit and analogue preparation, as well as preliminary cellular in vitro assay results highlighting sub micromolar inhibition of the FGF-23 signaling sequence at a concentration well below cytotoxicity are reported herein.

Osteocytic FGF23 and Its Kidney Function

Osteocytes, which represent up to 95% of adult skeletal cells, are deeply embedded in bone. These cells exhibit important interactive abilities with other bone cells such as osteoblasts and osteoclasts to control skeletal formation and resorption. Beyond this local role, osteocytes can also influence the function of distant organs due to the presence of their sophisticated lacunocanalicular system, which connects osteocyte dendrites directly to the vasculature. Through these networks, osteocytes sense changes in circulating metabolites and respond by producing endocrine factors to control homeostasis. One critical function of osteocytes is to respond to increased blood phosphate and 1,25(OH)2 vitamin D (1,25D) by producing fibroblast growth factor-23 (FGF23). FGF23 acts on the kidneys through partner fibroblast growth factor receptors (FGFRs) and the co-receptor Klotho to promote phosphaturia via a downregulation of phosphate transporters, as well as the control of vitamin D metabolizing enzymes to reduce blood 1,25D. In the first part of this review, we will explore the signals involved in the positive and negative regulation of FGF23 in osteocytes. In the second portion, we will bridge bone responses with the review of current knowledge on FGF23 endocrine functions in the kidneys.

Supercomputer-Based Ensemble Docking Drug Discovery Pipeline with Application to Covid-19

We present a supercomputer-driven pipeline for in-silico drug discovery using enhanced sampling molecular dynamics (MD) and ensemble docking. We also describe preliminary results obtained for 23 systems involving eight protein targets of the proteome of SARS CoV-2. THe MD performed is temperature replica-exchange enhanced sampling, making use of the massively parallel supercomputing on the SUMMIT supercomputer at Oak Ridge National Laboratory, with which more than 1ms of enhanced sampling MD can be generated per day. We have ensemble docked repurposing databases to ten configurations of each of the 23 SARS CoV-2 systems using AutoDock Vina. We also demonstrate that using Autodock-GPU on SUMMIT, it is possible to perform exhaustive docking of one billion compounds in under 24 hours. Finally, we discuss preliminary results and planned improvements to the pipeline, including the use of quantum mechanical (QM), machine learning, and AI methods to cluster MD trajectories and rescore docking poses.

FGF23 expression is stimulated in transgenic α-Klotho longevity mouse model

Observations in transgenic α-Klotho (Kl) mice (KlTg) defined the antiaging role of soluble Klotho (sKL130). A genetic translocation that elevates sKL levels in humans is paradoxically associated with increased circulating fibroblast growth factor 23 (FGF23) levels and the potential of both membrane KL (mKL135) and sKL130 to act as coreceptors for FGF23 activation of fibroblast growth factor receptors (FGFRs). Neither FGF23 expression nor the contributions of FGF23, mKL135, and sKL130 codependent and independent functions have been investigated in KlTg mice. In the current study, we examined the effects of Kl overexpression on FGF23 levels and functions in KlTg mice. We found that mKL135 but not sKL130 stimulated FGF23 expression in osteoblasts, leading to elevated Fgf23 bone expression and circulating levels in KlTg mice. Elevated FGF23 suppressed 1,25(OH)2D and parathyroid hormone levels but did not cause hypophosphatemic rickets in KlTg mice. KlTg mice developed low aldosterone-associated hypertension but not left ventricular hypertrophy. Mechanistically, we found that mKL135 and sKL130 are essential cofactors for FGF23-mediated ERK activation but that they inhibited FGF23 stimulation of PLC-γ and PI3K/AKT signaling. Thus, increased longevity in KlTg mice occurs in the presence of excess FGF23 that interacts with mKL and sKL to bias FGFR pathways.

