Immunophilin FKBP12 Research Articles

A functional interplay between the two BMP-SMAD pathway inhibitors TMPRSS6 and FKBP12 regulates hepcidin expression in vivo.

The Activin A Receptor type I (ALK2) is a critical component of BMP-SMAD signaling that, in the presence of ligands, phosphorylates cytosolic SMAD1/5/8 and modulates important biological processes, including bone formation and iron metabolism. In hepatocytes, the BMP-SMAD pathway controls the expression of hepcidin, the liver peptide hormone that regulates body iron homeostasis via the BMP receptors ALK2 and ALK3, and the hemochromatosis proteins. The main negative regulator of the pathway in the liver is transmembrane serine protease 6 (TMPRSS6), which downregulates hepcidin by cleaving the BMP coreceptor hemojuvelin. ALK2 function is inhibited also by the immunophilin FKBP12, which maintains the receptor in an inactive conformation. FKBP12 sequestration by tacrolimus or its silencing upregulates hepcidin in primary hepatocytes and in vivo in acute but not chronic settings. Interestingly, gain-of-function mutations in ALK2 that impair FKBP12 binding to the receptor and activate the pathway cause a bone phenotype in patients affected by Fibrodysplasia Ossificans Progressiva but not hepcidin and iron metabolism dysfunction. This observation suggests that additional mechanisms are active in the liver to compensate for the increased BMP-SMAD signaling. Here we demonstrate that Fkbp12 downregulation in hepatocytes by antisense oligonucleotide treatment upregulates the expression of the main hepcidin inhibitor Tmprss6, thus counteracting the ALK2-mediated activation of the pathway. Combined downregulation of both Fkbp12 and Tmprss6 blocks this compensatory mechanism. Our findings reveal a previously unrecognized functional cross talk between FKBP12 and TMPRSS6, the main BMP-SMAD pathway inhibitors, in the control of hepcidin transcription.NEW & NOTEWORTHY This study uncovers a previously unrecognized mechanism of hepcidin and BMP-SMAD pathway regulation in hepatocytes mediated by the immunophilin FKBP12 and the transmembrane serine protease TMPRSS6.

Tacrolimus impairs airway mucociliary clearance of rats

ObjectivesTacrolimus (TAC) is the most widely used immunosuppressive agent after lung transplantation. Considering that the ciliary beat frequency (CBF) mainly depends on the cytoplasmic calcium concentration and that TAC can affect this due to its binding with the intracellular immunophilin FKBP12, we hypothesized that TAC could also impair the airway mucociliary clearance of rats. MethodsSixty rats were divided into two groups (n = 30 each): Control = water; TAC = tacrolimus. After 7, 15 or 30 days of treatment, ten animals from each group were euthanized and the following parameters were studied: mucus transportability, CBF, mucociliary transport velocity (MCTV), and neutral and acid mucus production. ResultsThere was a significant decrease in CBF (Control vs TAC: 7 days, p = 0.008; 15 days, p = 0.007; 30 days, p = 0.001) and MCTV (Control vs TAC: 7 days, p = 0.004; 15 days, p < 0.001; 30 days, p < 0.001) in all immunosuppressed animals. TAC therapy also caused an increase in acid mucus production at all treatment times (Control vs TAC: 7 days, p = 0.001; 15 days, p = 0.043; 30 days, p = 0.001). ConclusionsTAC impairs airway mucociliary clearance of rats.

FKBP12 inhibits hepcidin expression by modulating BMP receptors interaction and ligand responsiveness in hepatocytes.

The expression of the iron regulatory hormone hepcidin in hepatocytes is regulated by the BMP-SMAD pathway through the type I receptors ALK2 and ALK3, the type II receptors ACVR2A and BMPR2, and the ligands BMP2 and BMP6. We previously identified the immunophilin FKBP12 as a new hepcidin inhibitor that acts by blocking ALK2. Both the physiologic ALK2 ligand BMP6 and the immunosuppressive drug Tacrolimus (TAC) displace FKBP12 from ALK2 and activate the signaling. However, the molecular mechanism whereby FKBP12 regulates BMP-SMAD pathway activity and thus hepcidin expression remains unclear. Here, we show that FKBP12 acts by modulating BMP receptor interactions and ligand responsiveness. We first demonstrate that in primary murine hepatocytes TAC regulates hepcidin expression exclusively via FKBP12. Downregulation of the BMP receptors reveals that ALK2, to a lesser extent ALK3, and ACVR2A are required for hepcidin upregulation in response to both BMP6 and TAC. Mechanistically, TAC and BMP6 increase ALK2 homo-oligomerization and ALK2-ALK3 hetero-oligomerization and the interaction between ALK2 and the type II receptors. By acting on the same receptors, TAC and BMP6 cooperate in BMP pathway activation and hepcidin expression both in vitro and in vivo. Interestingly, the activation state of ALK3 modulates its interaction with FKBP12, which may explain the cell-specific activity of FKBP12. Overall, our results identify the mechanism whereby FKBP12 regulates the BMP-SMAD pathway and hepcidin expression in hepatocytes, and suggest that FKBP12-ALK2 interaction is a potential pharmacologic target in disorders caused by defective BMP-SMAD signaling and characterized by low hepcidin and high BMP6 expression.

Chaperoning of specific tau structure by immunophilin FKBP12 regulates the neuronal resilience to extracellular stress.

Alzheimer's disease and related tauopathies are characterized by the pathogenic misfolding and aggregation of the microtubule-associated protein tau. Understanding how endogenous chaperones modulate tau misfolding could guide future therapies. Here, we show that the immunophilin FKBP12, the 12-kDa FK506-binding protein (also known as FKBP prolyl isomerase 1A), regulates the neuronal resilience by chaperoning a specific structure in monomeric tau. Using a combination of mouse and cell experiments, in vitro aggregation experiments, nuclear magnetic resonance-based structural analysis of monomeric tau, site-specific phosphorylation and mutation, as well as structure-based analysis using the neural network-based structure prediction program AlphaFold, we define the molecular factors that govern the binding of FKBP12 to tau and its influence on tau-induced neurotoxicity. We further demonstrate that tyrosine phosphorylation of tau blocks the binding of FKBP12 to two highly specific structural motifs in tau. Our data together with previous results demonstrating FKBP12/tau colocalization in neurons and neurofibrillary tangles support a critical role of FKBP12 in regulating tau pathology.

