H-Ferritin Is Preferentially Incorporated by Human Erythroid Cells through Transferrin Receptor 1 in a Threshold-Dependent Manner

The heavy subunit of ferritin stimulates NLRP3 inflammasomes in hepatic stellate cells through ICAM-1 to drive hepatic inflammation

Serum ferritin concentrations increase during hepatic inflammation and correlate with the severity of chronic liver disease. Here, we report a molecular mechanism whereby the heavy subunit of ferritin (FTH) contributes to hepatic inflammation. We found that FTH induced activation of the NLRP3 inflammasome and secretion of the proinflammatory cytokine interleukin-1β (IL-1β) in primary rat hepatic stellate cells (HSCs) through intercellular adhesion molecule-1 (ICAM-1). FTH-ICAM-1 stimulated the expression of Il1b, NLRP3 inflammasome activation, and the processing and secretion of IL-1β in a manner that depended on plasma membrane remodeling, clathrin-mediated endocytosis, and lysosomal destabilization. FTH-ICAM-1 signaling at early endosomes stimulated Il1b expression, implying that this endosomal signaling primed inflammasome activation in HSCs. In contrast, lysosomal destabilization was required for FTH-induced IL-1β secretion, suggesting that lysosomal damage activated inflammasomes. FTH induced IL-1β production in liver slices from wild-type mice but not in those from Icam1-/- or Nlrp3-/- mice. Thus, FTH signals through its receptor ICAM-1 on HSCs to activate the NLRP3 inflammasome. We speculate that this pathway contributes to hepatic inflammation, a key process that stimulates hepatic fibrogenesis associated with chronic liver disease.

A novel view of ferritin in cancer

Since its discovery more than 85 years ago, ferritin has principally been known as an iron storage protein. However, new roles, beyond iron storage, are being uncovered. Novel processes involving ferritin such as ferritinophagy and ferroptosis and as a cellular iron delivery protein not only expand our thinking on the range of contributions of this protein but present an opportunity to target these pathways in cancers. The key question we focus on within this review is whether ferritin modulation represents a useful approach for treating cancers. We discussed novel functions and processes of this protein in cancers. We are not limiting this review to cell intrinsic modulation of ferritin in cancers, but also focus on its utility in the trojan horse approach in cancer therapeutics. The novel functions of ferritin as discussed herein realize the multiple roles of ferritin in cell biology that can be probed for therapeutic opportunities and further research.

Efficacy of Antibodies Targeting TfR1 in Xenograft Mouse Models of AIDS-Related Non-Hodgkin Lymphoma

Transferrin receptor 1 (TfR1), also known as CD71, is a transmembrane protein involved in the cellular uptake of iron and the regulation of cell growth. This receptor is expressed at low levels on a variety of normal cells, but is upregulated on cells with a high rate of proliferation, including malignant cells and activated immune cells. Infection with the human immunodeficiency virus (HIV) leads to the chronic activation of B cells, resulting in high expression of TfR1, B-cell dysfunction, and ultimately the development of acquired immunodeficiency syndrome-related B-cell non-Hodgkin lymphoma (AIDS-NHL). Importantly, TfR1 expression is correlated with the stage and prognosis of NHL. Thus, it is a meaningful target for antibody-based NHL therapy. We previously developed a mouse/human chimeric IgG3 specific for TfR1 (ch128.1/IgG3) and showed that this antibody exhibits antitumor activity in an in vivo model of AIDS-NHL using NOD-SCID mice challenged intraperitoneally with 2F7 human Burkitt lymphoma (BL) cells that harbor the Epstein-Barr virus (EBV). We have also developed an IgG1 version of ch128.1 that shows significant antitumor activity in SCID-Beige mouse models of disseminated multiple myeloma, another B-cell malignancy. Here, we aim to explore the utility of ch128.1/IgG1 and its humanized version (hu128.1) in mouse models of AIDS-NHL. To accomplish this goal, we used the 2F7 cell line variant 2F7-BR44, which is more aggressive than the parental cell line and forms metastases in the brain of mice after systemic (intravenous) administration. We also used the human BL cell line JB, which in contrast to 2F7, is EBV-negative, allowing us to study both EBV-infected and non-infected NHL tumors. Treatment with ch128.1/IgG1 or hu128.1 of SCID-Beige mice challenged locally (subcutaneously) with 2F7-BR44 or JB cells results in significant antitumor activity against different stages of disease. Treatment of mice challenged systemically (intravenously) with either 2F7-BR44 or JB cells also showed significant antitumor activity, including long-term survival. Taken together, our results suggest that targeting TfR1 with antibodies, such as ch128.1/IgG1 or hu128.1, has potential as an effective therapy for AIDS-NHL.

