Evidence that a salt bridge in the light chain contributes to the physical stability difference between heavy and light human ferritins

A New Age of Drug Delivery: A Comparative Perspective of Ferritin-Drug Conjugates (FDCs) and Antibody-Drug Conjugates (ADCs)

Ferritin-drug conjugates (FDCs) and antibody-drug conjugates (ADCs) respectively represent the innovative and traditional mainstream approaches in drug delivery systems, each offering unique advantages and challenges. This viewpoint delves into the evolving landscape of drug delivery technologies, specifically focusing on FDCs and ADCs. Each method exhibits unique advantages and inherent challenges, shaping their roles in therapeutic applications. The article provides a comparative analysis of two delivery systems, FDCs and ADCs, in terms of targeting accuracy, drug loading capacity, and the nature of the payload itself. This comparison offers valuable insights into the distinct advantages and disadvantages associated with each system, enabling a clearer understanding of their potential applications and limitations in therapeutic contexts. This analysis is crucial for optimizing the use of these delivery systems across varying medical contexts, offering a comprehensive overview of their impact on the field of drug delivery.

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.

Hyperthermostable recombinant human heteropolymer ferritin derived from a novel plasmid design

Mammalian ferritins are predominantly heteropolymeric species consisting of 2 structurally similar, but functionally and genetically distinct subunit types, called H (Heavy) and L (Light). The two subunits co-assemble in different H and L ratios to form 24-mer shell-like protein nanocages where thousands of iron atoms can be mineralized inside a hollow cavity. Here, we use differential scanning calorimetry (DSC) to study ferritin stability and understand how various combinations of H and L subunits confer aspects of protein structure-function relationships. Using a recently engineered plasmid design that enables the synthesis of complex ferritin nanostructures with specific H to L subunit ratios, we show that homopolymer L and heteropolymer L-rich ferritins have a remarkable hyperthermostability (Tm = 115 ± 1°C) compared to their H-ferritin homologues (Tm = 93 ± 1°C). Our data reveal a significant linear correlation between protein thermal stability and the number of L subunits present on the ferritin shell. A strong and unexpected iron-induced protein thermal destabilization effect (ΔTm up to 20°C) is observed. To our knowledge, this is the first report of recombinant human homo- and hetero-polymer ferritins that exhibit surprisingly high dissociation temperatures, the highest among all known ferritin species, including many known hyperthermophilic proteins and enzymes. This extreme thermostability of our L and L-rich ferritins may have great potential for biotechnological applications.

Production of Recombinant Human Hybrid Ferritin with Heavy Chain and Light Chain in Escherichia coli and its Characterization

BACKGROUND

Natural human ferritin generally contains 24 subunits with different ratios of heavy chain to light chain, and the ratio of both subunits varies depending on tissue distribution and pathological conditions. However, the production of recombinant hybrid ferritin with both subunits is more challenging.

OBJECTIVE

This study aimed to prepare the recombinant hybrid ferritin for prokaryotic expression and characterize its structure and physicochemical properties.

METHODS

A prokaryotic expression vector of pACYCDuet-1 harboring the two individual genes of human ferritin heavy chain and light chain (FTH/FTL-pACYCDuet-1) was constructed and transfected into Escherichia coli bacteria. Then the genes were co-induced by IPTG to express.

RESULTS

The ferritin was purified by hydrophobic interaction chromatography combining size exclusion chromatography and verified by mass spectrometry and characterized by spectral and morphological analysis.

CONCLUSION

FTH and FTL subunits were successfully co-assembled into a hybrid ferritin nanoparticle (rhFTH/L). The structure of rhFTH/L was demonstrated highly ordered and fairly compact. Besides, the hybrid rhFTH/L nanoparticle was shown more sensitive to thermal stress and reduced stability when compared with that of both individual rhFTH and rhFTL.

Time-Resolved Small-Angle X-Ray Scattering of Protein Cage Assembly

Recent improvements in X-ray detectors and synchrotron light sources have made it possible to measure time-resolved small-angle X-ray scattering (TR-SAXS) at millisecond time resolution. As an example, in this chapter we describe the beamline setup, experimental scheme, and the points that should be noted in stopped-flow TR-SAXS experiments for investigating the ferritin assembly reaction.

Conversion of recombinant human ferritin light chain inclusion bodies into uniform nanoparticles in Escherichia coli for facile production

Prokaryotic expression systems are widely used to produce many types of biologics because of their extreme conveniences and unmatchable cost. However, production of recombinant human ferritin light chain (rhFTL) protein is largely restrained because its expression in Escherichia coli tends to form inclusion bodies (IBs). In this study, a prokaryotic expression vector (FTL-pBV220) harboring the rhFTL gene was constructed using a pBV220 plasmid. The tag-free rhFTL was highly expressed and almost entirely converted to soluble form, and thus the rhFTL was successfully self-assembled into uniform nanoparticles in E. coli. To establish a simplified downstream process, a precipitation procedure based on the optimized incubation temperature, pH condition, and ionic strength was developed to remove impurities from the crude lysate supernatant. The rhFTL retained in the clarified supernatant was subsequently purified in a single step using Capto Butyl column resulting in a considerable recovery and high purity. The purified rhFTL was characterized and verified by mass spectrometry and spectral and morphological analyses. The results revealed that rhFTL exhibited highly ordered and fairly compact structures and the spherical structures were preserved.