FKBP12 is a major regulator of ALK2 activity in multiple myeloma cells

BackgroundThe immunophilin FKBP12 binds to TGF-β family type I receptors, including the BMP type I receptor ALK2. FKBP12 keeps the type I receptor in an inactive state and controls signaling activity. Removal of FKBP12 with drugs such as the FKBP-ligand FK506 enhances BMP activity in various cell types. In multiple myeloma cells, activation of SMAD1/5/8 leads to apoptosis. We hypothesized that removing FKBP12 from ALK2 in myeloma cells would potentiate BMP-induced ALK2-SMAD1/5/8 activity and in consequence cell death.MethodsMultiple myeloma cell lines were treated with FK506, or other FKBP-binding compounds, combined with different BMPs before analyzing SMAD1/5/8 activity and cell viability. SMAD1/5/8 activity was also investigated using a reporter cell line, INA-6 BRE-luc. To characterize the functional signaling receptor complex, we genetically manipulated receptor expression by siRNA, shRNA and CRISPR/Cas9 technology.ResultsFK506 potentiated BMP-induced SMAD1/5/8 activation and apoptosis in multiple myeloma cell lines. By using FKBP-binding compounds with different affinity profiles, and siRNA targeting FKBP12, we show that the FK506 effect is mediated by binding to FKBP12. Ligands that typically signal via ALK3 in myeloma cells, BMP2, BMP4, and BMP10, did not induce apoptosis in cells lacking ALK3. Notably, BMP10 competed with BMP6 and BMP9 and antagonized their activity via ALK2. However, upon addition of FK506, we saw a surprising shift in specificity, as the ALK3 ligands gained the ability to signal via ALK2 and induce apoptosis. This indicates that the receptor complex can switch from an inactive non-signaling complex (NSC) to an active one by adding FK506. This gain of activity was also seen in other cell types, indicating that the observed effects have broader relevance. BMP2, BMP4 and BMP10 depended on BMPR2 as type II receptor to signal, which contrasts with BMP6 and BMP9, that activate ALK2 more potently when BMPR2 is knocked down.ConclusionsIn summary, our data suggest that FKBP12 is a major regulator of ALK2 activity in multiple myeloma cells, partly by switching an NSC into an active signaling complex. FKBP12 targeting compounds devoid of immunosuppressing activity could have potential in novel treatment strategies aiming at reducing multiple myeloma tumor load.AQgv4xH-kso9r-wigViCjMVideo

Light-Activating PROTACs in Cancer: Chemical Design, Challenges, and Applications

Nonselective cell damage remains a significant limitation of radiation therapies in cancer. Decades of successful integration of radiation therapies with other medicinal chemistry strategies significantly improved therapeutic benefits in cancer. Advancing in such technologies also led to the development of specific photopharmcology-based approaches that improved the cancer cell selectivity and provided researchers with spatiotemporal control over the degradation of highly expressed proteins in cancer (proteolysis targeting chimeras, PROTACs) using a monochrome wavelength light source. Two specific strategies that have achieved notable successes are photocage and photoswitchable PROTACs. Photocaged PROTACs require a photolabile protecting group (PPG) that, when radiated with a specific wavelength of light, irreversibly release PPG and induce protein degradation. Thus far, diethylamino coumarin for estrogen-related receptor α (ERRα), nitropiperonyloxymethyl (BRD4 bromodomain protein), and 4,5-dimethoxy-2-nitrobenzyl for (BRD4 bromodomain protein, as well as BTK kinase protein) were successfully incorporated in photocaged PROTACs. On the other hand, photoswitches of photoswitchable PROTACs act as an actual ON/OFF switch to target specific protein degradation in cancer. The ON/OFF function of photoswitches in PROTACs (as photoswitchable PROTACs) provide spatiotemporal control over protein degradation, and to an extent are correlated with their photoisomeric state (cis/trans-configuration), showcasing an application of the photochemistry concept in precision medicine. This study compiles the photoswitchable PROTACs targeted to bromodomain proteins: BRD 2, 3, and 4; kinases (BCR-ABL fusion protein, ABL); and the immunophilin FKBP12. Photocaging of PROTACs found successes in selective light-controlled degradation of kinase proteins, bromodomain-containing proteins, and estrogen receptors in cancer cells.

Structure-Guided Synthesis of FK506 and FK520 Analogs with Increased Selectivity Exhibit In Vivo Therapeutic Efficacy against Cryptococcus.

ABSTRACTCalcineurin is an essential virulence factor that is conserved across human fungal pathogens, including Cryptococcus neoformans, Aspergillus fumigatus, and Candida albicans. Although an excellent target for antifungal drug development, the serine-threonine phosphatase activity of calcineurin is conserved in mammals, and inhibition of this activity results in immunosuppression. FK506 (tacrolimus) is a naturally produced macrocyclic compound that inhibits calcineurin by binding to the immunophilin FKBP12. Previously, our fungal calcineurin-FK506-FKBP12 structure-based approaches identified a nonconserved region of FKBP12 that can be exploited for fungus-specific targeting. These studies led to the design of an FK506 analog, APX879, modified at the C-22 position, which was less immunosuppressive yet maintained antifungal activity. We now report high-resolution protein crystal structures of fungal FKBP12 and a human truncated calcineurin-FKBP12 bound to a natural FK506 analog, FK520 (ascomycin). Based on information from these structures and the success of APX879, we synthesized and screened a novel panel of C-22-modified compounds derived from both FK506 and FK520. One compound, JH-FK-05, demonstrates broad-spectrum antifungal activity in vitro and is nonimmunosuppressive in vivo. In murine models of pulmonary and disseminated C. neoformans infection, JH-FK-05 treatment significantly reduced fungal burden and extended animal survival alone and in combination with fluconazole. Furthermore, molecular dynamic simulations performed with JH-FK-05 binding to fungal and human FKBP12 identified additional residues outside the C-22 and C-21 positions that could be modified to generate novel FK506 analogs with improved antifungal activity.