Fibroblast growth factor 23 and α-Klotho co-dependent and independent functions

PURPOSE OF REVIEW

The current review examines what is known about the FGF-23/α-Klotho co-dependent and independent pathophysiological effects, and whether FGF-23 and/or α-Klotho are potential therapeutic targets.

RECENT FINDINGS

FGF-23 is a hormone derived mainly from bone, and α-Klotho is a transmembrane protein. Together they form a trimeric signaling complex with FGFRs in target tissues to mediate the physiological functions of FGF-23. Local and systemic factors control FGF-23 release from osteoblast/osteocytes in bone, and circulating FGF-23 activates FGFR/α-Klotho complexes in kidney proximal and distal renal tubules to regulate renal phosphate excretion, 1,25 (OH)2D metabolism, sodium and calcium reabsorption, and ACE2 and α-Klotho expression. The resulting bone-renal-cardiac-immune networks provide a new understanding of bone and mineral homeostasis, as well as identify other biological effects FGF-23. Direct FGF-23 activation of FGFRs in the absence of α-Klotho is proposed to mediate cardiotoxic and adverse innate immune effects of excess FGF-23, particularly in chronic kidney disease, but this FGF-23, α-Klotho-independent signaling is controversial. In addition, circulating soluble Klotho (sKl) released from the distal tubule by ectodomain shedding is proposed to have beneficial health effects independent of FGF-23.

SUMMARY

Separation of FGF-23 and α-Klotho independent functions has been difficult in mammalian systems and understanding FGF-23/α-Klotho co-dependent and independent effects are incomplete. Antagonism of FGF-23 is important in treatment of hypophosphatemic disorders caused by excess FGF-23, but its role in chronic kidney disease is uncertain. Administration of recombinant sKl is an unproven therapeutic strategy that theoretically could improve the healt span and lifespan of patients with α-Klotho deficiency.

Molecular Control of Phosphorus Homeostasis and Precision Treatment of Hypophosphatemic Disorders

Purpose of Review

Serum phosphorus is maintained in a narrow range by balancing dietary phosphate absorption, influx and efflux of phosphorus from bone and intracellular stores, and renal reabsorption of filtered phosphate. Acute hypophosphatemia, typically caused by transient increases in cellular uptake, can lead to severe complications such as cardiopulmonary dysfunction and rhabdomyolysis that can warrant parenteral phosphate repletion. Chronic hypophosphatemia, however, generally represents true phosphate deficiency and may result in long-term metabolic and skeletal complications, particularly in children due to the critical importance of phosphorus to skeletal mineralization and longitudinal growth.

Recent Findings

In addition to the well characterized roles of vitamin D and parathyroid hormone (PTH), a new bone-kidney axis has been discovered that regulates phosphate homeostasis through the bone-derived hormone Fibroblast Growth Factor 23 (FGF23) and its phosphaturic actions that are mediated by activation of fibroblast growth factor receptors (FGFRs) complexed with α-Klotho in renal tubules. Chronic hypophosphatemia can now be classified as FGF23 dependent or independent.

Summary

In cases of FGF23 dependent hypophosphatemia, traditional non-specific treatments with elemental phosphorus and 1,25(OH)₂ vitamin D (calcitriol) can now be replaced with a targeted approach by using an FGF-23 blocking antibody (Burosumab).

Predicting how drug molecules bind to their protein targets

There have been substantial advances in the application of molecular modelling and simulation to drug discovery in recent years, as massive increases in computer power are coupled with continued development in the underlying methods and understanding of how to apply them. Here, we survey recent advances in one particular area-predicting how a known ligand binds to a particular protein. We focus on the four contributing classes of calculation: predicting where a binding site is on a protein; characterizing where chemical functional groups will bind to that site; molecular docking to generate a binding mode for a ligand and dynamics simulations to refine that pose and allow for protein conformation change. Examples of successful application are provided for each class.