Encapsulation of Gold Nanoparticles into Redesigned Ferritin Nanocages for the Assembly of Binary Superlattices Composed of Fluorophores and Gold Nanoparticles

Nanomaterials with a defined composition and structure can be synthesized by exploiting natural templates or biomolecular matrices. In the present work, we use protein nanocages derived from human ferritin as a nanoscale building block for the assembly of gold nanoparticles and fluorescent molecules in the solid state. As a generalizable strategy, we show that prior to material synthesis, the cargo can be encapsulated into the protein nanocages using a dis- and reassembly approach. Toward this end, a new ligand system for gold nanoparticles enables efficient encapsulation of these particles into the nanocages. The gold nanoparticle-loaded protein nanocages are co-assembled with fluorophore-loaded protein nanocages. Binary superlattices are formed because two oppositely charged ferritin nanocages are used as templates for the assembly. The binary crystals show strong exciton-plasmon coupling between the encapsulated fluorophores and gold nanoparticles, which was spatially resolved with fluorescence lifetime imaging. The strategy outlined here offers a modular approach toward binary nanomaterials with highly ordered building blocks.

Importance of the Subunit-Subunit Interface in Ferritin Disassembly: A Molecular Dynamics Study

Ferritin is a spherical cage-like protein that is useful for loading large functional particles for various applications. To our knowledge, how pH affects the interfaces inside ferritin and the mechanism of ferritin disassembly is far from complete. For this article, we conducted a series of molecular dynamics simulations (MD) at different pH values to study how interfaces affect ferritins' stability. It is shown that dimers are stable even at extremely low pH (pH 2.0), indicating that the dimer is the essential subunit for disassembly, and the slight swelling of the dimer resulting from monomer rotation inside a dimer is what triggers disassembly. During ferritin disassembly, there are two types of interfaces involved, and the interface between dimers is crucial. We also found that the driving forces for maintaining dimer stability are different when a dimer is inside ferritin and in an acidic solution. At low pH, the protonation of residues can lead to the loss of the salt bridge and the hydrogen bond between dimers, resulting in the disassembly of ferritin in an acidic environment. The above simulations reveal the possible mechanism of ferritin disassembly in an acidic solution, which can help us to design innovative and functional ferritin cages for different applications.

Protein-Based Nanoparticles for the Imaging and Treatment of Solid Tumors: The Case of Ferritin Nanocages, a Narrative Review

Protein nanocages have been studied extensively, due to their unique architecture, exceptional biocompatibility and highly customization capabilities. In particular, ferritin nanocages (FNs) have been employed for the delivery of a vast array of molecules, ranging from chemotherapeutics to imaging agents, among others. One of the main favorable characteristics of FNs is their intrinsic targeting efficiency toward the Transferrin Receptor 1, which is overexpressed in many tumors. Furthermore, genetic manipulation can be employed to introduce novel variants that are able to improve the loading capacity, targeting capabilities and bio-availability of this versatile drug delivery system. In this review, we discuss the main characteristics of FN and the most recent applications of this promising nanotechnology in the field of oncology with a particular emphasis on the imaging and treatment of solid tumors.

Bioengineered Ferritin Nanocarriers for Cancer Therapy

Ferritin naturally exists in most organisms and can specifically recognize the transferrin 1 receptor (TfR1), which is generally highly expressed on various types of tumor cells. The pH dependent reversible assembling and disassembling property of ferritin renders it as a suitable candidate for encapsulating a variety of anticancer drugs and imaging probes. Ferritins external surface is chemically and genetically modifiable which can serve as attachment site for tumor specific targeting peptides or moieties. Moreover, the biological origin of these protein cages makes it a biocompatible nanocarrier that stabilizes and protects the enclosed particles from the external environment without provoking any toxic or immunogenic responses. Recent studies, further establish ferritin as a multifunctional nanocarrier for targeted cancer chemotherapy and phototherapy. In this review, we introduce the favorable characteristics of ferritin drug carriers, the specific targeted surface modification and a multifunctional nanocarriers combined chemotherapy with phototherapy for tumor treatment. Taken together, ferritin is a potential ideal base of engineered nanoparticles for tumor therapy and still needs to explore more on its way.

Pathogenic mechanism and modeling of neuroferritinopathy

Neuroferritinopathy is a rare autosomal dominant inherited movement disorder caused by alteration of the L-ferritin gene that results in the production of a ferritin molecule that is unable to properly manage iron, leading to the presence of free redox-active iron in the cytosol. This form of iron has detrimental effects on cells, particularly severe for neuronal cells, which are highly sensitive to oxidative stress. Although very rare, the disorder is notable for two reasons. First, neuroferritinopathy displays features also found in a larger group of disorders named Neurodegeneration with Brain Iron Accumulation (NBIA), such as iron deposition in the basal ganglia and extrapyramidal symptoms; thus, the elucidation of its pathogenic mechanism may contribute to clarifying the incompletely understood aspects of NBIA. Second, neuroferritinopathy shows the characteristic signs of an accelerated process of aging; thus, it can be considered an interesting model to study the progress of aging. Here, we will review the clinical and neurological features of neuroferritinopathy and summarize biochemical studies and data from cellular and animal models to propose a pathogenic mechanism of the disorder.