Dissecting the Mechanisms of Hepcidin and BMP-SMAD Pathway Regulation By FKBP12

The BMP-SMAD pathway is activated when a dimeric ligand (BMP) interacts with a dimeric serine threonine kinase receptor (BMPRII) and triggers the activation of a dimeric BMP type I receptor (BMPRI). Catalytically active BMPRIs phosphorylate SMAD1/5/8 that, upon SMAD4 binding, translocate to the nucleus to regulate the expression of BMP target genes, including hepcidin. Hepcidin is the main regulator of iron homeostasis that controls body iron levels by binding and blocking the sole iron exporter ferroportin. In agreement, hepcidin expression is homeostatically activated by serum and liver iron, and its deficiency is a common hallmark of Hereditary Hemochromatosis (HH) and the major cause of iron overload in beta thalassemia. The components of the BMP-SMAD pathway relevant for hepcidin regulation are ALK2 and ALK3 (BMPRI); BMPR2 and ACVR2A (BMPRII), BMP2 and BMP6 (BMP ligands). Recently, we have identified the immunophilin FKBP12 as an inhibitor of hepcidin and demonstrated that FKBP12 binds ALK2 to avoid ligand-independent activation of the BMP-SMAD pathway.To investigate the mechanism of BMP-SMAD pathway and hepcidin regulation by FKBP12, we performed in vitro, ex vivo and in vivo studies. We found that FKBP12 sequestration by the immunosuppressive drug Tacrolimus (TAC) stabilizes ALK2-ALK2 homodimers and ALK2-ALK3 heterodimers in a transfected human hepatoma cell line. In addition, it increases the interaction of ALK2 with ACVR2A and BMPR2. To investigate the role of FKBP12 on BMP-SMAD signaling, BMPRI and II were silenced in murine primary hepatocytes.Despite FKBP12 co-immunoprecipitates only with ALK2, silencing of Alk2 and Alk3 completely blunts TAC-mediated BMP-SMAD pathway activation, suggesting that FKBP12 functionally interacts also with ALK3. Acvr2a silencing impairs TAC-dependent hepcidin upregulation, whereas Bmpr2 silencing does not. As expected, Fkbp12 silencing abrogates hepcidin upregulation by TAC, confirming the main role of this immunophilin in hepcidin regulation. In vivo, TAC treatment upregulates hepcidin in wild type and HH mouse models, but surprisingly, Fkbp12 mRNA downregulation by ASOs does not. Interestingly, Fkbp 2, 4 and 8 are highly expressed in murine hepatocytes and, according to literature data, are able to bind to TAC. Of note, Fkbp12 is the least expressed immunophilin in murine primary hepatocytes. To further investigate the FKBPs involved in TAC-dependent hepcidin regulation, Fkbp2, 4 and 8 were knockdown in murine primary HCs that were then treated with TAC. The TAC effect is preserved in siFkbp2- and siFkbp4-derived HCs, but abolished when Fkbp8 was downregulated.Overall these data suggest that: 1) FKBP12 regulates BMP-SMAD signaling by favoring ALK2-ALK3 homo and heterodimerization, and interaction with BMPRII in the absence of ligands; 2) TAC-mediated hepcidin upregulation is dependent upon ALK2, ALK3, ACVR2A, FKBP12 and FKBP8. 3) In vivo, TAC treatment upregulates hepcidin whereas Fkbp12 silencing does not, suggesting the existence of redundancy between the different FKBPs.Further studies are needed to dissect the role of FKBP8 in BMP-SMAD pathway and hepcidin regulation. DisclosuresAghajan: Ionis Pharmaceuticals, Inc.: Current Employment. Muckenthaler: Silence Therapeutics: Research Funding. Guo: Ionis Pharmaceuticals, Inc.: Current Employment.

ALK2 Receptor Kinase Association with FKBP12.6 Is Structurally Conserved with the ALK2-FKBP12 Complex.

The immunophilin FKBP12 is a known inhibitor of type I BMP and TGF-β receptors that competes for binding with their substrate SMADs. FKBP12 and the close paralog FKBP12.6 additionally assemble with ryanodine receptors to control Ca2+ release. Binding of FKBP12.6 to BMP/TGF-β receptors has yet to be investigated, but appears plausible given its high sequence similarity to FKBP12. Here, we found that FKBP12.6 can assemble with BMP and TGF-β-family type I receptors, but not with type II receptors. Cellular immunoprecipitation confirmed similar binding of FKBP12 and FKBP12.6 to the BMP receptor ALK2 (ACVR1), a known target of mutations in the congenital syndrome fibrodysplasia ossificans progressiva (FOP), as well as the pediatric brain tumor diffuse intrinsic pontine glioma (DIPG). SEC-MALS analyses using purified proteins indicated a direct 1:1 interaction between FKBP12.6 and the receptor’s cytoplasmic domains. The 2.17 Å structure of this ALK2-FKBP12.6 complex bound to the inhibitor dorsomorphin showed FKBP12.6 binding to the GS domain of ALK2 in a manner equivalent to the FKBP12 complex, with ALK2 residues Phe198 and Leu199 extending into the FK506-binding pocket of FKBP12.6. These findings suggest a level of redundancy in FKBP-family regulation of BMP and TGF-β signaling.

Azidothymidine (AZT) Inhibits Proliferation of Human Ovarian Cancer Cells by Regulating Cell Cycle Progression.