X-Linked Hypophosphatemia and FGF23-Related Hypophosphatemic Diseases: Prospect for New Treatment

Phosphate plays essential roles in many biological processes, and the serum phosphate level is tightly controlled. Chronic hypophosphatemia causes impaired mineralization of the bone matrix and results in rickets and osteomalacia. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that regulates phosphate metabolism. FGF23 excess induces hypophosphatemia via impaired phosphate reabsorption in the renal proximal tubules and decreased phosphate absorption in the intestines. There are several types of genetic and acquired FGF23-related hypophosphatemic diseases. Among these diseases, X-linked hypophosphatemia (XLH), which is caused by inactivating mutations in the phosphate-regulating endopeptidase homolog, X-linked (PHEX) gene, is the most prevalent form of genetic FGF23-related hypophosphatemic rickets. Another clinically relevant form of FGF23-related hypophosphatemic disease is tumor-induced osteomalacia (TIO), a paraneoplastic syndrome associated with FGF23-producing tumors. A combination of active vitamin D and phosphate salts is the current medical therapy used to treat patients with XLH and inoperative TIO. However, this therapy has certain efficacy- and safety-associated limitations. Several measures to inhibit FGF23 activity have been considered as possible new treatments for FGF23-related hypophosphatemic diseases. In particular, a humanized monoclonal antibody for FGF23 (burosumab) is a promising treatment in patients with XLH and TIO. This review will focus on the phosphate metabolism and the pathogenesis and treatment of FGF23-related hypophosphatemic diseases.

Ensemble Docking in Drug Discovery

Ensemble docking corresponds to the generation of an "ensemble" of drug target conformations in computational structure-based drug discovery, often obtained by using molecular dynamics simulation, that is used in docking candidate ligands. This approach is now well established in the field of early-stage drug discovery. This review gives a historical account of the development of ensemble docking and discusses some pertinent methodological advances in conformational sampling.

Ensemble docking to difficult targets in early-stage drug discovery: Methodology and application to fibroblast growth factor 23

Ensemble docking is now commonly used in early-stage in silico drug discovery and can be used to attack difficult problems such as finding lead compounds which can disrupt protein-protein interactions. We give an example of this methodology here, as applied to fibroblast growth factor 23 (FGF23), a protein hormone that is responsible for regulating phosphate homeostasis. The first small-molecule antagonists of FGF23 were recently discovered by combining ensemble docking with extensive experimental target validation data (Science Signaling, 9, 2016, ra113). Here, we provide a detailed account of how ensemble-based high-throughput virtual screening was used to identify the antagonist compounds discovered in reference (Science Signaling, 9, 2016, ra113). Moreover, we perform further calculations, redocking those antagonist compounds identified in reference (Science Signaling, 9, 2016, ra113) that performed well on drug-likeness filters, to predict possible binding regions. These predicted binding modes are rescored with the molecular mechanics Poisson-Boltzmann surface area (MM/PBSA) approach to calculate the most likely binding site. Our findings suggest that the antagonist compounds antagonize FGF23 through the disruption of protein-protein interactions between FGF23 and fibroblast growth factor receptor (FGFR).

Targeting Fibroblast Growth Factor 23 Signaling with Antibodies and Inhibitors, Is There a Rationale?

Fibroblast growth factor 23 (FGF23) is a phosphotropic hormone mainly produced by bone. FGF23 reduces serum phosphate by suppressing intestinal phosphate absorption through reducing 1,25-dihydroxyvitamin D and proximal tubular phosphate reabsorption. Excessive actions of FG23 result in several kinds of hypophosphatemic rickets/osteomalacia including X-linked hypophosphatemic rickets (XLH) and tumor-induced osteomalacia. While neutral phosphate and active vitamin D are standard therapies for child patients with XLH, these medications have several limitations both in their effects and adverse events. Several approaches that inhibit FGF23 actions including anti-FGF23 antibodies and inhibitors of FGF signaling have been shown to improve phenotypes of model mice for FG23-related hypophosphatemic diseases. In addition, clinical trials indicated that a humanized anti-FGF23 antibody increased serum phosphate and improved quality of life in patients with XLH. Furthermore, circulatory FGF23 is high in patients with chronic kidney disease (CKD). Many epidemiological studies indicated the association between high FGF23 levels and various adverse events especially in patients with CKD. However, it is not known whether the inhibition of FGF23 activities in patients with CKD is beneficial for these patients. In this review, recent findings concerning the modulation of FGF23 activities are discussed.