Ferritins in Chordata: Potential evolutionary trajectory marked by discrete selective pressures: History and reclassification of ferritins in chordates and geological events' influence on their evolution and radiation

Ferritins (FTs) are iron storage proteins that are involved in managing iron-oxygen balance. In our work, we present a hypothesis on the putative effect of geological changes that have affected the evolution and radiation of ferritin proteins. Based on sequence analysis and phylogeny reconstruction, we hypothesize that two significant factors have been involved in the evolution of ferritin proteins: fluctuations of atmospheric oxygen concentrations, altering redox potential, and changing availability of water rich in bioavailable ferric ions. Fish, ancient amphibians, reptiles, and placental mammals developed the broadest repertoire of singular FTs, attributable to embryonic growth in aquatic environments containing low oxygen levels and abundant forms of soluble iron. In contrast, oviparous land vertebrates, like reptiles and birds, that have developed in high oxygen levels and limited levels of environmental Fe²⁺ exhibit a lower diversity of singular FTs, but display a broad repertoire of subfamilies, particularly notable in early reptiles.

Intrinsically Magnetic Cells: A Review on Their Natural Occurrence and Synthetic Generation

The magnetization of non-magnetic cells has great potential to aid various processes in medicine, but also in bioprocess engineering. Current approaches to magnetize cells with magnetic nanoparticles (MNPs) require cellular uptake or adsorption through in vitro manipulation of cells. A relatively new field of research is "magnetogenetics" which focuses on in vivo production and accumulation of magnetic material. Natural intrinsically magnetic cells (IMCs) produce intracellular, MNPs, and are called magnetotactic bacteria (MTB). In recent years, researchers have unraveled function and structure of numerous proteins from MTB. Furthermore, protein engineering studies on such MTB proteins and other potentially magnetic proteins, like ferritins, highlight that in vivo magnetization of non-magnetic hosts is a thriving field of research. This review summarizes current knowledge on recombinant IMC generation and highlights future steps that can be taken to succeed in transforming non-magnetic cells to IMCs.

Biochemistry of mammalian ferritins in the regulation of cellular iron homeostasis and oxidative responses

Ferritin, an iron-storage protein, regulates cellular iron metabolism and oxidative stress. The ferritin structure is characterized as a spherical cage, inside which large amounts of iron are deposited in a safe, compact and bioavailable form. All ferritins readily catalyze Fe(II) oxidation by peroxides at the ferroxidase center to prevent free Fe(II) from participating in oxygen free radical formation via Fenton chemistry. Thus, ferritin is generally recognized as a cytoprotective stratagem against intracellular oxidative damage The expression of cytosolic ferritins is usually regulated by iron status and oxidative stress at both the transcriptional and post-transcriptional levels. The mechanism of ferritin-mediated iron recycling is far from clarified, though nuclear receptor co-activator 4 (NCOA4) was recently identified as a cargo receptor for ferritin-based lysosomal degradation. Cytosolic ferritins are heteropolymers assembled by H- and L-chains in different proportions. The mitochondrial ferritins are homopolymers and distributed in restricted tissues. They play protective roles in mitochondria where heme- and Fe/S-enzymes are synthesized and high levels of ROS are produced. Genetic ferritin disorders are mainly related to the L-chain mutations, which generally cause severe movement diseases. This review is focused on the biochemistry and function of mammalian intracellular ferritin as the major iron-storage and anti-oxidation protein.

Ferritin Nanocage: A Versatile Nanocarrier Utilized in the Field of Food, Nutrition, and Medicine

Compared with other nanocarriers such as liposomes, mesoporous silica, and cyclodextrin, ferritin as a typical protein nanocage has received considerable attention in the field of food, nutrition, and medicine owing to its inherent cavity size, excellent water solubility, and biocompatibility. Additionally, ferritin nanocage also serves as a versatile bio-template for the synthesis of a variety of nanoparticles. Recently, scientists have explored the ferritin nanocage structure for encapsulation and delivery of guest molecules such as nutrients, bioactive molecules, anticancer drugs, and mineral metal ions by taking advantage of its unique reversible disassembly and reassembly property and biomineralization. In this review, we mainly focus on the preparation and structure of ferritin-based nanocarriers, and regulation of their self-assembly. Moreover, the recent advances of their applications in food nutrient delivery and medical diagnostics are highlighted. Finally, the main challenges and future development in ferritin-directed nanoparticles’ synthesis and multifunctional applications are discussed.

Identification of FerLCH, isolation of ferritin and functional analysis related to interaction with pathogens in Eri-silkworm, Samia cynthia ricini

Ferritin is a ubiquitous and conserved iron storage protein that plays a significant role in host detoxification, iron storage, and immune response. Although ferritin has been studied in many species, little is known about its role in the Eri-silkworm (Samia cynthia ricini). In this study, the ferritin light-chain subunit gene, named ScFerLCH, was identified from S. c. ricini. The full-length gene, ScFerLCH, was 1,155 bp and encoded a protein consisting of 231 amino acids with a deduced molecular weight of 26.38 kDa. Higher ScFerLCH expression levels were found in the midgut, silk gland, and fat body by quantitative reverse-transcription polymerase chain reaction and western blot analysis. Injection of Staphylococcus aureus and Pseudomonas aeruginosa could induce upregulation of ScFerLCH in the hemolymph, fat body, and midgut, indicating that ScFerLCH may contribute to the host defense against invading pathogens. In addition, the native ferritin protein was isolated from S. c. ricini by native polyacrylamide gel electrophoresis and its two subunits, ferritin heavy-chain subunit (ScFerHCH) and ferritin light-chain subunit (ScFerLCH), were identified by mass spectrometry. Specifically, we found that recombinant ferritin subunits could self-assemble into a protein complex in vitro; moreover, both recombinant subunits and the protein complex were found to bind different bacteria, including Escherichia coli, P. aeruginosa, S. aureus, and Bacillus subtilis. However, bactericidal tests showed that the protein complex could not inhibit the growth of bacteria directly. Taken together, our results suggest that ScFerritin might play an important role in mediating molecular interaction with pathogens.