Drug resistance is a significant cause of high mortality in ovarian cancer (OC) patients. The reverse transcriptase inhibitor azidothymidine (AZT) has been utilized as a treatment for tumors, but its role in OC treatment has not been revealed. The aim of the present in vitro study was to examine the influence of AZT on the growth of human OC cells and the involved proteins. The proliferation, cell cycle distribution, extent of apoptosis, mitotic index, and terminal restriction fragment length were examined in three OC cell lines, CaOV3, TOV112D, and TOV21G, treated with AZT. AZT inhibited growth of the TOV21G and CaOV3 cell lines by regulating cell cycle distribution. Specifically, AZT caused G2/M phase arrest on TOV21G cells and S phase arrest on CaOV3 cells. In addition, AZT treatment induced up-regulation of p21 and p16 in the TOV21G and CaOV3 cell line, respectively. AZT inhibited cell proliferation in serous and clear cell OC via the regulation of cell cycle distribution.

Rapamycin restrains platelet procoagulant responses via FKBP-mediated protection of mitochondrial integrity

Background and PurposeRapamycin is a potent immunosuppressant and anti-proliferative agent used clinically to prevent organ transplant rejection and for coating coronary stents to counteract restenosis. Rapamycin complexes with the immunophilin FKBP12, which subsequently binds and inhibits mTORC1. Despite several reports demonstrating that rapamycin affects platelet-mediated responses, the underlying mechanism of how it alters platelet function is poorly characterised. This study aimed to elucidate the effect of rapamycin on platelet procoagulant responses. Experimental ApproachThe effect of rapamycin on platelet activation and signalling was investigated alongside the catalytic mTOR inhibitors KU0063794 and WYE-687, and the FKBP12-binding macrolide FK506. Key ResultsRapamycin affects platelet procoagulant responses by reducing externalisation of the procoagulant phospholipid phosphatidylserine, formation of balloon-like structures and local generation of thrombin. Catalytic mTOR kinase inhibitors did not alter platelet procoagulant processes, despite having a similar effect as rapamycin on Ca2+ signalling, demonstrating that the effect of rapamycin on procoagulant responses is independent of mTORC1 inhibition and not linked to a reduction in Ca2+ signalling. FK506, which also forms a complex with FKBP12 but does not target mTOR, reduced platelet procoagulant responses to a similar extent as rapamycin. Both rapamycin and FK506 prevented the loss of mitochondria integrity induced by platelet activation, one of the central regulatory events leading to PS externalisation. Conclusions and ImplicationsRapamycin suppresses platelet procoagulant responses by protecting mitochondrial integrity in a manner independent of mTORC1 inhibition. Rapamycin and other drugs targeting FKBP immunophilins could aid the development of novel complementary anti-platelet therapies.

CONSTANS-FKBP12 interaction contributes to modulation of photoperiodic flowering in Arabidopsis.

Flowering time is a key process in plant development. Photoperiodic signals play a crucial role in the floral transition in Arabidopsis thaliana, and the protein CONSTANS (CO) has a central regulatory function that is tightly regulated at the transcriptional and post-translational levels. The stability of CO protein depends on a light-driven proteasome process that optimizes its accumulation in the evening to promote the production of the florigen FLOWERING LOCUS T (FT) and induce seasonal flowering. To further investigate the post-translational regulation of CO protein we have dissected its interactome network employing in vivo and in vitro assays and molecular genetics approaches. The immunophilin FKBP12 has been identified in Arabidopsis as a CO interactor that regulates its accumulation and activity. FKBP12 and CO interact through the CCT domain, affecting the stability and function of CO. fkbp12 insertion mutants show a delay in flowering time, while FKBP12 overexpression accelerates flowering, and these phenotypes can be directly related to a change in accumulation of FT protein. The interaction is conserved between the Chlamydomonas algal orthologs CrCO-CrFKBP12, revealing an ancient regulatory step in photoperiod regulation of plant development.

The Immunophilin FKBP12 Inhibits Hepcidin By Modulating BMP Type I-Type II Receptors Interaction and Ligand Responsiveness

