A mouse model of juvenile hemochromatosis

Hypoferremic Response to Chronic Inflammation Is Controlled via the Hemojuvelin/Hepcidin/Ferroportin Axis and Does Not Involve Hepcidin-Independent Regulation of Fpn mRNA

The iron regulatory hormone hepcidin contributes to the pathogenesis of anemia of inflammation (AI) by inhibiting the iron exporter ferroportin in target cells, causing hypoferremia. Under acute inflammation, hepcidin induction requires hemojuvelin (Hjv), a bone morphogenetic protein co-receptor, while Fpn mRNA is also suppressed in a hepcidin-independent manner. However, it is unclear whether, during chronic inflammation, Hjv and hepcidin-independent Fpn mRNA regulation are critical for hypoferremia and AI. To address these questions, wild type and Hjv-/- mice, a model of hemochromatosis, were fed for 8 weeks an adenine-rich diet to develop chronic kidney disease (CKD). Renal inflammation, accessed by increased Il6 mRNA expression, did not differ among genotypes. Hjv disruption did not mitigate the severity of kidney injury but suppressed the inflammatory induction of liver hepcidin. CKD triggered hypoferremia and mild anemia in wild type mice; however, Hjv-/- littermates maintained high serum iron and normal hemoglobin, consistent with a protective effect of Hjv/hepcidin deficiency. Notably, tissue Fpn mRNA levels were not affected by the inflammatory milieu of CKD. Following injection of wild type or Hjv-/- mice with heat-killed Brucella abortus, Fpn mRNA was suppressed during the acute phase of inflammation but quickly recovered and persisted in the chronic phase. We conclude that Hjv deficiency reduces hepcidin levels and mitigates anemia in the CKD model, providing further support for pharmacological targeting of Hjv for the treatment of AI. Moreover, our data demonstrate that Fpn mRNA suppression only occurs under acute but not chronic inflammatory conditions and therefore cannot substantially contribute to AI pathogenesis.

Emerging Therapeutic Approaches for Anemia in Myelofibrosis

PURPOSE OF REVIEW

In this review, we highlight conventional agents and novel emerging therapeutic strategies to treat anemia in MF.

RECENT FINDINGS

Anemia is a common and challenging feature of myelofibrosis (MF). The pathobiology of anemia is multifactorial, including progressive bone marrow fibrosis, decreased erythropoiesis due to high hepcidin levels leading to iron sequestration in the reticuloendothelial system, hypersplenism, erythropoiesis inhibition by myelosuppressive JAK inhibitors (ruxolitinib, fedratinib), and others. MF-associated anemia has a negative impact on survival. Conventional agents to manage anemia include erythropoiesis-stimulating agents, danazol, corticosteroids, and immunomodulatory agents, but responses are infrequent and lack durability. Notable advancements have emerged in developing novel treatments for anemia in MF, including the regulatory approval of momelotinib (ACVR1/JAK1/2 inhibitor) in 2023 and development of novel promising agents targeting hemojuvelin and activins. Momelotinib and pacritinib (ACVR1/JAK2 inhibitor) are the preferred JAK inhibitors for patients with cytopenias (anemia, thrombocytopenia). Luspatercept and elritercept are activin receptor ligand traps, promoting erythroid maturation and late-stage erythropoiesis. Currently, luspatercept is being evaluated in a phase 3 trial (INDEPENDENCE™) for anemia in MF patients who are on a JAK2 inhibitor and require transfusions, and in a phase 2 trial (ODYSSEY) in combination with momelotinib in MF patients who are transfusion dependent, whether or not on a JAK inhibitor. Interim results of the RESTORE trial demonstrated that elritercept significantly decreased transfusions in MF patients. DISC-0974 is a first-in-class anti-hemojuvelin (positive hepcidin regulator) monoclonal antibody that decreased hepcidin expression, increased serum iron, and enhanced erythropoiesis in anemic patients with MF in a phase 1b/2 study. Burgeoning studies of novel anemia-targeted agents and combinations are significantly improving the quality of life and outcomes of patients with MF. The recent approval of momelotinib to treat MF with anemia and the emerging novel anemia-directed strategies in early and advanced clinical development have ushered in a new era in the treatment of MF-related anemia.

Hepcidin expression is associated with increased γ-secretase-mediated cleavage of neogenin in the liver

Neogenin (NEO1) is a ubiquitously expressed transmembrane protein. It interacts with hemojuvelin (HJV). Both NEO1 and HJV play pivotal roles in iron homeostasis by inducing hepcidin expression in the liver. Our previous studies demonstrated that this process depends on Neo1-Hjv interaction and showed that the Hjv-mediated hepcidin expression is correlated with the accumulation of a truncated and membrane-associated form of Neo1. In this study, we tested whether hepcidin expression is induced by increased γ-secretase-mediated cleavage of Neo1 in the liver. We found that Neo1 underwent cleavage of its ectodomain and intracellular domains by α- and γ-secretases, respectively, in hepatoma cells. Our in vitro studies suggest that γ-secretase is responsible for cleavage and release of the cytoplasmic domain of Neo1 in the Hjv-Neo1 complex. This process was enhanced by the inhibition of α-secretase proteolysis and by co-expression with the Neo1-binding partner, Alk3. Further in vivo studies indicated that Neo1 induction of hepcidin expression required γ-secretase cleavage. Interestingly, neither predicted form of γ-secretase-cleaved Neo1 was able to induce hepcidin when separately expressed in hepatocyte-specific Neo1 KO mice. These results imply that the function of Neo1 requires a de novo γ-secretase proteolysis. Additional studies revealed that in addition to the Hjv-binding domains, the function of Neo1 also required its C-terminal intracellular domain and the N-terminal immunoglobulin-like domains that are involved in Neo1 binding to Alk3. Together, our data support the idea that Neo1 induction of hepcidin is initiated as a full-length form and requires a de novo γ-secretase cleavage of Neo1's cytoplasmic domain.

Investigational drugs in early phase trials for myelofibrosis

INTRODUCTION

Myelofibrosis (MF) is a chronic myeloproliferative neoplasm characterized by bone marrow fibrosis, cytopenias, and organomegaly. Four JAK inhibitors are US-FDA approved for treatment of MF. While these drugs reduce symptom burden and spleen size to varying degrees, they do not affect the natural disease course or decrease the risk of leukemic transformation. Therefore, there is a strong need for newer therapies to further advance the field and improve the outcomes of MF. In this review, we cover novel therapies for MF currently in early stages of development.

AREAS COVERED

We present the latest data from early phase clinical trials in MF using drugs with diverse therapeutic mechanisms, including novel JAK-STAT pathway inhibitors, epigenetic therapies, antifibrotic agents, and immunotherapeutic strategies. Additionally, we cover drugs targeted toward anemia improvement in MF.

EXPERT OPINION

Numerous agents representing diverse drug classes are in clinical development for MF. While deeper and durable improvements in splenomegaly, symptoms, and anemia are the main clinical objectives, a number of putative biomarkers are being assessed as measures of potential 'disease modification.' Although JAK inhibitor monotherapy represents the current standard, it is hoped that JAK inhibitor-based rational combinations and driver mutation-specific therapies will soon usher in a new era.

Bone phenotyping of murine hemochromatosis models with deficiencies of Hjv, Alk2, or Alk3: The influence of sex and the bone compartment

Osteopenia is frequently observed in patients with iron overload, especially in those with HFE-dependent hereditary hemochromatosis (HH). Interestingly, not all mouse models of HH show bone loss, suggesting that iron overload alone may not suffice to induce bone loss. In this study, the bone phenotypes of Hjv-/- and hepatocyte-specific Alk2- and Alk3-deficient mice as additional mouse models of HH were investigated to further clarify, how high iron levels lead to bone loss and which signaling mechanisms are operational. Neither male nor female 12-week-old Hjv-/- mice had an altered trabecular or cortical bone mass or bone turnover, despite severe iron overload. Male 12-month-old Hjv-/- mice even presented with a higher femoral trabecular bone volume compared to wildtype mice. Similarly, female mice with hepatocyte-specific Alk2 or Alk3 deficiency did not show an altered bone phenotype at 3, 6, and 12 months of age. Male hepatocyte-specific Alk3-deficient mice also had a normal trabecular bone mass at all ages analyzed, despite showing increased bone resorption and decreased bone formation parameters. Interestingly, hepatocyte-specific Alk2-deficient mice showed reduced femoral trabecular bone at 6 months of age due to suppressed bone formation. Cortical thickness at the femur was reduced in both, 6-month-old male hepatocyte-specific Alk2- and Alk3-deficient mice. Raising hepatocyte-specific Alk2-deficient male mice on an iron-deficient diet rescued the bone phenotype. Taken together, despite iron overload, trabecular bone microarchitecture was not altered in mice deficient of Hjv or Alk3. Only male hepatocyte-specific Alk2-deficient mice showed site-specific lower trabecular and cortical bone mass at the femur, which was dependent on iron. Thus, bone loss does not correlate with the extent of iron overload in these mouse models, but may relate to the amount of iron-loaded macrophages, as precursors of osteoclasts, in the bone marrow.