Ferritins in Kidney Disease

Ferritins are evolutionarily conserved proteins that regulate cellular iron metabolism. It is the only intracellular protein that is capable of storing large quantities of iron. Although the ratio of different subunits determines the iron content of each ferritin molecule, the exact mechanism that dictates organization of these subunits still is unclear. In this review, we address renal ferritin expression and its implication in kidney disease. Specifically, we address the role of ferritin subunits in preventing kidney injury and also promoting tolerance against infection-associated kidney injury. We describe functions for ferritin that are independent of its ability to ferroxidize and store iron. We further discuss the implications of ferritin in body fluids, including blood and urine, during inflammation and kidney disease. Although there are several in-depth review articles on ferritin in the context of iron metabolism, we chose to focus on the role of ferritin particularly in kidney health and disease and highlight unanswered questions in the field.

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.

Identification and functional analysis of an iron-binding protein, ferritin heavy chain subunit, from the swallowtail butterfly, Papilio xuthus

Ferritin, which is ubiquitous among all living organisms, plays a crucial role in maintaining iron homeostasis, immune response, and detoxification. In the present research, we identified an iron-binding protein, ferritin heavy chain subunit, from Papilio xuthus and named PxFerHCH. The complete complementary DNA of PxFerHCH was 1,252 bp encoding a sequence of 211 amino acids, which includes an iron-responsive element. Phylogenetic analysis showed that PxFerHCH is clustered with Manduca sexta and Galleria mellonella ferritin heavy chain subunits. Expression levels of PxFerHCH in various tissues were analyzed by reverse transcription quantitative polymerase chain reaction, and the results exhibited that PxFerHCH was expressed in all tissues with the highest expression in the fat body. The relative expression level of PxFerHCH in response to bacterial (Escherichia coli and Staphylococcus aureus) challenges sharply increased by about 12 hr postinfection (hpi) and then decreased at 24 hpi. In addition, the iron-binding capacity and antioxidation activity of recombinant PxFerHCH protein were also investigated. These results reveal that PxFerHCH might play an important role in defense against bacterial infection.

Repression of ferritin light chain translation by human eIF3

A central problem in human biology remains the discovery of causal molecular links between mutations identified in genome-wide association studies (GWAS) and their corresponding disease traits. This challenge is magnified for variants residing in non-coding regions of the genome. Single-nucleotide polymorphisms (SNPs) in the 5ʹ untranslated region (5ʹ-UTR) of the ferritin light chain (FTL) gene that cause hyperferritinemia are reported to disrupt translation repression by altering iron regulatory protein (IRP) interactions with the FTL mRNA 5ʹ-UTR. Here, we show that human eukaryotic translation initiation factor 3 (eIF3) acts as a distinct repressor of FTL mRNA translation, and eIF3-mediated FTL repression is disrupted by a subset of SNPs in FTL that cause hyperferritinemia. These results identify a direct role for eIF3-mediated translational control in a specific human disease.

Bacterioferritin of Magnetospirillum gryphiswaldense Is a Heterotetraeicosameric Complex Composed of Functionally Distinct Subunits but Is Not Involved in Magnetite Biomineralization

The biomineralization pathway of magnetite in magnetotactic bacteria is still poorly understood and a matter of intense debates. In particular, the existence, nature, and location of possible mineral precursors of magnetite are not clear. One possible precursor has been suggested to be ferritin-bound ferrihydrite. To clarify its role for magnetite biomineralization, we analyzed and characterized ferritin-like proteins from the magnetotactic alphaproteobacterium Magnetospirillum gryphiswaldense MSR-1, employing genetic, biochemical, and spectroscopic techniques. Transmission Mössbauer spectroscopy of the wild type (WT) and a bacterioferritin (bfr) deletion strain uncovered that the presence of ferrihydrite in cells is coupled to the presence of Bfr. However, bfr and dps deletion mutants, encoding another ferritin-like protein, or even mutants with their codeletion had no impact on magnetite formation in MSR-1. Thus, ferritin-like proteins are not involved in magnetite biomineralization and Bfr-bound ferrihydrite is not a precursor of magnetite biosynthesis. Using transmission electron microscopy and bacterial two-hybrid and electrophoretic methods, we also show that MSR-1 Bfr is an atypical representative of the Bfr subfamily, as it forms tetraeicosameric complexes from two distinct subunits. Furthermore, our analyses revealed that these subunits are functionally divergent, with Bfr1 harboring a ferroxidase activity while only Bfr2 contributes to heme binding. Because of this functional differentiation and the poor formation of homooligomeric Bfr1 complexes, only heterooligomeric Bfr protects cells from oxidative stress in vivo. In summary, our results not only provide novel insights into the biomineralization of magnetite but also reveal the unique properties of so-far-uncharacterized heterooligomeric bacterioferritins.IMPORTANCE Magnetotactic bacteria like Magnetospirillum gryphiswaldense are able to orient along magnetic field lines due to the intracellular formation of magnetite nanoparticles. Biomineralization of magnetite has been suggested to require a yet-unknown ferritin-like ferrihydrite component. Here, we report the identification of a bacterioferritin as the source of ferrihydrite in M. gryphiswaldense and show that, contrary to previous reports, bacterioferritin is not involved in magnetite biomineralization but required for oxidative stress resistance. Additionally, we show that bacterioferritin of M. gryphiswaldense is an unusual member of the bacterioferritin subfamily as it is composed of two functionally distinct subunits. Thus, our findings extend our understanding of the bacterioferritin subfamily and also solve a longstanding question about the magnetite biomineralization pathway.