Transferrin receptor 1 targeted nanomedicine for brain tumor therapy

Achieving effective drug delivery to traverse the blood-brain barrier (BBB) and target tumor cells remains the greatest challenge for brain tumor therapy. Importantly, the overexpressed membrane receptors on the brain endothelial cells, especially transferrin receptor 1 (TfR1), which mediate their ligands/antibodies to overcome the BBB by transcytosis, have been emerging as promising targets for brain tumor therapy. By employing ligands (e.g., transferrin, H-ferritin), antibodies or targeting peptides of TfR1 or aptamers, various functional nano-formulations have been developed in the last decade. These agents showed great potential for the treatment of brain diseases due to their ideal size, high loading capacity, controlled drug release and suitable pharmacokinetics. Herein, we summarize the latest advances on TfR1-targeted nanomedicine for brain tumor therapy. Moreover, we also discuss the strategies of improving stability, targeting ability and accumulation of nano-formulations in brain tumors for better outcomes. In this review, we hope to provide inspiration for the rational design of TfR1-targeted nanomedicine against brain tumors.

Reversible atransferrinemia in a patient with chronic enteropathy: is transferrin mandatory for iron transport?

Herein, we report the case of a 42-year-old woman, hospitalized in a French tertiary hospital for a relapse of a chronic enteropathy, who was found on admission to have no detectable serum transferrin. Surprisingly, she only exhibited mild anaemia. This atransferrinemia persisted for two months throughout her hospitalization, during which her haemoglobin concentration remained broadly stable. Based on her clinical history and evolution, we concluded to an acquired atransferrinemia secondary to chronic undernutrition, inflammation and liver failure. We discuss the investigations performed in this patient, and hypotheses regarding the relative stability of her haemoglobin concentration despite the absence of detectable transferrin.

Serum Ferritin Has Limited Prognostic Value on Mortality Risk in Patients with Decompensated Cirrhosis: A Propensity Score Matching Analysis

OBJECTIVE

The prognostic value of serum ferritin remains elusive in the literature. We aimed to examine the association between serum ferritin and mortality risk in cirrhosis.

METHODS

A total of 257 cirrhotic patients were recruited. The cut-off of serum ferritin was determined by X-tile. The Cox regression and Kaplan-Meier method were used. A 1:1 propensity score matching (PSM) was performed to diminish the impacts of selection bias and possible confounders.

RESULTS

The difference regarding mortality was mostly significant for serum ferritin >158 ng/mL. Before PSM, serum ferritin >158 ng/mL was an independent predictor of mortality. However, the clinical relevance of high ferritin level for prognostication was blunted after PSM (survival rate: 86.8% vs 96.3%, P = .078). Cox regression indicated that model for end-stage liver disease remains only independent risk factor of 180-day mortality after PSM.

CONCLUSION

Serum ferritin may not serve as an independent prognostic indicator of mortality risk in decompensated cirrhotic patients.

Molecular mimicry of the receptor-binding domain of the SARS-CoV-2 spike protein: from the interaction of spike-specific antibodies with transferrin and lactoferrin to the antiviral effects of human recombinant lactoferrin