Iron homeostasis and ferroptosis in human diseases: mechanisms and therapeutic prospects

Iron, an essential mineral in the body, is involved in numerous physiological processes, making the maintenance of iron homeostasis crucial for overall health. Both iron overload and deficiency can cause various disorders and human diseases. Ferroptosis, a form of cell death dependent on iron, is characterized by the extensive peroxidation of lipids. Unlike other kinds of classical unprogrammed cell death, ferroptosis is primarily linked to disruptions in iron metabolism, lipid peroxidation, and antioxidant system imbalance. Ferroptosis is regulated through transcription, translation, and post-translational modifications, which affect cellular sensitivity to ferroptosis. Over the past decade or so, numerous diseases have been linked to ferroptosis as part of their etiology, including cancers, metabolic disorders, autoimmune diseases, central nervous system diseases, cardiovascular diseases, and musculoskeletal diseases. Ferroptosis-related proteins have become attractive targets for many major human diseases that are currently incurable, and some ferroptosis regulators have shown therapeutic effects in clinical trials although further validation of their clinical potential is needed. Therefore, in-depth analysis of ferroptosis and its potential molecular mechanisms in human diseases may offer additional strategies for clinical prevention and treatment. In this review, we discuss the physiological significance of iron homeostasis in the body, the potential contribution of ferroptosis to the etiology and development of human diseases, along with the evidence supporting targeting ferroptosis as a therapeutic approach. Importantly, we evaluate recent potential therapeutic targets and promising interventions, providing guidance for future targeted treatment therapies against human diseases.

First-in-Human Phase 1 Study Evaluating the Safety, Pharmacokinetics, and Pharmacodynamics of DISC-0974, an Anti-Hemojuvelin Antibody, in Healthy Participants

Pathologic elevations in hepcidin, a key regulator of iron homeostasis, contribute to anemia of inflammation in chronic disease. DISC-0974 is a monoclonal antibody that binds to hemojuvelin and blocks bone morphogenetic protein signaling, thereby suppressing hepcidin production. Reduction of systemic hepcidin levels is predicted to increase iron absorption and mobilize stored iron into circulation, where it may be utilized by red blood cell (RBC) precursors in the bone marrow to improve hemoglobin levels and to potentially alleviate anemia of inflammation. We conducted a first-in-human, double-blind, placebo-controlled, single-ascending dose study to evaluate safety, pharmacokinetics, and pharmacodynamics of DISC-0974 in healthy participants. Overall, 42 participants were enrolled and received a single dose of placebo or DISC-0974 at escalating dose levels (7-56 mg), administered intravenously (IV) or subcutaneously (SC). DISC-0974 was well tolerated, with a safety profile comparable to that of placebo. Pharmacokinetic data was dose and route related, with a terminal half-life of approximately 7 days. The bioavailability of SC dosing was ∼50%. Pharmacodynamic data showed dose-dependent decreases in serum hepcidin, with reductions of nearly 75% relative to baseline at the highest dose level tested, and corresponding increases in serum iron in response to DISC-0974 administration. Dose-dependent changes in serum ferritin and hematology parameters were also observed, indicating mobilization of iron stores and downstream effects of enhanced hemoglobinization and production of RBCs. Altogether, these data are consistent with the mechanism of action of DISC-0974 and support the selection of a biologically active dose range for evaluation in clinical trials for individuals with anemia of inflammation.

Hepcidin contributes to largemouth bass (Micropterus salmoides) against bacterial infections

The increasing emergence and dissemination of bacterial pathogens in largemouth bass culture accelerate the desire for new treatment measures. Antimicrobial peptides as the host's antimicrobial source dominate the preferred molecules for discovering antibacterial agents. Here, the potential of Hepcidin-1 from largemouth bass (Micropterus salmoides) (MsHep-1) against bacterial infection is demonstrated. MsHep-1 not only improved the survival rate in infection experiments involving Nocardia seriolae (12 %) and Aeromonas hydrophila (18 %) but also coped with iron overload conditions in vivo. Moreover, the antibacterial activity of MsHep-1 in vitro was identified against both gram-negative and gram-positive bacteria. Mechanistic studies show MsHep-1 leads to bacterial death by changing the bacterial membrane potential and disrupting the bacterial membrane structure. These findings demonstrate that MsHep-1 may play an important role in the host response to bacterial infection. It provides promising strategies in the application of immunosuppression prevention and control in fish. AMPs may be a promising and available reservoir for treating the current bacterial diseases.

Comparison of the effects of high dietary iron levels on bone microarchitecture responses in the mouse strains 129/Sv and C57BL/6J

Iron is an essential nutrient for all living organisms. Both iron deficiency and excess can be harmful. Bone, a highly metabolic active organ, is particularly sensitive to fluctuations in iron levels. In this study, we investigated the effects of dietary iron overload on bone homeostasis with a specific focus on two frequently utilized mouse strains: 129/Sv and C57BL/6J. Our findings revealed that after 6 weeks on an iron-rich diet, 129/Sv mice exhibited a decrease in trabecular and cortical bone density in both vertebral and femoral bones, which was linked to reduced bone turnover. In contrast, there was no evidence of bone changes associated with iron overload in age-matched C57BL/6J mice. Interestingly, 129/Sv mice exposed to an iron-rich diet during their prenatal development were protected from iron-induced bone loss, suggesting the presence of potential adaptive mechanisms. Overall, our study underscores the critical role of genetic background in modulating the effects of iron overload on bone health. This should be considered when studying effects of iron on bone.

Mechanisms controlling cellular and systemic iron homeostasis

In mammals, hundreds of proteins use iron in a multitude of cellular functions, including vital processes such as mitochondrial respiration, gene regulation and DNA synthesis or repair. Highly orchestrated regulatory systems control cellular and systemic iron fluxes ensuring sufficient iron delivery to target proteins is maintained, while limiting its potentially deleterious effects in iron-mediated oxidative cell damage and ferroptosis. In this Review, we discuss how cells acquire, traffick and export iron and how stored iron is mobilized for iron-sulfur cluster and haem biogenesis. Furthermore, we describe how these cellular processes are fine-tuned by the combination of various sensory and regulatory systems, such as the iron-regulatory protein (IRP)-iron-responsive element (IRE) network, the nuclear receptor co-activator 4 (NCOA4)-mediated ferritinophagy pathway, the prolyl hydroxylase domain (PHD)-hypoxia-inducible factor (HIF) axis or the nuclear factor erythroid 2-related factor 2 (NRF2) regulatory hub. We further describe how these pathways interact with systemic iron homeostasis control through the hepcidin-ferroportin axis to ensure appropriate iron fluxes. This knowledge is key for the identification of novel therapeutic opportunities to prevent diseases of cellular and/or systemic iron mismanagement.

Matriptase-2 regulates iron homeostasis primarily by setting the basal levels of hepatic hepcidin expression through a nonproteolytic mechanism

Matriptase-2 (MT2), encoded by TMPRSS6, is a membrane-anchored serine protease. It plays a key role in iron homeostasis by suppressing the iron-regulatory hormone, hepcidin. Lack of functional MT2 results in an inappropriately high hepcidin and iron-refractory iron-deficiency anemia. Mt2 cleaves multiple components of the hepcidin-induction pathway in vitro. It is inhibited by the membrane-anchored serine protease inhibitor, Hai-2. Earlier in vivo studies show that Mt2 can suppress hepcidin expression independently of its proteolytic activity. In this study, our data indicate that hepatic Mt2 was a limiting factor in suppressing hepcidin. Studies in Tmprss6-/- mice revealed that increases in dietary iron to ∼0.5% were sufficient to overcome the high hepcidin barrier and to correct iron-deficiency anemia. Interestingly, the increased iron in Tmprss6-/- mice was able to further upregulate hepcidin expression to a similar magnitude as in wild-type mice. These results suggest that a lack of Mt2 does not impact the iron induction of hepcidin. Additional studies of wild-type Mt2 and the proteolytic-dead form, fMt2S762A, indicated that the function of Mt2 is to lower the basal levels of hepcidin expression in a manner that primarily relies on its nonproteolytic role. This idea is supported by the studies in mice with the hepatocyte-specific ablation of Hai-2, which showed a marginal impact on iron homeostasis and no significant effects on iron regulation of hepcidin. Together, these observations suggest that the function of Mt2 is to set the basal levels of hepcidin expression and that this process is primarily accomplished through a nonproteolytic mechanism.