Introduction The liver hormone hepcidin is the master regulator of iron metabolism that modulates iron release into the circulation by binding and blocking the iron exporter ferroportin (Nemeth et al., 2004). Hepcidin expression is under the control of the BMP-SMAD pathway (Babitt et al., 2006), whose activation requires the formation of a hexameric complex composed of a dimer of BMP receptors type I (BMPR-Is), a dimer of BMPR type II (BMPR-IIs) and dimeric ligands. ALK2 and ALK3, as BMPR-Is (Steinbiecker et al., 2011), BMPR2 and ACVR2A, as BMPR-IIs (Mayeur et al., 2014), and BMP2 (Koch et al, 2017) and BMP6 (Meynard et al., 2009), as ligands, control hepcidin expression in vivo. We previously demonstrated that the immunophilin FKBP12 limits hepcidin expression in hepatocytes by binding ALK2 (Colucci et al., 2017). However, the molecular mechanism whereby FKBP12 regulates ALK2 and its relationship with BMPR-IIs and ligands in the regulation of the BMP-SMAD pathway and hepcidin expression are still unclear. Methods: BMPR-Is dimerization was evaluated by co-immunoprecipitation (CoIP) experiments performed in the HuH7 human hepatoma cell line. BMP-SMAD pathway and hepcidin promoter activation were analyzed by using a reporter vector with the luciferase under the control of BMP responsive elements or of the human hepcidin promoter, respectively. Endogenous hepcidin expression was measured by qRT-PCR. Results: Since BMPRIs act as dimers, we first tested whether FKBP12 modulates the dimerization process. MYC- and FLAG-tagged ALK2 or ALK3 were transfected in HuH7 cells in the presence of FKBP12. Cells were treated or not with tacrolimus (TAC), an immunosuppressive drug that sequesters FKBP12 from ALK2. FKBP12 promotes ALK2 homodimers, functionally inactive in the absence of ligands, with no effect on ALK3 homodimerization. TAC promotes increased ALK2 homodimerization and SMAD1/5/8 phosphorylation, demonstrating that in the absence of FKBP12, ALK2 homodimers are stabilized and functionally active. We next focused on BMP6, the physiologic ligand that binds preferentially ALK2 and plays a fundamental role in hepcidin regulation in vivo. In HuH7 cells transfected with FKBP12 and ALK2, BMP6 treatment reduced FKBP12-ALK2 binding and increased ALK2 homodimers. In agreement, SMAD1/5/8 phosphorylation was increased, indicating that FKBP12 displacement allows the formation of functional receptor complexes responsive to BMP6. BMPR-Is activate SMAD1/5/8 following BMPR-IIs phosphorylation. Since TAC induces SMAD1/5/8 phosphorylation in the absence of ligands, we hypothesized that FKBP12 displacement also affects the formation of BMPR-I/BMPR-II oligomers. HuH7 cells were transfected with ALK2, BMPR2 or ACVR2A and FKBP12, and treated or not with TAC. FKBP12 sequestration by TAC enhances the ALK2-BMPR2 and ALK2-ACVR2A interaction and accordingly activates SMAD1/5/8 signaling. Given that FKBP12 modulates BMP receptor interaction, we wondered how this functionally impacts on the response to BMP ligands, as BMP2, that guarantees basal hepcidin levels by binding ALK3, and BMP6, upregulated in iron overload that signals preferentially through ALK2. ALK3 upregulates the BMP pathway and hepcidin expression in a similar way in response to BMP2 and BMP6, in agreement with the evidence that both ligands bind ALK3. ALK2, which failed to activate the pathway in the absence ligands, leads to a greater response to BMP6, consistent with the fact that it is the BMP6 receptor. Thus FKBP12 quantitatively, rather than qualitatively, modulates the BMP-SMAD pathway activation in response to BMP ligands. Conclusions: Altogether our results clarify the molecular mechanisms of hepcidin regulation demonstrating that: 1) FKBP12 limits hepcidin expression by inducing the formation of inactive ALK2 homodimers in the absence of ligands. 2) Decreased FKBP12 binding to ALK2, by TAC or BMP6, favors the formation of active ALK2 homodimers. 3) FKBP12 sequestration increases the binding of ALK2 with the BMPR-IIs, thus favoring SMAD1/5/8 phosphorylation and pathway activation. 4) FKBP12 quantitatively modulates the response of BMPRIs to the ligands BMP2 and BMP6. Disclosures Camaschella: Vifor Iron Core: Consultancy; Celgene: Consultancy; Novartis: Consultancy.

S1628 TARGETING THE IMMUNOPHILIN FKBP12 BY DRUG REPURPOSING OR RNASE‐BASED APPROACHES FOR BMP‐SMAD PATHWAY AND HEPCIDIN UPREGULATION IN HEREDITARY HEMOCHROMATOSIS

Background:The main regulator of iron homeostasis, the liver hormone hepcidin, is inappropriately low in Hereditary Hemochromatosis (HH). HH is caused by mutations in HJV, TFR2 and HFE that negatively affect the liver BMP‐SMAD pathway, the main signaling pathway regulating hepcidin, thus causing iron overload. Current phlebotomy‐based therapy is symptomatic, does not correct hepcidin deficiency and is not well tolerated by all patients. We recently identified a new function for a cytosolic protein, the immunophilin FKBP12, and demonstrated that it negatively controls the BMP‐SMAD pathway by binding the type I receptor ALK2 (Colucci et al., 2017). Therefore, FKBP12 may be a novel therapeutic target for increasing hepcidin expression.Aims:Since sequestration of FKBP12 by Tacrolimus (TAC) upregulates hepcidin in wild type (WT) mice, the aim of this work is to investigate whether reducing FKBP12 may restore hepcidin expression in HH mouse models.Methods:Hepatocytes were isolated from WT, Tfr2 and Hjv KO mice and treated with TAC. Hjv KO mice were treated for 28 days with TAC delivered by subcutaneous miniosmotic pumps. In vivo RNAse‐based Fkbp12 downregulation was achieved using antisense oligonucleotides (ASOs). Hepcidin, Id1, Smad7, Erfe expression were evaluated by qRT‐PCR. Iron parameters, CBC and terminal erythroid differentiation were analyzed by standard methods.Results:FKBP12 sequestration by TAC upregulates hepcidin through BMP‐SMAD pathway activation in Hjv and Tfr2 KO primary hepatocytes, suggesting that FKBP12 targeting may be effective in HH. Due to the transient effect of TAC‐mediated hepcidin upregulation, we treated Hjv KO mice with chronic TAC administration by miniosmotic pumps. TAC upregulated hepcidin through BMP‐SMAD pathway activation and caused spleen iron retention, demonstrating that the FKBP12 inhibitory function on ALK2 is also preserved in an in vivo HH model. Since TAC is an immunosuppressive drug, we are exploring Fkbp12 downregulation by ASOs. ASO‐Fkbp12 treatment of WT mice decreased Fkbp12 expression in liver (78.4%), spleen (66.3%) and kidney (56.4%) but not in the BM. Fkbp12 inactivation mildly upregulated Hamp, Id1 and Smad7, consistent with BMP‐SMAD pathway activation. Although hepcidin expression was not significantly increased, we observed a reduction in transferrin saturation and serum iron and increased spleen iron retention. Hemoglobin and MCV were reduced in ASO‐Fkbp12‐treated mice, characterized also by a trend towards a reduction in RBC number and HCT. Spleen size and EPO levels remained unchanged. Analysis of terminal erythroid differentiation demonstrated an increased percentage of immature erythroid precursors with comparable Ter119+ cells in the spleen of ASO‐Fkbp12‐treated mice but not in the BM, and accordingly Erfe, the erythroid hepcidin inhibitor, is upregulated.Summary/Conclusion:Fkbp12 downregulation in the liver upregulates the BMP‐SMAD pathway, although hepcidin expression is minimally affected. Fkbp12 downregulation in the spleen impairs erythropoiesis increasing Erfe expression and decreasing serum iron concentration. Considering the presence of two inhibitory signals (high Erfe, low iron), hepcidin is inappropriately high in ASO‐Fkbp12 treated mice. We conclude that to increase hepcidin expression in HH, FKBP12 must be specifically downregulated in the liver. In addition, our results reveal a potential effect of FKBP12 on erythropoiesis, uncovering a novel role for the immunophilin in the regulation of erythropoiesis.