The pathogenesis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection involves dysregulations of iron metabolism, and although the mechanism of this pathology is not yet fully understood, correction of iron metabolism pathways seems a promising pharmacological target. The previously observed effect of inhibiting SARS-CoV-2 infection by ferristatin II, an inducer of transferrin receptor 1 (TfR1) degradation, prompted the study of competition between Spike protein and TfR1 ligands, especially lactoferrin (Lf) and transferrin (Tf). We hypothesized molecular mimicry of Spike protein as cross-reactivity of Spike-specific antibodies with Tf and Lf. Thus, strong positive correlations (R² > 0.95) were found between the level of Spike-specific IgG antibodies present in serum samples of COVID-19-recovered and Sputnik V-vaccinated individuals and their Tf-binding activity assayed with peroxidase-labeled anti-Tf. In addition, we observed cross-reactivity of Lf-specific murine monoclonal antibody (mAb) towards the SARS-CoV-2 Spike protein. On the other hand, the interaction of mAbs produced to the receptor-binding domain (RBD) of the Spike protein with recombinant RBD protein was disrupted by Tf, Lf, soluble TfR1, anti-TfR1 aptamer, as well as by peptides RGD and GHAIYPRH. Furthermore, direct interaction of RBD protein with Lf, but not Tf, was observed, with affinity of binding estimated by K_D to be 23 nM and 16 nM for apo-Lf and holo-Lf, respectively. Treatment of Vero E6 cells with apo-Lf and holo-Lf (1–4 mg/mL) significantly inhibited SARS-CoV-2 replication of both Wuhan and Delta lineages. Protective effects of Lf on different arms of SARS-CoV-2-induced pathogenesis and possible consequences of cross-reactivity of Spike-specific antibodies are discussed.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Structural and functional relationship of mammalian and nematode ferritins

Ferritin is a unique buffering protein in iron metabolism. By storing or releasing iron in a tightly controlled manner, it prevents the negative effects of free ferrous ions on biomolecules in all domains of life - from bacteria to mammals. This review focuses on the structural features and activity of the ferritin protein family with an emphasis on nematode ferritins and the similarities in their biological roles with mammalian ferritins. The conservative characteristic of the ferritin family across the species originates from the ferroxidase activity against redox-active iron. The antioxidative function of these proteins translates into their involvement in a wide range of important biological processes, e.g., aging, fat metabolism, immunity, anticancer activity, and antipathogenic activity. Moreover, disturbances in ferritin expression lead to severe iron-associated diseases. Research on the Caenorhabditis elegans model organism may allow us to better understand the wide spectrum of mechanisms involving ferritin activity.

HCV-Induced Immunometabolic Crosstalk in a Triple-Cell Co-Culture Model Capable of Simulating Systemic Iron Homeostasis

Iron is crucial to the regulation of the host innate immune system and the outcome of many infections. Hepatitis C virus (HCV), one of the major viral human pathogens that depends on iron to complete its life cycle, is highly skilled in evading the immune system. This study presents the construction and validation of a physiologically relevant triple-cell co-culture model that was used to investigate the input of iron in HCV infection and the interplay between HCV, iron, and determinants of host innate immunity. We recorded the expression patterns of key proteins of iron homeostasis involved in iron import, export and storage and examined their relation to the iron regulatory hormone hepcidin in hepatocytes, enterocytes and macrophages in the presence and absence of HCV. We then assessed the transcriptional profiles of pro-inflammatory cytokines Interleukin-6 (IL-6) and interleukin-15 (IL-15) and anti-inflammatory interleukin-10 (IL-10) under normal or iron-depleted conditions and determined how these were affected by infection. Our data suggest the presence of a link between iron homeostasis and innate immunity unfolding among liver, intestine, and macrophages, which could participate in the deregulation of innate immune responses observed in early HCV infection. Coupled with iron-assisted enhanced viral propagation, such a mechanism may be important for the establishment of viral persistence and the ensuing chronic liver disease.

A holistic view of mammalian (vertebrate) cellular iron uptake

Cell iron uptake in mammals is commonly distinguished by whether the iron is presented to the cell as transferrin-bound or not: TBI or NTBI. This generic perspective conflates TBI with canonical transferrin receptor, endosomal iron uptake, and NTBI with uptake supported by a plasma membrane-localized divalent metal ion transporter, most often identified as DMT1. In fact, iron uptake by mammalian cells is far more nuanced than this somewhat proscribed view suggests. This view fails to accommodate the substantial role that ZIP8 and ZIP14 play in iron uptake, while adhering to the traditional premise that a relatively high endosomal [H+] is thermodynamically required for release of iron from holo-Tf. The canonical view of iron uptake also does not encompass the fact that plasma membrane electron transport - PMET - has long been linked to cell iron uptake. In fact, the known mammalian metallo-reductases - Dcytb and the STEAP proteins - are members of this cohort of cytochrome-dependent oxido-reductases that shuttle reducing equivalents across the plasma membrane. A not commonly appreciated fact is the reduction potential of ferric iron in holo-Tf is accessible to cytoplasmic reducing equivalents - reduced pyridine and flavin mono- and di-nucleotides and dihydroascorbic acid. This allows for the reductive release of Fe2+ at the extracellular surface of the PM and subsequent transport into the cytoplasm by a neutral pH transporter - a ZIP protein. What this perspective emphasizes is that there are two TfR-dependent uptake pathways, one which does and one which does not involve clathrin-dependent, endolysosomal trafficking. This raises the question as to the selective advantage of having two Tf, TfR-dependent routes of iron accumulation. This review of canonical and non-canonical iron uptake uses cerebral iron trafficking as a point of discussion, a focus that encourages inclusion also of the importance of ferritin as a circulating 'chaperone' of ferric iron.