Differential Modulation of miR-122 Transcription by TGFβ1/BMP6: Implications for Nonresolving Inflammation and Hepatocarcinogenesis

Chronic inflammation is widely recognized as a significant factor that promotes and worsens the development of malignancies, including hepatocellular carcinoma. This study aimed to explore the potential role of microRNAs in inflammation-associated nonresolving hepatocarcinogenesis. By conducting a comprehensive analysis of altered microRNAs in animal models with liver cancer of various etiologies, we identified miR-122 as the most significantly downregulated microRNA in the liver of animals with inflammation-associated liver cancer. Although previous research has indicated the importance of miR-122 in maintaining hepatocyte function, its specific role as either the trigger or the consequence of underlying diseases remains unclear. Through extensive analysis of animals and in vitro models, we have successfully demonstrated that miR-122 transcription is differentially regulated by the immunoregulatory cytokines, by the transforming growth factor-beta 1 (TGFβ1), and the bone morphogenetic protein-6 (BMP6). Furthermore, we presented convincing evidence directly linking reduced miR-122 transcription to inflammation and in chronic liver diseases. The results of this study strongly suggest that prolonged activation of pro-inflammatory signaling pathways, leading to disruption of cytokine-mediated regulation of miR-122, may significantly contribute to the onset and exacerbation of chronic liver disease.

Cardiac Hamp mRNA Is Predominantly Expressed in the Right Atrium and Does Not Respond to Iron

Hepcidin is a liver-derived hormone that controls systemic iron traffic. It is also expressed in the heart, where it acts locally. We utilized cell and mouse models to study the regulation, expression, and function of cardiac hepcidin. Hepcidin-encoding Hamp mRNA was induced upon differentiation of C2C12 cells to a cardiomyocyte-like phenotype and was not further stimulated by BMP6, BMP2, or IL-6, the major inducers of hepatic hepcidin. The mRNAs encoding hepcidin and its upstream regulator hemojuvelin (Hjv) are primarily expressed in the atria of the heart, with ~20-fold higher Hamp mRNA levels in the right vs. left atrium and negligible expression in the ventricles and apex. Hjv−/− mice, a model of hemochromatosis due to suppression of liver hepcidin, exhibit only modest cardiac Hamp deficiency and minor cardiac dysfunction. Dietary iron manipulations did not significantly affect cardiac Hamp mRNA in the atria of wild-type or Hjv−/− mice. Two weeks following myocardial infarction, Hamp was robustly induced in the liver and heart apex but not atria, possibly in response to inflammation. We conclude that cardiac Hamp is predominantly expressed in the right atrium and is partially regulated by Hjv; however, it does not respond to iron and other inducers of hepatic hepcidin.

Genetic Iron Overload Hampers Development of Cutaneous Leishmaniasis in Mice

The survival, growth, and virulence of Leishmania spp., a group of protozoan parasites, depends on the proper access and regulation of iron. Macrophages, Leishmania's host cell, may divert iron traffic by reducing uptake or by increasing the efflux of iron via the exporter ferroportin. This parasite has adapted by inhibiting the synthesis and inducing the degradation of ferroportin. To study the role of iron in leishmaniasis, we employed Hjv^-/- mice, a model of hemochromatosis. The disruption of hemojuvelin (Hjv) abrogates the expression of the iron hormone hepcidin. This allows unrestricted iron entry into the plasma from ferroportin-expressing intestinal epithelial cells and tissue macrophages, resulting in systemic iron overload. Mice were injected with Leishmania major in hind footpads or intraperitoneally. Compared with wild-type controls, Hjv^-/- mice displayed transient delayed growth of L. major in hind footpads, with a significant difference in parasite burden 4 weeks post-infection. Following acute intraperitoneal exposure to L. major, Hjv^-/- peritoneal cells manifested increased expression of inflammatory cytokines and chemokines (Il1b, Tnfa, Cxcl2, and Ccl2). In response to infection with L. infantum, the causative agent of visceral leishmaniasis, Hjv^-/- and control mice developed similar liver and splenic parasite burden despite vastly different tissue iron content and ferroportin expression. Thus, genetic iron overload due to hemojuvelin deficiency appears to mitigate the early development of only cutaneous leishmaniasis.

Amlodipine rescues advanced iron overload cardiomyopathy in hemojuvelin knockout murine model: Clinical implications

Background

Iron overload cardiomyopathy (IOC) is a major co-morbidity of genetic hemochromatosis and secondary iron overload with limited therapeutic options. We aim to investigate mechanisms of rescue action of amlodipine in the murine model of iron overload, characterize changes in human cardiac tissue due to IOC, and compare them to the changes in the animal model of IOC.

Methods and results

As an animal model, we used male hemojuvelin knockout (HJVKO) mice, which lacked hemojuvelin (a co-receptor protein for hepcidin expression). The mice were fed a high-iron diet from 4 weeks to 1 year of age. As a rescue, iron-fed mice received the Ca²⁺ channel blocker, amlodipine, from 9 to 12 months. Iron overload resulted in systolic and diastolic dysfunctions and changes in the cardiac tissue similar to the changes in the explanted human heart with IOC. An IOC patient (β-thalassemia) with left-ventricular ejection fraction (LVEF) 25% underwent heart transplantation. The murine model and the explanted heart showed intra-myocyte iron deposition, fibrosis, hypertrophy, oxidative stress, remodeling of Ca²⁺ cycling proteins, and metabolic kinases typical of heart failure. Single-myocyte contractility and Ca²⁺ release were diminished in the murine model. The amlodipine-treated group exhibited normalization of cellular function and reversed fibrosis, hypertrophy, oxidative stress, and metabolic remodeling. We also report a clinical case of primary hemochromatosis successfully treated with amlodipine.

Conclusions

The aged HJVKO murine model on the iron-rich diet reproduced many features of the human case of IOC. The use of amlodipine in the murine model and clinical case reversed IOC remodeling, demonstrating that amlodipine is effective adjuvant therapy for IOC.

Haemochromatosis revisited

Haemochromatosis is a genetic disease caused by hepcidin deficiency, responsible for an increase in intestinal iron absorption. Haemochromatosis is associated with homozygosity for the HFE p.Cys282Tyr mutation. However, rare cases of haemochromatosis (non-HFE haemochromatosis) can also be caused by pathogenic variants in other genes (such as HJV, HAMP, TFR2 and SLC40A1). A working group of the International Society for the Study of Iron in Biology and Medicine (BIOIRON Society) has concluded that the classification based in different molecular subtypes is difficult to be adopted in clinical practice and has proposed a new classification approaching clinical questions and molecular complexity. The aim of the present review is to provide an update on classification, pathophysiology and therapeutic recommendations.

A crosstalk between hepcidin and IRE/IRP pathways controls ferroportin expression and determines serum iron levels in mice

The iron hormone hepcidin is transcriptionally activated by iron or inflammation via distinct, partially overlapping pathways. We addressed how iron affects inflammatory hepcidin levels and the ensuing hypoferremic response. Dietary iron overload did not mitigate hepcidin induction in lipopolysaccharide (LPS)-treated wild type mice but prevented effective inflammatory hypoferremia. Likewise, LPS modestly decreased serum iron in hepcidin-deficient Hjv^-/- mice, model of hemochromatosis. Synthetic hepcidin triggered hypoferremia in control but not iron-loaded wild type animals. Furthermore, it dramatically decreased hepatic and splenic ferroportin in Hjv^-/- mice on standard or iron-deficient diet, but only triggered hypoferremia in the latter. Mechanistically, iron antagonized hepcidin responsiveness by inactivating IRPs in the liver and spleen to stimulate ferroportin mRNA translation. Prolonged LPS treatment eliminated ferroportin mRNA and permitted hepcidin-mediated hypoferremia in iron-loaded mice. Thus, de novo ferroportin synthesis is a critical determinant of serum iron and finetunes hepcidin-dependent functional outcomes. Our data uncover a crosstalk between hepcidin and IRE/IRP systems that controls tissue ferroportin expression and determines serum iron levels. Moreover, they suggest that hepcidin supplementation therapy is more efficient when combined with iron depletion.