Novel Insights Into Systemic Iron Regulation

Iron, an essential element in mammals, is absorbed by duodenal enterocytes, enters the circulation through the iron exporter ferroportin, (FPN), circulates bound to transferrin and is uptaken through Transferrin Receptor 1. If in excess, iron is stored in macrophages and hepatocytes and released when needed. To maintain systemic iron homeostasis and to avoid the formation of “non transferrin bound iron” (NTBI), a highly reactive form which causes organ damage, the liver synthetizes hepcidin that, binding FPN, blocks iron export to the circulation. Hepcidin integrates signals from body iron, erythropoiesis and inflammatory cytokines. Defective hepcidin production causes iron overload and organ failure in Hereditary Hemochromatosis and Thalassemia; hepcidin excess leads to anemia in Iron Refractory iron Deficiency Anemia (IRIDA) and Anemia of Inflammation (AI).In hepatocytes hepcidin is under the control of the BMP-SMAD pathway, which is activated in a paracrine manner by BMP2 and BMP6 produced by liver sinusoidal endothelial cells. BMP2 maintains hepcidin basal levels, while BMP6 controls its expression in response to iron. The two ligands have different affinity for BMP type I receptors ALK2 and ALK3, suggesting two distinct branches of the hepcidin activation pathway. This possibility is consistent with the non-redundant function of BMP2 and BMP6, the different iron phenotype of hepatocyte-conditional ALK2 and ALK3 KO mice and the residual ability of BMP6 to activate hepcidin in hemochromatosis mice. Moreover ALK2, but not ALK3, is inhibited by the immunophilin FKBP12 in the absence of ligands. The BMP pathway activation depends upon the coreceptor hemojuvelin (HJV), the MHC class I protein HFE and the second transferrin receptor (TFR2). Mutations of all these proteins lead to decreased hepcidin expression in hemochromatosis.Hepcidin expression is inhibited in iron deficiency, hypoxia and when erythropoiesis is increased. Inhibitors are the liver transmembrane serine protease TMPRSS6, whose genetic inactivation causes IRIDA, and the erythroid hormone erythroferrone (ERFE), which is released by erythropoietin-stimulated erythroblasts. The mechanism of hepcidin inhibition by ERFE is unclear; still to allow ERFE function the BMP-SMAD pathway has not to be hyperactive. Intriguingly, both iron deficiency and erythropoiesis require epigenetic modifications at the hepcidin locus with HDAC3-dependent reversible loss of H3K9ac and H3K4me3.Hepcidin also acts as an antimicrobial peptide since its expression, increased by proinflammatory cytokines, such as IL6 through JAK2-STAT3 signaling, restricts iron availability for microbial growth. This first-line of defense against infections negatively influences erythropoiesis since chronic hepcidin activation causes AI. Despite persistent JAK2-STAT3 activation, inhibition of the BMP-SMAD pathway reduces hepcidin activation in AI experimental rodent models, suggesting that hepcidin activation in inflammation requires a functional BMP-SMAD pathway. Independently from hepcidin, inflammation also reduces FPN mRNA levels, favoring macrophage iron sequestration.The identification of hepcidin-ferroportin axis molecular players has translational implications. In primary and secondary iron overload hepcidin agonists (hepcidin peptides or mimics, agents that inhibit the hepcidin inhibitor TMPRSS6 and likely the ALK2-inhibitor FKBP12) and ferroportin inhibitors are potentially useful to prevent iron overload and/or to favor iron redistribution to macrophages. In case of AI, hepcidin antagonists (including anti-hepcidin, anti-HJV and anti-BMP6 monoclonal antibodies, L-enantiomeric oligonucleotides targeting hepcidin, siRNA against hepcidin, non-anticoagulant heparins, the ALK2 inhibitor momelotinib) might improve erythropoiesis increasing iron availability.The effect of some agents that have now entered the clinical phase will become apparent in the coming years. DisclosuresCamaschella:vifor Pharma: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Inhibiting the Immunophilin FKBP12: A Potential Therapeutic Approach to Iron Overload/Low Hepcidin Disorders