Apoferritin nanocages for targeted delivery of idarubicin against breast cancer cells

In recent years, nanotechnology has attracted attention for its capability to diagnose and remedy diverse tumors successfully.  Protein nanocarriers as a platform of targeted drug delivery can be used to reduce toxicity and improve the effect of anticancer drugs. Idarubicin (IDR) is a chemotherapy drug that is classified as an anthracycline antitumor. In this study, IDR was encapsulated within horse spleen apoferritin (HsAFr) nanocarriers. Encapsulation was obtained through disassembling apoferritin into subunits at pH 2 and subsequently reassembling it at pH 7.4 in the presence of IDR. Transmission electron microscopy, UV-vis, and fluorescence spectroscopy techniques showed that drug molecules are loaded within apoferritin. Intrinsic fluorescence information exhibited that the encapsulation does not have any effects on the tertiary structure of the protein. Drug loading and entrapment efficiency were found to be 7.15% and 84.75%, respectively. Comparison of anticancer activities in HsAFr-IDR and free drug IDR was made via the MTT viability technique in a human breast cancer cell line (MCF-7).

Antibodies Targeting the Transferrin Receptor 1 (TfR1) as Direct Anti-cancer Agents

The transferrin receptor 1 (TfR1), also known as cluster of differentiation 71 (CD71), is a type II transmembrane glycoprotein that binds transferrin (Tf) and performs a critical role in cellular iron uptake through the interaction with iron-bound Tf. Iron is required for multiple cellular processes and is essential for DNA synthesis and, thus, cellular proliferation. Due to its central role in cancer cell pathology, malignant cells often overexpress TfR1 and this increased expression can be associated with poor prognosis in different types of cancer. The elevated levels of TfR1 expression on malignant cells, together with its extracellular accessibility, ability to internalize, and central role in cancer cell pathology make this receptor an attractive target for antibody-mediated therapy. The TfR1 can be targeted by antibodies for cancer therapy in two distinct ways: (1) indirectly through the use of antibodies conjugated to anti-cancer agents that are internalized by receptor-mediated endocytosis or (2) directly through the use of antibodies that disrupt the function of the receptor and/or induce Fc effector functions, such as antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cell-mediated phagocytosis (ADCP), or complement-dependent cytotoxicity (CDC). Although TfR1 has been used extensively as a target for antibody-mediated cancer therapy over the years, interest continues to increase for both targeting the receptor for delivery purposes and for its use as direct anti-cancer agents. This review focuses on the developments in the use of antibodies targeting TfR1 as direct anti-tumor agents.

Abnormal ferritin levels predict development of poor outcomes in cirrhotic outpatients: a cohort study

Background

Both iron overload and iron deficient anemia can associate with cirrhosis. At the same time, inflammation might be continuously present in cirrhotic patients due to bacterial translocation and patients’ susceptibility to infections. Ferritin is a sensitive and widely available marker of iron homeostasis, in addition it acts as an acute phase protein. Therefore, we evaluated the prognostic potential of serum ferritin in the long-term follow-up of cirrhotic outpatients.

Methods

A cohort of 244 cirrhotic outpatients was recruited and followed for 2 years. We measured their serum ferritin levels in our routine laboratory unit at enrolment and investigated its association with clinical outcomes.