Hemojuvelin deficiency promotes liver mitochondrial dysfunction and predisposes mice to hepatocellular carcinoma

Hemojuvelin (HJV) enhances signaling to the iron hormone hepcidin and its deficiency causes iron overload, a risk factor for hepatocellular carcinoma (HCC). We utilized Hjv^−/− mice to dissect mechanisms for hepatocarcinogenesis. We show that suboptimal treatment with diethylnitrosamine (DEN) triggers HCC only in Hjv^−/− but not wt mice. Liver proteomics data were obtained by mass spectrometry. Hierarchical clustering analysis revealed that Hjv deficiency and DEN elicit similar liver proteomic responses, including induction of mitochondrial proteins. Dietary iron overload of wt mice does not recapitulate the liver proteomic phenotype of Hjv^−/− animals, which is only partially corrected by iron depletion. Consistent with these data, primary Hjv^−/− hepatocytes exhibit mitochondrial hyperactivity, while aged Hjv^−/− mice develop spontaneous HCC. Moreover, low expression of HJV or hepcidin (HAMP) mRNAs predicts poor prognosis in HCC patients. We conclude that Hjv has a hepatoprotective function and its deficiency in mice promotes mitochondrial dysfunction and hepatocarcinogenesis.

Hemojuvelin (HJV), a BMP co-receptor promoting hepcidin expression in the liver, has a hepatoprotective function and its deficiency in mice triggers mitochondrial dysfunction and hepatocarcinogenesis.

Yersiniabactin contributes to overcoming zinc restriction during Yersinia pestis infection of mammalian and insect hosts

Yersinia pestis causes human plague and colonizes both a mammalian host and a flea vector during its transmission cycle. A key barrier to bacterial infection is the host's ability to actively sequester key biometals (e.g., iron, zinc, and manganese) required for bacterial growth. This is referred to as nutritional immunity. Mechanisms to overcome nutritional immunity are essential virulence factors for bacterial pathogens. Y. pestis produces an iron-scavenging siderophore called yersiniabactin (Ybt) that is required to overcome iron-mediated nutritional immunity and cause lethal infection. Recently, Ybt has been shown to bind to zinc, and in the absence of the zinc transporter ZnuABC, Ybt improves Y. pestis growth in zinc-limited medium. These data suggest that, in addition to iron acquisition, Ybt may also contribute to overcoming zinc-mediated nutritional immunity. To test this hypothesis, we used a mouse model defective in iron-mediated nutritional immunity to demonstrate that Ybt contributes to virulence in an iron-independent manner. Furthermore, using a combination of bacterial mutants and mice defective in zinc-mediated nutritional immunity, we identified calprotectin as the primary barrier for Y. pestis to acquire zinc during infection and that Y. pestis uses Ybt to compete with calprotectin for zinc. Finally, we discovered that Y. pestis encounters zinc limitation within the flea midgut, and Ybt contributes to overcoming this limitation. Together, these results demonstrate that Ybt is a bona fide zinc acquisition mechanism used by Y. pestis to surmount zinc limitation during the infection of both the mammalian and insect hosts.

Runx3 regulates iron metabolism via modulation of BMP signalling

Objectives

Runx3, a member of the Runx family of transcription factors, has been studied as a tumour suppressor and key player of organ development. In a previous study, we reported differentiation failure and excessive angiogenesis in the liver of Runx3 knock‐out (KO) mice. Here, we examined a function of the Runx3 in liver, especially in iron metabolism.

Methods

We performed histological and immunohistological analyses of the Runx3 KO mouse liver. RNA‐sequencing analyses were performed on primary hepatocytes isolated from Runx3 conditional KO (cKO) mice. The effect of Runx3 knock‐down (KD) was also investigated using siRNA‐mediated KD in functional human hepatocytes and human hepatocellular carcinoma cells.

Result

We observed an iron‐overloaded liver with decreased expression of hepcidin in Runx3 KO mice. Expression of BMP6, a regulator of hepcidin transcription, and activity of the BMP pathway were decreased in the liver tissue of Runx3 KO mice. Transcriptome analysis on primary hepatocytes isolated from Runx3 cKO mice also revealed that iron‐induced increase in BMP6 was mediated by Runx3. Similar results were observed in Runx3 knock‐down experiments using HepaRG cells and HepG2 cells. Finally, we showed that Runx3 enhanced the activity of the BMP6 promoter by responding to iron stimuli in the hepatocytes.

Conclusion

In conclusion, we suggest that Runx3 plays important roles in iron metabolism of the liver through regulation of BMP signalling.

Tissue-Specific Regulation of Ferroportin in Wild-Type and Hjv-/- Mice Following Dietary Iron Manipulations

Hepcidin is a liver‐derived peptide hormone that limits iron egress from tissues to the bloodstream. It operates by binding to the iron exporter ferroportin, which blocks iron transport and tags ferroportin for degradation. Genetic hepcidin inactivation leads to hereditary hemochromatosis, a disease of iron overload. We used wild‐type and Hjv‐/‐ mice, a model of hemochromatosis, to examine the expression of ferroportin and other proteins of iron metabolism in hepcidin target tissues. The animals were previously subjected to dietary iron manipulations. In Hjv‐/‐ mice, hepcidin messenger RNA correlated significantly with hepatic iron load (r = 0.8211, P < 0.001), but was substantially lower compared with wild‐type controls. Duodenal ferroportin and divalent metal transporter 1 (DMT1), as well as splenic and hepatic ferroportin, were overexpressed in these animals. A high‐iron diet (2% carbonyl iron) suppressed duodenal DMT1 levels in both wild‐type and Hjv‐/‐ mice; however, it did not affect duodenal ferroportin expression in Hjv‐/‐ mice, and only reduced it in wild‐type mice. In contrast, the high‐iron diet decreased splenic ferroportin exclusively in Hjv‐/‐ mice, whereas it induced hepatic ferroportin exclusively in wild‐type mice. Conclusion: Our data show that dietary iron differentially affects ferroportin expression in mouse tissues and are consistent with hepcidin‐dependent and hepcidin‐independent mechanisms for ferroportin regulation. In the Hjv‐/‐ mouse model of hemochromatosis, duodenal ferroportin remains unresponsive to iron but DMT1 is appropriately iron‐regulated.

RNF217 regulates iron homeostasis through its E3 ubiquitin ligase activity by modulating ferroportin degradation

Ferroportin (FPN), the body's sole iron exporter, is essential for maintaining systemic iron homeostasis. In response to either increased iron or inflammation, hepatocyte-secreted hepcidin binds to FPN, inducing its internalization and subsequent degradation. However, the E3 ubiquitin ligase that underlies FPN degradation has not been identified. Here, we report the identification and characterization of a novel mechanism involving the RNF217-mediated degradation of FPN. A combination of 2 different E3 screens revealed that the Rnf217 gene is a target of Tet1, mediating the ubiquitination and subsequent degradation of FPN. Interestingly, loss of Tet1 expression causes an accumulation of FPN and an impaired response to iron overload, manifested by increased iron accumulation in the liver together with decreased iron in the spleen and duodenum. Moreover, we found that the degradation and ubiquitination of FPN could be attenuated by mutating RNF217. Finally, using 2 conditional knockout mouse lines, we found that knocking out Rnf217 in macrophages increases splenic iron export by stabilizing FPN, whereas knocking out Rnf217 in intestinal cells appears to increase iron absorption. These findings suggest that the Tet1-RNF217-FPN axis regulates iron homeostasis, revealing new therapeutic targets for FPN-related diseases.