Ferritin cage for encapsulation and delivery of bioactive nutrients: From structure, property to applications

Ferritin is a class of naturally occurring iron storage proteins, which is distributed widely in animal, plant, and bacteria. It usually consists of 24 subunits that form a hollow protein shell with high symmetry. One holoferritin molecule can store up to 4500 iron atom within its inner cavity, and it becomes apoferritin upon removal of iron from the cavity. Recently, scientists have subverted these nature functions and used reversibly self-assembled property of apoferritin cage controlled by pH for the encapsulation and delivery of bioactive nutrients or anticancer drug. In all these cases, the ferritin cages shield their cargo from the influence of external conditions and provide a controlled microenvironment. More importantly, upon encapsulation, ferritin shell greatly improved the water solubility, thermal stability, photostability, and cellular uptake activity of these small bioactive compounds. This review aims to highlight recent advances in applications of ferritin cage as a novel vehicle in the field of food science and nutrition. Future outlooks are highlighted with the aim to suggest a research line to follow for further studies.

Iron Oxidation and Core Formation in Recombinant Heteropolymeric Human Ferritins

In animals, the iron storage and detoxification protein, ferritin, is composed of two functionally and genetically distinct subunit types, H (heavy) and L (light), which co-assemble in various ratios with tissue specific distributions to form shell-like protein structures of 24 subunits within which a mineralized iron core is stored. The H-subunit possesses a ferroxidase center (FC) that catalyzes Fe(II) oxidation, whereas the L-subunit does not. To assess the role of the L-subunit in iron oxidation and core formation, two human recombinant heteropolymeric ferritins, designated H-rich and L-rich with ratios of ∼20H:4L and ∼22L:2H, respectively, were employed and compared to the human homopolymeric H-subunit ferritin (HuHF). These heteropolymeric ferritins have a composition similar to the composition of those found in hearts and brains (i.e., H-rich) and in livers and spleens (i.e., L-rich). As for HuHF, iron oxidation in H-rich ferritin was found to proceed with a 2:1 Fe(II):O₂ stoichiometry at an iron level of 2 Fe(II) atoms/H-subunit with the generation of H₂O₂. The H₂O₂ reacted with additional Fe(II) in a 2:1 Fe(II):H₂O₂ ratio, thus avoiding the production of hydroxyl radical. A μ-1,2-peroxo-diFe(III) intermediate was observed at the FC of H-rich ferritin as for HuHF. Importantly, the H-rich protein regenerated full ferroxidase activity more rapidly than HuHF did and additionally formed larger iron cores, indicating dual roles for the L-subunit in facilitating iron turnover at the FC and in mineralization of the core. The L-rich ferritin, while also facilitating iron oxidation at the FC, additionally promoted oxidation at the mineral surface once the iron binding capacity of the FC was exceeded.

Ferritin M of Paralichthys olivaceus possesses antimicrobial and antioxidative properties

Ferritin is an evolutionarily conserved protein that plays a vital role in maintaining iron homeostasis. In this study, we identified a ferritin M (PoFerM) from Japanese flounder (Paralichthys olivaceus) and analyzed its biological property. PoFerM is composed of 176 amino acid residues and contains the conserved ferroxidase diiron center and the ferrihydrite nucleation center typical of M ferritins. Expression of PoFerM occurred in multiple tissues and was most abundant in blood. Bacterial infection upregulated PoFerM expression in head kidney, spleen, and liver in a time-dependent manner. Recombinant PoFerM (rPoFerM) purified from Escherichia coli exhibited iron-chelating activity and inhibited bacterial growth, whereas rPoFerMM, the mutant protein that bears alanine substitution at two conserved residues of the ferroxidase center and the ferrihydrite nucleation center, failed to do so. Oxidative protection analysis showed that rPoFerM, but not rPoFerMM, was able to alleviate the deleterious effect of H2O2-induced free radicals on plasmid DNA and primary flounder cells. Together these results indicate that PoFerM is an iron chelator with antimicrobial and antioxidative properties, all which depend on the conserved ferroxidase center and the ferrihydrite nucleation site.