Role of Fibroblast Growth Factor-23 in Innate Immune Responses

Fibroblast growth factor-23 (FGF-23) is a bone-derived hormone that activates FGFR/α-Klotho binary complexes in the kidney renal tubules to regulate phosphate reabsorption and vitamin D metabolism. The objective of this review is to discuss the emerging data that show that FGF-23 has functions beyond regulation of mineral metabolism, including roles in innate immune and hemodynamic responses. Excess FGF-23 is associated with inflammation and adverse infectious outcomes, as well as increased morbidity and mortality, particularly in patients with chronic kidney disease. Enhancer elements in the FGF-23 promoter have been identified that mediate the effects of inflammatory cytokines to stimulate FGF-23 gene transcription in bone. In addition, inflammation induces ectopic expression of FGF-23 and α-Klotho in macrophages that do not normally express FGF-23 or its binary receptor complexes. These observations suggest that FGF-23 may play an important role in regulating innate immunity through multiple potential mechanisms. Circulating FGF-23 acts as a counter-regulatory hormone to suppress 1,25D production in the proximal tubule of the kidney. Since vitamin D deficiency may predispose infectious and cardiovascular diseases, FGF-23 effects on innate immune responses may be due to suppression of 1,25D production. Alternatively, systemic and locally produced FGF-23 may modulate immune functions through direct interactions with myeloid cells, including macrophages and polymorphonuclear leukocytes to impair immune cell functions. Short-acting small molecules that reversibly inhibit FGF-23 offer the potential to block pro-inflammatory and cardiotoxic effects of FGF-23 with less side effects compared with FGF-23 blocking antibodies that have the potential to cause hyperphosphatemia and soft tissue calcifications in animal models. In conclusion, there are several mechanisms by which FGF-23 impacts the innate immune system and further investigation is critical for the development of therapies to treat diseases associated with elevated FGF-23.

Polycystin-1 interacts with TAZ to stimulate osteoblastogenesis and inhibit adipogenesis

The molecular mechanisms that transduce the osteoblast response to physical forces in the bone microenvironment are poorly understood. Here, we used genetic and pharmacological experiments to determine whether the polycystins PC1 and PC2 (encoded by Pkd1 and Pkd2) and the transcriptional coactivator TAZ form a mechanosensing complex in osteoblasts. Compound-heterozygous mice lacking 1 copy of Pkd1 and Taz exhibited additive decrements in bone mass, impaired osteoblast-mediated bone formation, and enhanced bone marrow fat accumulation. Bone marrow stromal cells and osteoblasts derived from these mice showed impaired osteoblastogenesis and enhanced adipogenesis. Increased extracellular matrix stiffness and application of mechanical stretch to multipotent mesenchymal cells stimulated the nuclear translocation of the PC1 C-terminal tail/TAZ (PC1-CTT/TAZ) complex, leading to increased runt-related transcription factor 2-mediated (Runx2-mediated) osteogenic and decreased PPARγ-dependent adipogenic gene expression. Using structure-based virtual screening, we identified a compound predicted to bind to PC2 in the PC1:PC2 C-terminal tail region with helix:helix interaction. This molecule stimulated polycystin- and TAZ-dependent osteoblastogenesis and inhibited adipogenesis. Thus, we show that polycystins and TAZ integrate at the molecular level to reciprocally regulate osteoblast and adipocyte differentiation, indicating that the polycystins/TAZ complex may be a potential therapeutic target to increase bone mass.