Hepatocyte neogenin is required for hemojuvelin-mediated hepcidin expression and iron homeostasis in mice

Neogenin (NEO1) is a ubiquitously expressed multifunctional transmembrane protein. It interacts with hemojuvelin (HJV), a BMP coreceptor that plays a pivotal role in hepatic hepcidin expression. Earlier studies suggest that the function of HJV relies on its interaction with NEO1. However, the role of NEO1 in iron homeostasis remains controversial because of the lack of an appropriate animal model. Here, we generated a hepatocyte-specific Neo1 knockout (Neo1fl/fl;Alb-Cre+) mouse model that circumvented the developmental and lethality issues of the global Neo1 mutant. Results show that ablation of hepatocyte Neo1 decreased hepcidin expression and caused iron overload. This iron overload did not result from altered iron utilization by erythropoiesis. Replacement studies revealed that expression of the Neo1L1046E mutant that does not interact with Hjv, was unable to correct the decreased hepcidin expression and high serum iron in Neo1fl/fl;Alb-Cre+ mice. In Hjv-/- mice, expression of HjvA183R mutant that has reduced interaction with Neo1, also displayed a blunted induction of hepcidin expression. These observations indicate that Neo1-Hjv interaction is essential for hepcidin expression. Further analyses suggest that the Hjv binding triggered the cleavage of the Neo1 cytoplasmic domain by a protease, which resulted in accumulation of truncated Neo1 on the plasma membrane. Additional studies did not support that Neo1 functions by inhibiting Hjv shedding as previously proposed. Together, our data favor a model in which Neo1 interaction with Hjv leads to accumulation of cleaved Neo1 on the plasma membrane, where Neo1 acts as a scaffold to induce the Bmp signaling and hepcidin expression.

Physiological and pathophysiological mechanisms of hepcidin regulation: clinical implications for iron disorders

The discovery of hepcidin has provided a solid foundation for understanding the mechanisms of systemic iron homeostasis and the aetiologies of iron disorders. Hepcidin assures the balance of circulating and stored iron levels for multiple physiological processes including oxygen transport and erythropoiesis, while limiting the toxicity of excess iron. The liver is the major site where regulatory signals from iron, erythropoietic drive and inflammation are integrated to control hepcidin production. Pathologically, hepcidin dysregulation by genetic inactivation, ineffective erythropoiesis, or inflammation leads to diseases of iron deficiency or overload such as iron-refractory iron-deficiency anaemia, anaemia of inflammation, iron-loading anaemias and hereditary haemochromatosis. In the present review, we discuss recent insights into the molecular mechanisms governing hepcidin regulation, how these pathways are disrupted in iron disorders, and how this knowledge is being used to develop novel diagnostic and therapeutic strategies.

Iron aggravates hepatic insulin resistance in the absence of inflammation in a novel db/db mouse model with iron overload

OBJECTIVE

The molecular pathogenesis of late complications associated with type 2 diabetes mellitus (T2DM) is not yet fully understood. While high glucose levels indicated by increased HbA1c only poorly explain disease progression and late complications, a pro-inflammatory status, oxidative stress, and reactive metabolites generated by metabolic processes were postulated to be involved. Individuals with metabolic syndrome (MetS) frequently progress to T2DM, whereby 70% of patients with T2DM show non-alcoholic fatty liver disease (NAFLD), the hepatic manifestation of MetS, and insulin resistance (IR). Epidemiological studies have shown that T2DM and steatosis are associated with alterations in iron metabolism and hepatic iron accumulation. Excess free iron triggers oxidative stress and a switch towards a macrophage pro-inflammatory status. However, so far it remains unclear whether hepatic iron accumulation plays a causative role in the generation of IR and T2DM or whether it is merely a manifestation of altered hepatic metabolism. To address this open question, we generated and characterized a mouse model of T2DM with IR, steatosis, and iron overload.

METHODS

Lepr^db/db mice hallmarked by T2DM, IR and steatosis were crossed with Fpn^wt/C326S mice with systemic iron overload to generate Lepr^db/db/Fpn^wt/C326S mice. The resulting progeny was characterized for major diabetic and iron-related parameters.

RESULTS

We demonstrated that features associated with T2DM in Lepr^db/db mice, such as obesity, steatosis, or IR, reduce the degree of tissue iron overload in Fpn^wt/C326S mice, suggesting an 'iron resistance' phenotype. Conversely, we observed increased serum iron levels that strongly exceeded those in the iron-overloaded Fpn^wt/C326S mice. Increased hepatic iron levels induced oxidative stress and lipid peroxidation and aggravated IR, as indicated by diminished IRS1 phosphorylation and AKT activation. Additionally, in the liver, we observed gene response patterns indicative of de novo lipogenesis and increased gluconeogenesis as well as elevated free glucose levels. Finally, we showed that iron overload in Lepr^db/db/Fpn^wt/C326S mice enhances microvascular complications observed in retinopathy, suggesting that iron accumulation can enhance diabetic late complications associated with the liver and the eye.

CONCLUSION

Taken together, our data show that iron causes the worsening of symptoms associated with the MetS and T2DM. These findings imply that iron depletion strategies together with anti-diabetic drugs may ameliorate IR and diabetic late complications.

Matriptase-2 and Hemojuvelin in Hepcidin Regulation: In Vivo Immunoblot Studies in Mask Mice

Matriptase-2, a serine protease expressed in hepatocytes, is a negative regulator of hepcidin expression. The purpose of the study was to investigate the interaction of matriptase-2 with hemojuvelin protein in vivo. Mice lacking the matriptase-2 proteolytic activity (mask mice) display decreased content of hemojuvelin protein. Vice versa, the absence of hemojuvelin results in decreased liver content of matriptase-2, indicating that the two proteins interact. To further characterize the role of matriptase-2, we investigated iron metabolism in mask mice fed experimental diets. Administration of iron-enriched diet increased liver iron stores as well as hepcidin expression. Treatment of iron-overloaded mask mice with erythropoietin increased hemoglobin and hematocrit, indicating that the response to erythropoietin is intact in mask mice. Feeding of an iron-deficient diet to mask mice significantly increased spleen weight as well as the splenic content of erythroferrone and transferrin receptor proteins, indicating stress erythropoiesis. Liver hepcidin expression was decreased; expression of Id1 was not changed. Overall, the results suggest a complex interaction between matriptase-2 and hemojuvelin, and demonstrate that hepcidin can to some extent be regulated even in the absence of matriptase-2 proteolytic activity.

Iron overload in aging Bmp6‑/‑ mice induces exocrine pancreatic injury and fibrosis due to acinar cell loss

The relationship between hemochromatosis and diabetes has been well established, as excessive iron deposition has been reported to result in impaired function of the endocrine and exocrine pancreas. Therefore, the objective of the present study was to analyze the effects of iron accumulation on the pancreata and glucose homeostasis in a bone morphogenetic protein 6-knockout (Bmp6^−/−) mouse model of hemochromatosis. The sera and pancreatic tissues of wild-type (WT) and Bmp6^−/− mice (age, 3 and 10 months) were subjected to biochemical and histological analyses. In addition, ¹⁸F-fluorodeoxyglucose biodistribution was evaluated in the liver, muscle, heart, kidney and adipose tissue of both animal groups. The results demonstrated that 3-month-old Bmp6^−/− mice exhibited iron accumulation preferentially in the exocrine pancreas, with no signs of pancreatic injury or fibrosis. No changes were observed in the glucose metabolism, as pancreatic islet diameter, insulin and glucagon secretion, blood glucose levels and glucose uptake in the liver, muscle and adipose tissue remained comparable with those in the WT mice. Aging Bmp6^−/− mice presented with progressive iron deposits in the exocrine pancreas, leading to pancreatic degeneration and injury that was characterized by acinar atrophy, fibrosis and the infiltration of inflammatory cells. However, the aging mice exhibited unaltered blood glucose levels and islet structure, normal insulin secretion and moderately increased α-cell mass compared with those in the age-matched WT mice. Additionally, iron overload and pancreatic damage were not observed in the aging WT mice. These results supported a pathogenic role of iron overload in aging Bmp6^−/− mice leading to iron-induced exocrine pancreatic deficiency, whereas the endocrine pancreas retained normal function.

Genes Regulating Spermatogenesis and Sperm Function Associated With Rare Disorders

Spermatogenesis is a cell differentiation process that ensures the production of fertilizing sperm, which ultimately fuse with an egg to form a zygote. Normal spermatogenesis relies on Sertoli cells, which preserve cell junctions while providing nutrients for mitosis and meiosis of male germ cells. Several genes regulate normal spermatogenesis, some of which are not exclusively expressed in the testis and control multiple physiological processes in an organism. Loss-of-function mutations in some of these genes result in spermatogenesis and sperm functionality defects, potentially leading to the insurgence of rare genetic disorders. To identify genetic intersections between spermatogenesis and rare diseases, we screened public archives of human genetic conditions available on the Genetic and Rare Diseases Information Center (GARD), the Online Mendelian Inheritance in Man (OMIM), and the Clinical Variant (ClinVar), and after an extensive literature search, we identified 22 distinct genes associated with 21 rare genetic conditions and defective spermatogenesis or sperm function. These protein-coding genes regulate Sertoli cell development and function during spermatogenesis, checkpoint signaling pathways at meiosis, cellular organization and shape definition during spermiogenesis, sperm motility, and capacitation at fertilization. A number of these genes regulate folliculogenesis and oogenesis as well. For each gene, we review the genotype–phenotype association together with associative or causative polymorphisms in humans, and provide a description of the shared molecular mechanisms that regulate gametogenesis and fertilization obtained in transgenic animal models.