Mutagenesis study to disrupt electrostatic interactions on the twofold symmetry interface of Escherichia coli bacterioferritin

Ferritins and other cage proteins have been utilized as models to understand the fundamentals of protein folding and self-assembly. The bacterioferritin (BFR) from Escherichia coli, a maxi-ferritin made up of 24 subunits, was chosen as the basis for a mutagenesis study to investigate the role of electrostatic intermolecular interactions mediated through charged amino acids. Through structural and computational analyses, three charged amino acids R30, D56 and E60 which involved in an electrostatic interaction network were mutated to the opposite charge. Four mutants, R30D, D56R, E60H and D56R-E60H, were expressed, purified and characterized. All of the mutants fold into α-helical structures. Consistent with the computational prediction, they all show a lowered thermostability; double mutant D56R-E60H was found to be 16°C less stable than the wild type. Except for the mutant E60H, all the other mutations completely shut down the formation of protein cages to favour the dimer state in solution. The mutants, however, retain their ability to form cage-like nanostructures in the dried, surface immobilized conditions of transmission electron microscopy. Our findings confirm that even a single charge-inversion mutation at the 2-fold interface of BFR can affect the quaternary structure of its dimers and their ability to self-assemble into cage structures.

Neuroferritinopathy: From ferritin structure modification to pathogenetic mechanism

Neuroferritinopathy is a rare, late-onset, dominantly inherited movement disorder caused by mutations in L-ferritin gene. It is characterized by iron and ferritin aggregate accumulation in brain, normal or low serum ferritin levels and high variable clinical feature. To date, nine causative mutations have been identified and eight of them are frameshift mutations determined by nucleotide(s) insertion in the exon 4 of L-ferritin gene altering the structural conformation of the C-terminus of the L-ferritin subunit. Acting in a dominant negative manner, mutations are responsible for an impairment of the iron storage efficiency of ferritin molecule. Here, we review the main characteristics of neuroferritinopathy and present a computational analysis of some representative recently defined mutations with the purpose to gain new information about the pathogenetic mechanism of the disorder. This is particularly important as neuroferritinopathy can be considered an interesting model to study the relationship between iron, oxidative stress and neurodegeneration.

Loop electrostatics modulates the intersubunit interactions in ferritin

Functional ferritins are 24-mer nanocages that self-assemble with extended contacts between pairs of 4-helix bundle subunits coupled in an antiparallel fashion along the C2 axes. The largest intersubunit interaction surface in the ferritin nanocage involves helices, but contacts also occur between groups of three residues midway in the long, solvent-exposed L-loops of facing subunits. The anchor points between intersubunit L-loop pairs are the salt bridges between the symmetry-related, conserved residues Asp80 and Lys82. The resulting quaternary structure of the cage is highly soluble and thermostable. Substitution of negatively charged Asp80 with a positively charged Lys in homopolymeric M ferritin introduces electrostatic repulsions that inhibit the oligomerization of the ferritin subunits. D80K ferritin was present in inclusion bodies under standard overexpressing conditions in E. coli, contrasting with the wild type protein. Small amounts of fully functional D80K nanocages formed when expression was slowed. The more positively charged surface results in a different solubility profile and D80K crystallized in a crystal form with a low density packing. The 3D structure of D80K variant is the same as wild type except for the side chain orientations of Lys80 and facing Lys82. When three contiguous Lys groups are introduced in D80KI81K ferritin variant the nanocage assembly is further inhibited leading to lower solubility and reduced thermal stability. Here, we demonstrate that the electrostatic pairing at the center of the L-loops has a specific kinetic role in the self-assembly of ferritin nanocages.

Identification and functional characterization of a novel ferritin subunit from the tropical sea cucumber, Stichopus monotuberculatus

Ferritin is one of the major non-harm iron storage proteins that found in most cell types of animals, plants and microorganisms. In this study, a ferritin subunit named StmFer was identified from the sea cucumber (Stichopus monotuberculatus) and characterized functionally. The full-length cDNA of StmFer is 1184 bp in size with a 5'-untranslated region (UTR) of 131 bp, a 3'-UTR of 531 bp and an open reading frame of 522 bp that encoding a protein of 173 amino acids with a deduced molecular weight of 19.95 kDa. StmFer possesses both the ferroxidase center of vertebrate ferritin heavy subunit and iron nucleation sites of vertebrate ferritin light subunit. For the gene structure, StmFer contains only three exons separated by two introns. Higher levels of mRNA expression were noticed in intestine and coelomocyte of S. monotuberculatus by northern blot analysis. In in vitro experiments performed in coelomocytes, transcriptional expression of StmFer showed the strongest response to polyriboinosinic polyribocytidylic acid [Poly (I:C)] (9.08 fold up-regulation), followed by lipopolysaccharides (LPS), ferrous chloride (FeCl2) and inactivated bacteria (Vibrio alginolyticus) (7.84, 7.41 and 4.90 fold up-regulation, respectively) after 3 h post-challenge. In addition, the anti-oxidation activity and iron binding capacity of recombinant ferritin protein were demonstrated in this study. As a whole, our study suggested that the ferritin from sea cucumber may play critical roles not only in the cellular and organismic iron homeostasis, but also in the innate immune defense.

Coupling heme and iron metabolism via ferritin H chain

SIGNIFICANCE

Inflammation and immunity can be associated with varying degrees of heme release from hemoproteins, eventually leading to cellular and tissue iron (Fe) overload, oxidative stress, and tissue damage. Presumably, these deleterious effects contribute to the pathogenesis of systemic infections.