Results

Ferritin serum level was higher in males and older patients than in females (median: 152.6 vs. 75 μg/L, p < 0.001) or younger individuals (median: 142.9 vs. 67.9 μg/L, p = 0.002). Patients who previously survived variceal bleeding had lower ferritin levels (median: 43.1 vs. 146.6 μg/L, p < 0.001). In multivariate regression models, including laboratory and clinical factors, lower (< 40 μg/L) ferritin concentration was associated with the development of decompensated clinical stage in patients with previously compensated cirrhosis (sHR: 3.762, CI 1.616–8.760, p = 0.002), while higher (> 310 μg/L) circulating ferritin levels were associated with increased risks of bacterial infections in decompensated patients (sHR: 2.335, CI 1.193–4.568, p = 0.013) and mortality in the whole population (HR: 2.143, CI 1.174–3.910, p = 0.013).

Conclusion

We demonstrated usefulness of serum ferritin as a prognostic biomarker in cirrhosis, pointing out that both low and high concentrations need attention in these patients.

New Insights into the Role of Ferritin in Iron Homeostasis and Neurodegenerative Diseases

Growing evidence has indicated that iron deposition is one of the key factors leading to neuronal death in the neurodegenerative diseases. Ferritin is a hollow iron storage protein composed of 24 subunits of two types, ferritin heavy chain (FTH) and ferritin light chain (FTL), which plays an important role in maintaining iron homeostasis. Recently, the discovery of extracellular ferritin and ferritin in exosomes indicates that ferritin might be not only an iron storage protein within the cell, but might also be an important factor in the regulation of tissue and body iron homeostasis. In this review, we first described the structural characteristics, regulation and the physiological functions of ferritin. Secondly, we reviewed the current evidence concerning the mechanisms underlying the secretion of ferritin and the possible role of secreted ferritin in the brain. Then, we summarized the relationship between ferritin and the neurodegenerative diseases including Parkinson's disease (PD), Alzheimer's disease (AD) and neuroferritinopathy (NF). Given the importance and relationship between iron and neurodegenerative diseases, understanding the role of ferritin in the brain can be expected to contribute to our knowledge of iron dysfunction and neurodegenerative diseases.

Transferrin Receptors in Erythropoiesis

Erythropoiesis is a highly dynamic process giving rise to red blood cells from hematopoietic stem cells present in the bone marrow. Red blood cells transport oxygen to tissues thanks to the hemoglobin comprised of α- and β-globin chains and of iron-containing hemes. Erythropoiesis is the most iron-consuming process to support hemoglobin production. Iron delivery is mediated via transferrin internalization by the endocytosis of transferrin receptor type 1 (TFR1), one of the most abundant membrane proteins of erythroblasts. A second transferrin receptor—TFR2—associates with the erythropoietin receptor and has been implicated in the regulation of erythropoiesis. In erythroblasts, both transferrin receptors adopt peculiarities such as an erythroid-specific regulation of TFR1 and a trafficking pathway reliant on TFR2 for iron. This review reports both trafficking and signaling functions of these receptors and reassesses the debated role of TFR2 in erythropoiesis in the light of recent findings. Potential therapeutic uses targeting the transferrin-TFR1 axis or TFR2 in hematological disorders are also discussed.

Ferritin: An Inflammatory Player Keeping Iron at the Core of Pathogen-Host Interactions

Iron is an essential element for virtually all cell types due to its role in energy metabolism, nucleic acid synthesis and cell proliferation. Nevertheless, if free, iron induces cellular and organ damage through the formation of free radicals. Thus, iron levels must be firmly controlled. During infection, both host and microbe need to access iron and avoid its toxicity. Alterations in serum and cellular iron have been reported as important markers of pathology. In this regard, ferritin, first discovered as an iron storage protein, has emerged as a biomarker not only in iron-related disorders but also in inflammatory diseases, or diseases in which inflammation has a central role such as cancer, neurodegeneration or infection. The basic research on ferritin identification and functions, as well as its role in diseases with an inflammatory component and its potential as a target in host-directed therapies, are the main considerations of this review.

Ferritin Nanocages for Protein Delivery to Tumor Cells

The delivery of therapeutic proteins is one of the greatest challenges in the treatment of human diseases. In this frame, ferritins occupy a very special place. Thanks to their hollow spherical structure, they are used as modular nanocages for the delivery of anticancer drugs. More recently, the possibility of encapsulating even small proteins with enzymatic or cytotoxic activity is emerging. Among all ferritins, particular interest is paid to the Archaeoglobus fulgidus one, due to its peculiar ability to associate/dissociate in physiological conditions. This protein has also been engineered to allow recognition of human receptors and used in vitro for the delivery of cytotoxic proteins with extremely promising results.