Bone morphogenic proteins in iron homeostasis

The bone morphogenetic protein (BMP)-SMAD signaling pathway plays a central role in regulating hepcidin, which is the master hormone governing systemic iron homeostasis. Hepcidin is produced by the liver and acts on the iron exporter ferroportin to control iron absorption from the diet and iron release from body stores, thereby providing adequate iron for red blood cell production, while limiting the toxic effects of excess iron. BMP6 and BMP2 ligands produced by liver endothelial cells bind to BMP receptors and the coreceptor hemojuvelin (HJV) on hepatocytes to activate SMAD1/5/8 signaling, which directly upregulates hepcidin transcription. Most major signals that influence hepcidin production, including iron, erythropoietic drive, and inflammation, intersect with the BMP-SMAD pathway to regulate hepcidin transcription. Mutation or inactivation of BMP ligands, BMP receptors, HJV, SMADs or other proteins that modulate the BMP-SMAD pathway result in hepcidin dysregulation, leading to iron-related disorders, such as hemochromatosis and iron refractory iron deficiency anemia. Pharmacologic modulators of the BMP-SMAD pathway have shown efficacy in pre-clinical models to regulate hepcidin expression and treat iron-related disorders. This review will discuss recent insights into the role of the BMP-SMAD pathway in regulating hepcidin to control systemic iron homeostasis.

Juvenile Hemochromatosis: A Case Report and Review of the Literature

Juvenile hemochromatosis (JH), type 2A hemochromatosis, is a rare autosomal recessive disorder of systemic iron overload due to homozygous mutations of HJV (HFE2), which encodes hemojuvelin, an essential regulator of the hepcidin expression, causing liver fibrosis, diabetes, and heart failure before 30 years of age, often with fatal outcomes. We report two Japanese sisters of 37 and 52 years of age, with JH, who showed the same homozygous HJV I281T mutation and hepcidin deficiency and who both responded well to phlebotomy on an outpatient basis. When all reported cases of JH with homozygous HJV mutations in the relevant literature were reviewed, we found—for the first time—that JH developed in females and males at a ratio of 3:2, with no age difference in the two groups. Furthermore, we found that the age of onset of JH may depend on the types of HJV mutations. In comparison to patients with the most common G320V/G320V mutation, JH developed earlier in patients with L101P/L101P or R385X/R385X mutations and later in patients with I281T/I281T mutations.

Repulsive guidance molecules lock growth differentiation factor 5 in an inhibitory complex

Repulsive guidance molecules (RGMs) are cell surface proteins that regulate the development and homeostasis of many tissues and organs, including the nervous, skeletal, and immune systems. They control fundamental biological processes, such as migration and differentiation by direct interaction with the Neogenin (NEO1) receptor and function as coreceptors for the bone morphogenetic protein (BMP)/growth differentiation factor (GDF) family. We determined crystal structures of all three human RGM family members in complex with GDF5, as well as the ternary NEO1-RGMB-GDF5 assembly. Surprisingly, we show that all three RGMs inhibit GDF5 signaling, which is in stark contrast to RGM-mediated enhancement of signaling observed for other BMPs, like BMP2. Despite their opposite effect on GDF5 signaling, RGMs occupy the BMP type 1 receptor binding site similar to the observed interactions in RGM-BMP2 complexes. In the NEO1-RGMB-GDF5 complex, RGMB physically bridges NEO1 and GDF5, suggesting cross-talk between the GDF5 and NEO1 signaling pathways. Our crystal structures, combined with structure-guided mutagenesis of RGMs and BMP ligands, binding studies, and cellular assays suggest that RGMs inhibit GDF5 signaling by competing with GDF5 type 1 receptors. While our crystal structure analysis and in vitro binding data initially pointed towards a simple competition mechanism between RGMs and type 1 receptors as a possible basis for RGM-mediated GDF5 inhibition, further experiments utilizing BMP2-mimicking GDF5 variants clearly indicate a more complex mechanism that explains how RGMs can act as a functionality-changing switch for two structurally and biochemically similar signaling molecules.

The functional role of hemojuvelin in acute ischemic stroke

Our study aimed to establish the role of hemojuvelin (HJV) in acute ischemic stroke (AIS). We performed immunohistochemistry for HJV expression in human brain tissues from 10 AIS and 2 non-stroke autopsy subjects. Plasma HJV was measured in 112 AIS patients within 48 h after stroke. The results showed significantly increased HJV expression in brain tissues from AIS patients compare to non-stroke subjects. After adjusting for clinical variables, plasma levels of HJV within 48 h after stroke were an independent predictor of poor functional outcome three months post-stroke (OR:1.78, 95% CI: 1.03-3.07; P = 0.038). In basic part, Western blotting showed that HJV expression in mice brains was apparent at 3 h after middle cerebral artery occlusion (MCAO), and increased significantly at 72 h. In cultured cortical neurons, expression of HJV protein increased remarkably 24 h after oxygen glucose deprivation (OGD), and small interfering RNAs (siHJV) transfected OGD neurons had a lower apoptotic rate. Importantly, 72 h post-MCAO, HJV knockout mice had significantly smaller infarcts and less expression of cleaved caspase-3 protein compared with wild-type mice. In summary, HJV participates in the mechanisms of post-stroke neuronal injury, and that plasma HJV levels can be a potential early outcome indicator for AIS patients.

Iron Overload Mimicking Conditions Skews Bone Marrow Dendritic Cells Differentiation into MHCIIlowCD11c+CD11b+F4/80+ Cells

Iron overload is an undesired effect of frequent blood transfusions or genetic diseases. Myelodysplastic syndrome (MDS) patients become transfusion dependent, but due to the combination of ineffective haematopoiesis and repeated blood transfusions they are often subject to iron overload. In this study, we demonstrate that iron-overload mimicking condition alters bone marrow progenitor differentiation towards dendritic cells (DCs). Cells cultured in iron-enriched culture medium for seven days fail to differentiate into conventional CD11c⁺MHCII^hi DCs and fail to efficiently respond to LPS (Lipopolysaccharides). Cells appear smaller than control DCs but vital and able to perform FITC-dextran (Fluorescein isothiocyanate-dextran) endocytosis. At molecular level, cells cultured in iron-enriched conditions show increased ARG1 and PU.1, and decreased IRF8 expression.

Animal Models of Normal and Disturbed Iron and Copper Metabolism

Research on the interplay between iron and copper metabolism in humans began to flourish in the mid-20th century, and diseases associated with dysregulated homeostasis of these essential trace minerals are common even today. Iron deficiency is the most frequent cause of anemia worldwide, leading to significant morbidity, particularly in developing countries. Iron overload is also quite common, usually being the result of genetic mutations which lead to inappropriate expression of the iron-regulatory hormone hepcidin. Perturbations of copper homeostasis in humans have also been described, including rare genetic conditions which lead to severe copper deficiency (Menkes disease) or copper overload (Wilson disease). Historically, the common laboratory rat (Rattus norvegicus) was the most frequently utilized species to model human physiology and pathophysiology. Recently, however, the development of genetic-engineering technology combined with the worldwide availability of numerous genetically homogenous (i.e., inbred) mouse strains shifted most research on iron and copper metabolism to laboratory mice. This created new opportunities to understand the function of individual genes in the context of a living animal, but thoughtful consideration of whether mice are the most appropriate models of human pathophysiology was not necessarily involved. Given this background, this review is intended to provide a guide for future research on iron- and copper-related disorders in humans. Generation of complementary experimental models in rats, swine, and other mammals is now facile given the advent of newer genetic technologies, thus providing the opportunity to accelerate the identification of pathogenic mechanisms and expedite the development of new treatments to mitigate these important human disorders.