IntroductionHepcidin negatively regulates body iron by binding and degrading the iron exporter ferroportin. Its expression is mainly controlled by the liver BMP-SMAD pathway whose activation requires BMP ligands (BMP2 and BMP6), constitutively active BMP type II receptors and the type I receptors ALK2 and ALK3. The co-receptor hemojuvelin (HJV) further potentiates the signaling. Mutations in key genes of the pathway impair hepcidin synthesis in Hereditary Hemochromatosis (HH), one of the most severe form being due to HJV mutations. As current therapies are symptomatic and do not correct hepcidin deficiency, alternative targeted therapies to increase hepcidin production are needed.We have demonstrated that the immunophilin FKBP12 binds ALK2 in hepatoma cells to prevent uncontrolled activation of the BMP pathway in the absence of ligands. We also demonstrated that FKBP12 sequestration by immunosuppressive drugs, such as FK506 (tacrolimus, TAC) or rapamycin, upregulate hepcidin in vitro. The role of FKBP12 is maintained in vivo since acute TAC treatment of wild type (WT) mice increases hepcidin expression (Colucci et al., Blood 2017).The aim of this study is to investigate whether pharmacologic inactivation of FKBP12, by TAC or antisense oligonucleotides (ASO), upregulates hepcidin for therapeutic purposes.MethodsPrimary hepatocytes isolated from WT, Hjv and Tfr2 KO mice (sv129/j background) were treated with increasing concentrations of TAC. Hepcidin and Id1 expression was investigated by qRT-PCR. Nine-weeks-old Hjv KO male mice were treated for 28 days with TAC (0.37 mg/h) delivered through surgically implanted mini-osmotic pumps. Six-weeks-old WT mice, were treated twice a week for 6 weeks with 50 mg/kg of Fkbp12 or control ASO. Mice were sacrificed and analyzed for iron, CBC, erythropoiesis and liver expression of hepcidin and BMP-SMAD target genes.ResultsTAC treatment of primary HCs from Hjv KO (Colucci et al., Blood 2018) and Tfr2 KO mice upregulates hepcidin as in WT mice, suggesting that HJV and TFR2 are dispensable for FKBP12-dependent hepcidin regulation and providing the proof of principle for FKBP12 targeting in HH. First, we explored a drug repurposing approach in the severe Hjv KO mice by chronic subcutaneous delivery of suboptimal, non-immunosuppressive TAC doses. Treatment of Hjv KO mice with TAC upregulates hepcidin via BMP-SMAD pathway activation, as assessed by Id1 and Smad7 upregulation. Since TAC also inhibits calcineurin upon FKBP12 sequestration and to avoid potential off-targets effect, FKBP12 was inactivated by ASO. Fkbp12 ASO treatment of WT mice decreases Fkbp12 expression by about 70-80% in liver, spleen and kidney, but not in the bone marrow. Fkbp12 ASO-treated mice exhibit microcytic anemia, decreased serum iron and upregulation of liver BMP-SMAD target genes. However, hepcidin remains inappropriately high considering the low serum iron. Fkbp12 ASO-treated mice show increased spleen immature erythroid precursors and increased expression of erythroferrone (Erfe). This is due to the unexpected effect of FKBP12 on spleen erythropoiesis and likely explains the lack of hepcidin upregulation.ConclusionsChronic TAC treatment in Hjv KO mice, which causes FKBP12 sequestration, improves hepcidin expression via BMP-SMAD pathway and favors spleen iron retention, suggesting that this “drug repurposing approach” may be beneficial for all iron overload disorders exhibiting low hepcidin.ASO-Fkbp12 in WT mice efficiently downregulates hepatic Fkbp12, upregulates the BMP-SMAD pathway but leaves hepcidin unchanged compared to control mice. We hypothesize that this result reflects the concomitant hepcidin inhibition due to decreased serum iron and increased Erfe expression. The latter is a consequence of spleen Fkbp12 reduction in ASO-treated mice, suggesting that a hepatocyte targeting Fkbp12 ASO is required for therapeutic purposes.Since Fkbp12 binds and inhibits ALK2 (Colucci et al., Blood 2017), our in vitro and in vivo data suggest that HJV and TFR2 functionally interact with ALK3 but not with ALK2. DisclosuresAghajan:Ionis Pharmaceuticals, Inc: Employment. Guo:Ionis Pharmaceuticals, Inc: Employment. Camaschella:vifor Pharma: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Rapamycin and FK506 derivative TH2849 could ameliorate neurodegenerative diseases through autophagy with low immunosuppressive effect.

Autophagy is an essential cellular process concern with cellular homeostasis down-regulated by mTOR, whose activity can be modulated by rapamycin, a kind of lipophilic macrolide antibiotic, through forming a complex with immunophilin FKBP12 essential for mTOR regulation to induce autophagy. Therefore, rapamycin is normally used as a neuron protective agent. The immunophilin FKBP12 binding ligand FK506 is well known as an immunosuppressive agent by inhibiting the calcineurin expression. In this study, we synthesized a series of modified compounds based on the FKBP12 binding moiety to as same as the binding structure of rapamycin and FK506 particularly. We removed the other binding regions of the complex that has the property of immunosuppression. We found that a novel small molecule named TH2849 from these derivative compounds has a significant binding connection with mTOR by comparing to calcineurin. The effects of TH2849 on calcineurin/NFAT were not as significant as FK506, and weak effects on IL2/p34cdc2 /cyclin signaling pathway were also found. Moreover, TH2849 also shows mitochondrial protective effect through stabilizing the mitochondrial structure and transmembrane potential (ΔΨm) and could rescue dopaminergic neurons in MPTP-treated zebrafishes as well as mice models with less immunosuppressive effect. Our present study shows that TH2849 works as a neuroprotective agent possibly by inducing autophagy and low immunosuppressive effect.

The immunophilin FKBP12 inhibits hepcidin expression by binding the BMP type I receptor ALK2 in hepatocytes

AbstractThe expression of the key regulator of iron homeostasis hepcidin is activated by the BMP-SMAD pathway in response to iron and inflammation and among drugs, by rapamycin, which inhibits mTOR in complex with the immunophilin FKBP12. FKBP12 interacts with BMP type I receptors to avoid uncontrolled signaling. By pharmacologic and genetic studies, we identify FKBP12 as a novel hepcidin regulator. Sequestration of FKBP12 by rapamycin or tacrolimus activates hepcidin both in vitro and in murine hepatocytes. Acute tacrolimus treatment transiently increases hepcidin in wild-type mice. FKBP12 preferentially targets the BMP receptor ALK2. ALK2 mutants defective in binding FKBP12 increase hepcidin expression in a ligand-independent manner, through BMP-SMAD signaling. ALK2 free of FKBP12 becomes responsive to the noncanonical inflammatory ligand Activin A. Our results identify a novel hepcidin regulator and a potential therapeutic target to increase defective BMP signaling in disorders of low hepcidin.

FKBP12-Dependent Inhibition of Calcineurin Mediates Immunosuppressive Antifungal Drug Action in Malassezia.