RECENT ADVANCES

Heme release from hemoglobin sensitizes parenchyma cells to undergo programmed cell death in response to proinflammatory cytokines, such as tumor necrosis factor. This cytotoxic effect is driven by a mechanism involving intracellular accumulation of free radicals, which sustain the activation of the c-Jun N-terminal kinase (JNK) signaling transduction pathway. While heme catabolism by heme oxygenase-1 (HO-1) prevents programmed cell death, this cytoprotective effect requires the co-expression of ferritin H (heart/heavy) chain (FTH), which controls the pro-oxidant effect of labile Fe released from the protoporphyrin IX ring of heme. This antioxidant effect of FTH restrains JNK activation, whereas JNK activation inhibits FTH expression, a cross talk that controls metabolic adaptation to cellular Fe overload associated with systemic infections.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Identification and characterization of the mechanisms via which FTH provides metabolic adaptation to tissue Fe overload should provide valuable information to our current understanding of the pathogenesis of systemic infections as well as other immune-mediated inflammatory diseases.

Human L-ferritin deficiency is characterized by idiopathic generalized seizures and atypical restless leg syndrome

Human L-ferritin deficiency causes reduced cellular iron availability and increased ROS production with enhanced oxidized proteins, which results in idiopathic generalized seizures and atypical restless leg syndrome.

The ubiquitously expressed iron storage protein ferritin plays a central role in maintaining cellular iron homeostasis. Cytosolic ferritins are composed of heavy (H) and light (L) subunits that co-assemble into a hollow spherical shell with an internal cavity where iron is stored. The ferroxidase activity of the ferritin H chain is critical to store iron in its Fe3+ oxidation state, while the L chain shows iron nucleation properties. We describe a unique case of a 23-yr-old female patient affected by a homozygous loss of function mutation in the L-ferritin gene, idiopathic generalized seizures, and atypical restless leg syndrome (RLS). We show that L chain ferritin is undetectable in primary fibroblasts from the patient, and thus ferritin consists only of H chains. Increased iron incorporation into the FtH homopolymer leads to reduced cellular iron availability, diminished levels of cytosolic catalase, SOD1 protein levels, enhanced ROS production and higher levels of oxidized proteins. Importantly, key phenotypic features observed in fibroblasts are also mirrored in reprogrammed neurons from the patient’s fibroblasts. Our results demonstrate for the first time the pathophysiological consequences of L-ferritin deficiency in a human and help to define the concept for a new disease entity hallmarked by idiopathic generalized seizure and atypical RLS.

The significance of ferritin in cancer: anti-oxidation, inflammation and tumorigenesis

The iron storage protein ferritin has been continuously studied for over 70years and its function as the primary iron storage protein in cells is well established. Although the intracellular functions of ferritin are for the most part well-characterized, the significance of serum (extracellular) ferritin in human biology is poorly understood. Recently, several lines of evidence have demonstrated that ferritin is a multi-functional protein with possible roles in proliferation, angiogenesis, immunosuppression, and iron delivery. In the context of cancer, ferritin is detected at higher levels in the sera of many cancer patients, and the higher levels correlate with aggressive disease and poor clinical outcome. Furthermore, ferritin is highly expressed in tumor-associated macrophages which have been recently recognized as having critical roles in tumor progression and therapy resistance. These characteristics suggest ferritin could be an attractive target for cancer therapy because its down-regulation could disrupt the supportive tumor microenvironment, kill cancer cells, and increase sensitivity to chemotherapy. In this review, we provide an overview of the current knowledge on the function and regulation of ferritin. Moreover, we examine the literature on ferritin's contributions to tumor progression and therapy resistance, in addition to its therapeutic potential.

Human ferritin for tumor detection and therapy

Ferritin, a major iron storage protein found in most living organisms, is composed of a 24-subunit protein cage with a hollow interior cavity. Serum ferritin serves as a critical marker to detect total body iron status. However, recent research reveals a number of novel functions of ferritin besides iron storage; for example, a ferritin receptor, transferrin receptor 1 (TfR1), has been identified and serum ferritin levels are found to be elevated in tumors. A particular new finding is that magnetoferritin nanoparticles, biomimetically synthesized using H-chain ferritin to form a 24-subunit cage with an iron oxide core, possess intrinsic dual functionality, the protein shell specifically targeting tumors and the iron oxide core catalyzing peroxidase substrates to produce a color reaction allowing visualization of tumor tissues. Here we attempt to summarize current research on ferritin, particularly newly identified functions related to tumors, in order to address current challenges and highlight future directions.

Mass spectrometric characteristics and kinetics of iron release in visceral mass of Saccostrea cucullata

Ferritins with electrophoretic homogeneity were prepared from the visceral mass of Saccostrea cucullata in batch. The native PAGE approach showed similar electrophoretic mobility among pig pancreatic ferritin, liver ferritin of Dasyatis akajei, and visceral mass ferritin of Saccostrea cucullata. SDS-PAGE indicated that the Saccostrea cucullata visceral ferritin (SCVF) consisted of a single subunit type and had a molecular weight (MW) of approximately 20 kDa, suggesting that the protein shell in SCVF was composed of a single subunit. In addition, peptide mass fingerprinting and transmission electron microscopy were used to identify SCVF further, and to observe its molecular structure. We found that the molecular structure in SCVF was similar to that of most mammalian ferritins, which are composed of a protein shell and an iron core. The results of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry under the assistance of an acidic matrix, sinapic acid, also showed that SCVF was composed of a single subunit type and its subunit MW was calculated to be 19871.042 Da in the absence of heme. Kinetics analysis revealed that the complete process of iron release fitted the law of a first-order reaction, which is similar to that of most ferritins in mammals. Similar to bacterial ferritin, studies indicated that the shell consisted of a single subunit type and showed similar kinetics of iron release, suggesting that this subunit plays two important roles in iron release and storage, and that it shows different stability and intensity of interaction in carrying out its physiological functions in SCVF.