Low-molecular-mass iron complexes in blood plasma of iron-deficient pigs do not originate directly from nutrient iron

Nutrient iron entering the blood binds transferrin (TFN)d, which delivers iron to cells in the body. In healthy individuals, ∼30% of TFN is iron-bound while the remainder is unbound (apo-TFN). TFN saturates the plasma of individuals with iron-overload diseases such as hereditary hemochromatosis, prompting release of a poorly-defined low-molecular-mass (LMM) iron species called non-transferrin-bound iron (NTBI). An experiment was devised to directly detect NTBI in plasma of iron-deficient pigs and to assess the role of the liver which is known to bind NTBI. Catheters were surgically installed in the portal vein (PV) and either the caudal vena cava or the cranial vena cava. After the animals recovered, 57Fe II ascorbate was injected into the stomach via a feeding tube. Blood was removed through the catheters before and after injection; plasma became 57Fe-enriched after injection. 57Fe-enriched plasma was passed through a 10 kDa cutoff membrane and the flow-through solution (FTS) was subjected to size-exclusion liquid chromatography (LC). The eluent flowed into an ICP-MS where 56Fe and 57Fe were detected. Low-intensity iron peaks with masses of 400-1600 Da were observed, but none became enriched in 57Fe after injection. Rather, the injected 57Fe bound to apo-TFN. Viewed naively, this implies that nutrient-derived 57Fe in healthy mammals passes from the intestines to apo-TFN without first entering the blood as a LMM intermediate. In this case, nutrient iron exported from intestinal enterocytes of healthy individuals may quickly bind apo-TFN such that LMM iron species do not accumulate in blood plasma. Some 57Fe from the FTS may have adsorbed onto the column. In any event, the LMM iron species in plasma that eluted from the column must have originated from iron stored within the body, perhaps in macrophages - not directly from nutrient iron absorption. The liver absorbed and released LMM iron species, but the effect was modest, consistent with its role as a dynamic iron buffer. Passage through the liver also altered the distribution of different forms of TFN present in the PV.

3-Amidinophenylalanine-derived matriptase inhibitors can modulate hepcidin production in vitro

Matriptase-2 (MT-2) is a type II transmembrane serine protease and predominantly attached to the surface of hepatocytes. MT-2 decreases the production of hepcidin, a key regulator of iron homeostasis. In this study, the effects of four 3-amidinophenylalanine-derived combined matriptase-1/matriptase-2 (MT-1/2) inhibitors (MI-432, MI-441, MI-460, and MI-461) on hepcidin production were investigated in hepatocyte mono- and hepatocyte-Kupffer cell co-cultures. In MI-461-treated cell cultures, the extracellular hydrogen peroxide contents and the interleukin-6 and -8 (IL-6 and IL-8) levels were determined and compared to controls. Hepcidin overproduction was observed in hepatocytes upon treatment with MI-432, MI-441 and MI-461 at 50 μM. In contrast, extracellular hydrogen peroxide levels were not elevated significantly after matriptase inhibition with MI-461. Furthermore, MI-461 did not induce increases in IL-6 and IL-8 levels in these hepatic models. A model of the binding mode of inhibitor MI-461 in complex with MT-2 revealed numerous polar contacts contributing to the nanomolar potency of this compound. Based on the in vitro data on hepcidin regulation, treatment with MI-461 might be valuable in pathological states of iron metabolism without causing excessive oxidative stress.

Hemojuvelin is a novel suppressor for Duchenne muscular dystrophy and age-related muscle wasting

BACKGROUND

Muscle wasting occurs in response to various physiological and pathological conditions, including ageing and Duchenne muscular dystrophy (DMD). Transforming growth factor-β1 (TGF-β1) contributes to muscle pathogenesis in elderly people and DMD patients; inhibition of TGF-β1 signalling is a promising therapeutic strategy for muscle-wasting disorders. Hemojuvelin (HJV or Hjv as the murine homologue) is a membrane-bound protein that is highly expressed in skeletal muscle, heart, and liver. In hepatic cells, Hjv acts as a coreceptor for bone morphogenetic protein, a TGF-β subfamily member. The aim of this study was to investigate whether Hjv plays an essential role in muscle physiological and pathophysiological processes by acting as a coreceptor for TGF-β1 signalling.

METHODS

Conventional and conditional Hjv knockout mice as well as mdx and aged mice transfected with Hjv overexpression vector were used to study the role of Hjv in muscle physiology and pathophysiology. qRT-PCR, western blotting, and immunohistochemistry examinations were conducted to evaluate gene, protein, and structural changes in vivo and in vitro. Exercise endurance was determined using treadmill running test, and muscle force was detected by an isometric transducer. RNA interference, immunoprecipitation, and dual-luciferase reporter assays were utilized to explore the mechanism by which Hjv regulates TGF-β1 signalling in skeletal muscle.

RESULTS

Conventional and conditional Hjv knockout mice displayed muscle atrophy, fibrosis, reduced running endurance, and muscle force. HJV was significantly down-regulated in the muscles of DMD patients (n = 3, mean age: 11.7 ± 5.7 years) and mdx mice as well as in those of aged humans (n = 10, 20% women, mean age: 75.1 ± 9.5 years) and mice. Overexpression of Hjv rescued dystrophic and age-related muscle wasting. Unlike its function in hepatic cells, the bone morphogenetic protein downstream phosphorylated p-Smad1/5/8 signalling pathway was unchanged, but TGF-β1, TGF-β receptor II (TβRII), and p-Smad2/3 expression were increased in Hjv-deficient muscles. Mechanistically, loss of Hjv promoted activation of Smad3 signalling induced by TGF-β1, whereas Hjv overexpression inhibited TGF-β1/Smad3 signalling by directly interacting with TβRII on the muscle membrane.

CONCLUSIONS

Our findings identify an unrecognized role of HJV in skeletal muscle by regulating TGF-β1/Smad3 signalling as a coreceptor for TβRII. Unlike the TGF-β1/Smad3 pathway, HJV could be a reliable drug target as its expression is not widespread. Novel therapeutic strategies could potentially be devised to interfere only with the muscle function of HJV to treat DMD and age-related muscle wasting.

Low-molecular-mass iron in healthy blood plasma is not predominately ferric citrate

Blood contains a poorly characterized pool of labile iron called non-transferrin-bound iron (NTBI). In patients with iron-overload diseases such as hemochromatosis, NTBI accumulates in the liver, heart, and other organs. This material is probably nonproteinaceous and low molecular mass (LMM). However, the number, concentration, mass, and chemical composition of NTBI species remain unknown despite decades of effort. Here, solutions of plasma from humans, pigs, horses, and mice were passed through a 10 kDa cutoff membrane, affording flow-through solutions (FTSs) containing ∼1 μM iron. The FTSs were subjected to size-exclusion liquid chromatography at pH 8.5, 6.5, and 4.5. Iron was detected by an online inductively-coupled-plasma mass spectrometer. LC-ICP-MS chromatograms of the FTSs exhibited 2-6 iron-containing species with apparent masses between 400 and 2500 Da. Their approximate concentrations in plasma were 10-8-10-7 M. Not every FTS sample contained every LMM iron species, indicating individual variations. The most reproducible iron species had apparent masses of 400 and 500 Da. Chromatograms of the FTSs from established hemochromatosis patients exhibited no significant differences relative to controls. The peak positions and intensities depended on column pH. Some FTS iron adsorbed onto the column, especially at higher pH. Column-adsorbing-iron coordinated apo-transferrin whereas the more tightly coordinated iron species did not. Ferric citrate standards exhibited LMM iron peaks that were similar to but not the same as those obtained in FTSs. The results indicate that the LMM iron species in healthy blood plasma is not primarily ferric citrate; however, this may be one of many contributing complexes.

Recent progress on inhibitors of the type II transmembrane serine proteases, hepsin, matriptase and matriptase-2

Members of the type II transmembrane serine proteases (TTSP) family play a vital role in cell growth and development but many are also implicated in disease. Two of the well-studied TTSPs, matriptase and hepsin proteolytically process multiple protein substrates such as the inactive single-chain zymogens pro-HGF and pro-macrophage stimulating protein into the active heterodimeric forms, HGF and macrophage stimulating protein. These two proteases also have many other substrates which are associated with cancer and tumor progression. Another related TTSP, matriptase-2 is expressed in the liver and functions by regulating iron homoeostasis through the cleavage of hemojuvelin and thus is implicated in iron overload diseases. In the present review, we will discuss inhibitor design strategy and Structure activity relationships of TTSP inhibitors, which have been reported in the literature.