ABSTRACTThe genus Malassezia includes yeasts that are commonly found on the skin or hair of animals and humans as commensals and are associated with a number of skin disorders. We have previously developed an Agrobacterium tumefaciens transformation system effective for both targeted gene deletion and insertional mutagenesis in Malassezia furfur and M. sympodialis. In the present study, these molecular resources were applied to characterize the immunophilin FKBP12 as the target of tacrolimus (FK506), ascomycin, and pimecrolimus, which are calcineurin inhibitors that are used as alternatives to corticosteroids in the treatment of inflammatory skin disorders such as those associated with Malassezia species. While M. furfur and M. sympodialis showed in vitro sensitivity to these agents, fkb1Δ mutants displayed full resistance to all three of them, confirming that FKBP12 is the target of these calcineurin inhibitors and is essential for their activity. We found that calcineurin inhibitors act additively with fluconazole through an FKBP12-dependent mechanism. Spontaneous M. sympodialis isolates resistant to calcineurin inhibitors had mutations in the gene encoding FKBP12 in regions predicted to affect the interactions between FKBP12 and FK506 based on structural modeling. Due to the presence of homopolymer nucleotide repeats in the gene encoding FKBP12, an msh2Δ hypermutator of M. sympodialis was engineered and exhibited an increase of more than 20-fold in the rate of emergence of resistance to FK506 compared to that of the wild-type strain, with the majority of the mutations found in these repeats.

A Novel Crosstalk Between Iron Homeostasis and mTOR Mediated By the Immunophilin FKBP12

IntroductionHepcidin, the liver hormone that regulates iron homeostasis, is mainly activated via the BMP/SMAD pathway. Among other stimuli, as erythropoiesis expansion, inflammation and hypoxia, hepcidin expression is influenced by drugs, including the mTOR inhibitor rapamycin (RAPA) (Mleczko-Sanecka et al., Blood 2014). In the liver, BMP type I receptors, ALK2 and ALK3, are both essential for hepcidin regulation (Steinbiecker et al., Blood 2012), whereas BMP type II receptors, BMPR2 and ACVR2A, have a redundant role (Mayeur et al., Blood 2014). The signaling is activated when BMP type II receptors phosphorylate residues of the glycine/serine rich (GS) domain of BMP type I receptors, which then activate SMAD1/5/8. Treatment with RAPA increases hepcidin expression in murine hepatocytes and may cause microcytic anemia in patients (Przybylowski P. et al., Transplant Proc. 2013). However, the mechanisms involved in hepcidin and mTOR crosstalk are unknown.MethodsHepcidin, BMP-SMAD and mTOR target genes were analyzed in human hepatoma cell lines and murine primary hepatocytes (HCs), treated with RAPA (100 nM), Torin1 (100 nM), tacrolimus (TAC, 1 μg/ml) in the presence or absence of the BMP pathway inhibitor DMH1 (0.5 μg/ml) or of the ligands BMP6, BMP2, Activin B (ActB) and Activin A (ActA) (1-100 ng/ml). ALK2wt was mutagenized in the GS domain (R206H, Q207E) or close to the GS domain (R258S) to generate ALK2mut with reduce binding to FKBP12 (Taylor et al., Cancer Res. 2014). SMAD1/5/8 phosphorylation was analyzed by Western Blotting in cells transfected with SMAD1 and ALK2wt or ALK2mut and treated or not with TAC, BMP6 and ActA. Binding of FKBP12 to ALK2wt and ALK2mut was assessed by coimmunoprecipitation of tagged proteins in transfected cells treated as above. Eight weeks old C57BL/6 wild type male mice were treated with a single dose of TAC (10 mg/kg) or vehicle and sacrificed at different time points (3-18 hrs). Hepcidin expression, LIC, SIC, serum iron and hematological parameters were analyzed by standard methods.ResultsWe analyzed hepcidin expression in hepatoma cells and primary HCs treated with RAPA, an inhibitor of mTORC1, and Torin1, an ATP-competitive inhibitor of mTORC1 and 2. Hepcidin is increased by RAPA, but not Torin1, in a SMAD1/5/8 dependent way since DMH1 abrogates the effect. RAPA inhibits mTORC1 when complexed with FKBP12, an immunophilin that binds BMP receptors to avoid leakage activation of the pathway. To investigate whether hepcidin upregulation by RAPA is mediated by FKBP12 sequestration, we used genetic and pharmacologic approaches. First, we confirmed by coimmunoprecipitation that ALK2mut have a reduced ability to bind FKBP12. Then we transfected hepatoma cells with ALK2wt and ALK2mut and analyzed hepcidin and BMP pathway activation. Overexpression of ALK2mut increases hepcidin through SMAD1/5/8 as shown by high levels of SMAD1 phosphorylation, an effect abrogated by DMH1. Second, we treated hepatoma cells and primary HCs with TAC, a calcineurin inhibitor that binds FKBP12. This treatment increases hepcidin through SMAD1/5/8, suggesting a mechanism shared with RAPA. The same effect is observed in vivo since hepcidin is increased at 6 hrs post-injection in TAC-treated wt mice. Our results identify FKBP12 as a novel regulator of hepcidin. In addition, FKBP12 displacement alters the BMP receptor selectivity to ligands. Despite ALK2wt preferentially binds BMP6, ALK2mut become responsive to ActA, a TGF-β ligand that signals through SMAD2/3. Hepcidin activation by BMP6, BMP2 and ActB is comparable in ALK2wt and ALK2mut expressing cells. However, ActA upregulates hepcidin (through SMAD1/5/8) only in ALK2mut transfected cells. This effect is due to the impaired ability of ALK2mut to bind FKBP12, since it is observed even in ALK2wt transfected cells pretreated with TAC.ConclusionsFKBP12 contributes to hepcidin regulation both in vitro and in vivo, thus adding a new player to the BMP-dependent hepcidin activation and a potential pharmacologic target for disorders characterized by low hepcidin and iron overload. Furthermore the ability of ALK2 to respond to Activin A, which is released in inflammation, links the BMP pathway-hepcidin activation to the inflammatory response. DisclosuresNo relevant conflicts of interest to declare.