Ferritin drug carrier (FDC) for tumor targeting therapy

Ferritin is an iron storage protein that plays a key role in iron homeostasis and anti-oxidation of cells. Due to its unique architecture of 24 self-assembling subunits and hollow cavity capable of encapsulating drugs, and an outer surface that can be modified genetically and chemically for additional functionality, ferritin has recently emerged as a promising drug delivery vehicle. Recent research demonstrated that unmodified human heavy chain ferritin binds to its receptor, transferrin receptor 1 (TfR1), in different types of tumor tissues, including lung and breast cancer, thus highlighting the potential use of ferritin for tumor-targeting applications. In this review, we consider the many favorable characteristics of ferritin drug carriers (FDCs) for tumor drug delivery. In particular, compared with antibody-drug conjugates (ADCs), ferritin exhibits superiority in a range of attributes, including drug loading ability, thermostability, and ease of production. Thus, the emergence of FDCs may be the next step in targeted cancer therapy.

Generation and Molecular Characterization of Human Ring Sideroblasts: a Key Role of Ferrous Iron in Terminal Erythroid Differentiation and Ring Sideroblast Formation

Ring sideroblasts are a hallmark of sideroblastic anemia, although little is known about their characteristics. Here, we first generated mutant mice by disrupting the GATA-1 binding motif at the intron 1 enhancer of the ALAS2 gene, a gene responsible for X-linked sideroblastic anemia (XLSA). Although heterozygous female mice showed an anemic phenotype, ring sideroblasts were not observed in their bone marrow. We next established human induced pluripotent stem cell-derived proerythroblast clones harboring the same ALAS2 gene mutation. Through coculture with sodium ferrous citrate, mutant clones differentiated into mature erythroblasts and became ring sideroblasts with upregulation of metal transporters (MFRN1, ZIP8, and DMT1), suggesting a key role for ferrous iron in erythroid differentiation. Interestingly, holo-transferrin (holo-Tf) did not induce erythroid differentiation as well as ring sideroblast formation, and mutant cells underwent apoptosis. Despite massive iron granule content, ring sideroblasts were less apoptotic than holo-Tf-treated undifferentiated cells. Microarray analysis revealed upregulation of antiapoptotic genes in ring sideroblasts, a profile partly shared with erythroblasts from a patient with XLSA. These results suggest that ring sideroblasts exert a reaction to avoid cell death by activating antiapoptotic programs. Our model may become an important tool to clarify the pathophysiology of sideroblastic anemia.

Cryo-EM structure of the human ferritin-transferrin receptor 1 complex

Human transferrin receptor 1 (CD71) guarantees iron supply by endocytosis upon binding of iron-loaded transferrin and ferritin. Arenaviruses and the malaria parasite exploit CD71 for cell invasion and epitopes on CD71 for interaction with transferrin and pathogenic hosts were identified. Here, we provide the molecular basis of the CD71 ectodomain-human ferritin interaction by determining the 3.9 Å resolution single-particle cryo-electron microscopy structure of their complex and by validating our structural findings in a cellular context. The contact surfaces between the heavy-chain ferritin and CD71 largely overlap with arenaviruses and Plasmodium vivax binding regions in the apical part of the receptor ectodomain. Our data account for transferrin-independent binding of ferritin to CD71 and suggest that select pathogens may have adapted to enter cells by mimicking the ferritin access gate.

The human transferrin receptor 1 (CD71) is a transmembrane protein responsible for iron uptake. Here the authors present the 3.9 Å resolution cryo-EM structure of the CD71 ectodomain-human ferritin (H-Ft) complex and find that H-Ft binds a CD71 region different from the transferrin one that overlaps with the surface recognized by select pathogens.

Ferritin: A Platform for MRI Contrast Agents Delivery

The search for high relaxivities and increased specificity continues to be central to the development of paramagnetic contrast agents for magnetic resonance imaging (MRI). Ferritin, due to its unique surface properties, architecture, and biocompatibility, has emerged as a natural nanocage that can potentially help to reach both these goals. This review aims to highlight recent advances in the use of ferritin as a nanoplatform for the delivery of metal-based MRI contrast agents (containing Gd3+, Mn2+, or Fe2O3) alone or in combination with active molecules used for therapeutic purposes. The collected results unequivocally show that the use of ferritin for contrast agent delivery leads to more accurate imaging of cancer cells and a significantly improved targeted therapy.