Hepcidin as a therapeutic target for anemia and inflammation associated with chronic kidney disease

INTRODUCTION

Anemia is a common manifestation of chronic kidney disease (CKD). The pathogenesis of CKD-associated anemia is multifactorial. Our understanding of the molecular control of iron metabolism has improved dramatically because of the discovery of hepcidin and attempts to introduce new drugs to stimulate erythropoiesis or affect the hepcidin-ferroportin pathway have recently emerged. Areas covered: We examine the possible role of hepcidin in iron metabolism and regulation and the potential therapeutic options involving hepcidin and hepcidin-ferroportin axis in renal anemia treatment. We focus on therapeutic targeting of hepcidin, the hepcidin-ferroportin axis and key molecules such as anti-hepcidin antibodies, spigelmers, and anticalins. We also discuss compounds affecting the bone morphogenetic protein receptor [BMP/BMPR] complex and molecules that influence hepcidin, such as hypoxia-inducible factor 1 stabilizers. Expert opinion: Hepcidin is a key regulator of iron availability and is a potential future therapeutic target for managing anemia that is associated with CKD. There are potential risks and benefits associated with novel sophisticated therapies and there are several novel options on the horizon; however, clinical data are currently limited and need development. Inhibition of hepcidin via various pathways might be a viable adjunctive therapeutic option in other clinical situations.

Involvement of cytosolic and mitochondrial iron in iron overload cardiomyopathy: an update

Iron overload cardiomyopathy (IOC) is a major cause of death in patients with diseases associated with chronic anemia such as thalassemia or sickle cell disease after chronic blood transfusions. Associated with iron overload conditions, there is excess free iron that enters cardiomyocytes through both L- and T-type calcium channels thereby resulting in increased reactive oxygen species being generated via Haber-Weiss and Fenton reactions. It is thought that an increase in reactive oxygen species contributes to high morbidity and mortality rates. Recent studies have, however, suggested that it is iron overload in mitochondria that contributes to cellular oxidative stress, mitochondrial damage, cardiac arrhythmias, as well as the development of cardiomyopathy. Iron chelators, antioxidants, and/or calcium channel blockers have been demonstrated to prevent and ameliorate cardiac dysfunction in animal models as well as in patients suffering from cardiac iron overload. Hence, either a mono-therapy or combination therapies with any of the aforementioned agents may serve as a novel treatment in iron-overload patients in the near future. In the present article, we review the mechanisms of cytosolic and/or mitochondrial iron load in the heart which may contribute synergistically or independently to the development of iron-associated cardiomyopathy. We also review available as well as potential future novel treatments.

Variation in the repulsive guidance molecule family in human populations

Repulsive guidance molecules, RGMA, RGMB, and RGMC, are related proteins discovered independently through different experimental paradigms. They are encoded by single copy genes in mammalian and other vertebrate genomes, and are ~50% identical in amino acid sequence. The importance of RGM actions in human physiology has not been realized, as most research has focused on non-human models, although mutations in RGMC are the cause of the severe iron storage disorder, juvenile hemochromatosis. Here I show that repositories of human genomic and population genetic data can be used as starting points for discovery and for developing new testable hypotheses about each of these paralogs in human biology and disease susceptibility. Information was extracted, aggregated, and analyzed from the Ensembl and UCSC Genome Browsers, the Exome Aggregation Consortium, the Genotype-Tissue Expression project portal, the cBio portal for Cancer Genomics, and the National Cancer Institute Genomic Data Commons data site. Results identify extensive variation in gene expression patterns, substantial alternative RNA splicing, and possible missense alterations and other modifications in the coding regions of each of the three genes, with many putative mutations being detected in individuals with different types of cancers. Moreover, selected amino acid substitutions are highly prevalent in the world population, with minor allele frequencies of up to 37% for RGMA and up to 8% for RGMB. These results indicate that protein sequence variation is common in the human RGM family, and raises the possibility that individual variants will have a significant population impact on human physiology and/or disease predisposition.

ALK3 undergoes ligand-independent homodimerization and BMP-induced heterodimerization with ALK2

The bone morphogenetic protein (BMP) type I receptors ALK2 and ALK3 are essential for expression of hepcidin, a key iron regulatory hormone. In mice, hepatocyte-specific Alk2 deficiency leads to moderate iron overload with periportal liver iron accumulation, while hepatocyte-specific Alk3 deficiency leads to severe iron overload with centrilobular liver iron accumulation and a more marked reduction of basal hepcidin levels. The objective of this study was to investigate whether the two receptors have additive roles in hepcidin regulation. Iron overload in mice with hepatocyte-specific Alk2 and Alk3 (Alk2/3) deficiency was characterized and compared to hepatocyte-specific Alk3 deficient mice. Co-immunoprecipitation studies were performed to detect the formation of ALK2 and ALK3 homodimer and heterodimer complexes in vitro in the presence and absence of ligands. The iron overload phenotype of hepatocyte-specific Alk2/3-deficient mice was more severe than that of hepatocyte-specific Alk3-deficient mice. In vitro co-immunoprecipitation studies in Huh7 cells showed that ALK3 can homodimerize in absence of BMP2 or BMP6. In contrast, ALK2 did not homodimerize in either the presence or absence of BMP ligands. However, ALK2 did form heterodimers with ALK3 in the presence of BMP2 or BMP6. ALK3-ALK3 and ALK2-ALK3 receptor complexes induced hepcidin expression in Huh7 cells. Our data indicate that: (I) ALK2 and ALK3 have additive functions in vivo, as Alk2/3 deficiency leads to a greater degree of iron overload than Alk3 deficiency; (II) ALK3, but not ALK2, undergoes ligand-independent homodimerization; (III) the formation of ALK2-ALK3 heterodimers is ligand-dependent and (IV) both receptor complexes functionally induce hepcidin expression in vitro.

Liver iron sensing and body iron homeostasis

The liver orchestrates systemic iron balance by producing and secreting hepcidin. Known as the iron hormone, hepcidin induces degradation of the iron exporter ferroportin to control iron entry into the bloodstream from dietary sources, iron recycling macrophages, and body stores. Under physiologic conditions, hepcidin production is reduced by iron deficiency and erythropoietic drive to increase the iron supply when needed to support red blood cell production and other essential functions. Conversely, hepcidin production is induced by iron loading and inflammation to prevent the toxicity of iron excess and limit its availability to pathogens. The inability to appropriately regulate hepcidin production in response to these physiologic cues underlies genetic disorders of iron overload and deficiency, including hereditary hemochromatosis and iron-refractory iron deficiency anemia. Moreover, excess hepcidin suppression in the setting of ineffective erythropoiesis contributes to iron-loading anemias such as β-thalassemia, whereas excess hepcidin induction contributes to iron-restricted erythropoiesis and anemia in chronic inflammatory diseases. These diseases have provided key insights into understanding the mechanisms by which the liver senses plasma and tissue iron levels, the iron demand of erythrocyte precursors, and the presence of potential pathogens and, importantly, how these various signals are integrated to appropriately regulate hepcidin production. This review will focus on recent insights into how the liver senses body iron levels and coordinates this with other signals to regulate hepcidin production and systemic iron homeostasis.

Assessment of MR-based R2* and quantitative susceptibility mapping for the quantification of liver iron concentration in a mouse model at 7T

PURPOSE

To assess the feasibility of quantifying liver iron concentration (LIC) using R2* and quantitative susceptibility mapping (QSM) at a high field strength of 7 Tesla (T).

METHODS

Five different concentrations of Fe-dextran were injected into 12 mice to produce various degrees of liver iron overload. After mice were sacrificed, blood and liver samples were harvested. Ferritin enzyme-linked immunosorbent assay (ELISA) and inductively coupled plasma mass spectrometry were performed to quantify serum ferritin concentration and LIC. Multiecho gradient echo MRI was conducted to estimate R2* and the magnetic susceptibility of each liver sample through complex nonlinear least squares fitting and a morphology enabled dipole inversion method, respectively.

RESULTS

Average estimates of serum ferritin concentration, LIC, R2*, and susceptibility all show good linear correlations with injected Fe-dextran concentration; however, the standard deviations in the estimates of R2* and susceptibility increase with injected Fe-dextran concentration. Both R2* and susceptibility measurements also show good linear correlations with LIC (R2  = 0.78 and R2  = 0.91, respectively), and a susceptibility-to-LIC conversion factor of 0.829 ppm/(mg/g wet) is derived.

CONCLUSION

The feasibility of quantifying LIC using MR-based  R2* and QSM at a high field strength of 7T is demonstrated. Susceptibility quantification, which is an intrinsic property of tissues and benefits from being field-strength independent, is more robust than R2* quantification in this ex vivo study. A susceptibility-to-LIC conversion factor is presented that agrees relatively well with previously published QSM derived results obtained at 1.5T and 3T.