Transferrin and transferrin receptors update

In vertebrates, transferrin (Tf) safely delivers iron through circulation to cells. Tf-bound iron is incorporated through Tf receptor (TfR) 1-mediated endocytosis. TfR1 can mediate cellular uptake of both Tf and H-ferritin, an iron storage protein. New World arenaviruses, which cause hemorrhagic fever, and Plasmodium vivax use TfR1 for entry into host cells. Human TfR2, another receptor for Tf, is predominantly expressed in hepatocytes and erythroid precursors, and holo-Tf dramatically upregulates its expression. TfR2 forms a complex with hemochromatosis protein, HFE, and serves as a component of the iron sensing machinery in hepatocytes. Defects in TfR2 cause systemic iron overload, hemochromatosis, through down-regulation of hepcidin. In erythroid cells, TfR2 forms a complex with the erythropoietin receptor and regulates erythropoiesis. TfR2 facilitates iron transport from lysosomes to mitochondria in erythroblasts and dopaminergic neurons. Administration of apo-Tf, which scavenges free iron, has been explored for various clinical conditions including atransferrinemia, iron overload, and tissue ischemia. Apo-Tf has also been shown to ameliorate anemia in animal models of β-thalassemia. In this review, I provide an update and summary on our knowledge of mammalian Tf and its receptors.

Brain Iron Homeostasis: A Focus on Microglial Iron

Iron is an essential trace element required for important brain functions including oxidative metabolism, synaptic plasticity, myelination, and the synthesis of neurotransmitters. Disruptions in brain iron homeostasis underlie many neurodegenerative diseases. Increasing evidence suggests that accumulation of brain iron and chronic neuroinflammation, characterized by microglia activation and secretion of proinflammatory cytokines, are hallmarks of neurodegenerative disorders including Alzheimer’ s disease. While substantial efforts have led to an increased understanding of iron metabolism and the role of microglial cells in neuroinflammation, important questions still remain unanswered. Whether or not increased brain iron augments the inflammatory responses of microglial cells, including the molecular cues that guide such responses, is still unclear. How these brain macrophages accumulate, store, and utilize intracellular iron to carry out their various functions under normal and disease conditions is incompletely understood. Here, we describe the known and emerging mechanisms involved in microglial cell iron transport and metabolism as well as inflammatory responses in the brain, with a focus on AD.

Emerging and Dynamic Biomedical Uses of Ferritin

Ferritin, a ubiquitously expressed protein, has classically been considered the main iron cellular storage molecule in the body. Owing to the ferroxidase activity of the H-subunit and the nucleation ability of the L-subunit, ferritin can store a large amount of iron within its mineral core. However, recent evidence has demonstrated a range of abilities of ferritin that extends well beyond the scope of iron storage. This review aims to discuss novel functions and biomedical uses of ferritin in the processes of iron delivery, delivery of biologics such as chemotherapies and contrast agents, and the utility of ferritin as a biomarker in a number of neurological diseases.

Ferritin Nanocarrier Traverses the Blood Brain Barrier and Kills Glioma

Over the last decades, considerable efforts have been put into developing active nanocarrier systems that cross the blood brain barrier (BBB) to treat brain-related diseases such as glioma tumors. However, to date none have been approved for clinical usage. Here, we show that a human H-ferritin (HFn) nanocarrier both successfully crosses the BBB and kills glioma tumor cells. Its principle point of entry is the HFn receptor (transferrin receptor 1), which is overexpressed in both BBB endothelial cells (ECs) and glioma cells. Importantly, we found that HFn enters and exits the BBB via the endosome compartment. In contrast, upon specifically targeting and entering glioma cells, nearly all of the HFn accumulated in the lysosomal compartment, resulting in the killing of glioma tumor cells, with no HFn accumulation in the surrounding healthy brain tissue. Thus, HFn is an ideal nanocarrier for glioma therapy and possesses the potential to serve as a therapeutic approach against a broad range of central nervous system diseases.