Single subunit type of ferritin from visceral mass of Saccostrea cucullata: cloning, expression and cisplatin-subunit analysis

Ferritin, the iron storage protein, plays a key role in iron metabolism. Here, we have cloned an inducible ferritin cDNA with 516 bp within the open reading frame fragment from the visceral mass of Saccostrea cucullata. The subunit sequence of the ferritin was predicted to be a polypeptide of 171 amino acids with a molecular weight (MW) of 19.9182 kDa and an isoelectric point of 5.24. The cDNA sequence of S. cucullata ferritin was constructed into a pET-32a expression system for expressing its relative protein efficiently in the Escherichia coli BL21 strain under isopropyl-β-D-thiogalactoside (IPTG) induction. The recombinant ferritin, which was further purified on a Ni-NTA resin column and digested with enterokinase, was detected as a single subunit of approximately MW 20 kDa using both SDS-PAGE and mass spectrometry. S. cucullata ferritin (ScFer) showed 98% identity with Crassostrea gigas ferritin at the amino acid level. The secondary structure and phosphorylation sites of deduced amino acids were predicted with ExPASy proteomics tools and the NetPhos 2.0 server, respectively, and the subunit space structure of recombinant S. cucullata ferritin (rScFer) was built using the molecular operating environmental software system. The results of both in-gel digestion and identification using MALDI-TOF MS/MS showed that the recombinant protein was ScFer. ICP-MS indicated that rScFer subunit can directly bind to cisplatin[cis-Diaminedichloroplatinum(CDDP)], giving approximately 22.9 CDDP/ferritin subunit for forming a novel complex of CDDP-subunit, which suggests that it constructs a nanometer CDDP core-ferritin for developing a new drug of anti-cancer. The results of both the real-time PCR and Western blotting showed that the expression of ScFer mRNA was up-regulated in the oyster under the stress of Cd(2+). In addition, the expression increment of ScFer mRNA under bacterial challenge indicated that ferritin participated in the immune response of S. cucullata. The recombinant ScFer should prove to be useful for further study of the structure and function of ferritin in S. cucullata.

Structural and functional analyses of chicken liver ferritin

Characterization of ferritins from different species has provided insight into iron regulation mechanisms and evolutionary relationships. Here, we examined chicken liver ferritin, which comprises only H subunit and has 14.8 µg of Fe/100 µg of protein. The chicken H subunit apo homopolymer showed the same iron uptake rate as bovine H subunit homopolymer expressed with a baculovirus expression system (0.31 and 0.28 mmol of Fe/min per micromole of protein for chicken and bovine H subunit, respectively). Chicken H subunit apo homopolymer showed a significantly higher biotinylated hemin-binding activity than liver holoferritin. Although bovine spleen apoferritin, which has an L (liver or light):H (heart or heavy) subunit ratio of 1:1, also shows a significantly higher biotinylated hemin-binding activity than its holoferritin, these biotinylated hemin-binding activities were markedly lower than those of both chicken holo- and apoferritins. Binding of chicken holo- and apoferritin with biotinylated hemin was strongly inhibited by hemin but not iron-free hemin, protoporphyrin IX, or Zn-protoporphyrin. These findings demonstrate that chicken ferritin comprises only an H subunit, possesses ferroxidase activity as in mammalian ferritin H subunits, and binds heme more strongly than mammalian ferritins.

Ferritin M of Cynoglossus semilaevis: an iron-binding protein and a broad-spectrum antimicrobial that depends on the integrity of the ferroxidase center and nucleation center for biological activity

Ferritin is a major intracellular iron storage protein in higher vertebrates and plays an important role in iron metabolism. In this study, we identified and analyzed the biological activity of a ferritin M subunit (CsFerM) from half-smooth tongue sole (Cynoglossus semilaevis). The open reading frame (ORF) of CsFerM is 534 bp and encodes a protein that shares 79.7-86.4% overall sequence identities with the ferritin M subunits of a number of teleosts. In silico analysis identified in CsFerM a eukaryotic ferritin domain with conserved ferroxidase diiron center and ferrihydrite nucleation center. Quantitative real time RT-PCR analysis showed that under normal physiological conditions, expression of CsFerM was highest in liver, moderate in gill, spleen, and muscle, and low in gut, heart, and brain. Following experimental challenge with bacterial pathogens, CsFerM expression was significantly upregulated in kidney, spleen, and liver in time-dependent manners. Biological activity analysis showed that recombinant CsFerM purified from Escherichia coli exhibited apparent iron-binding activity and, when present in the culture medium of six different species of fish bacterial pathogens, completely inhibited bacterial growth. In contrast, a mutant CsFerM that bears alanine substitution at two conserved residues of the ferroxidase diiron center and ferrihydrite nucleation center was abolished in both iron-binding and antimicrobial capacity. These results demonstrate that CsFerM is a biologically active iron chelator with broad-spectrum antibacterial activity, which suggests a role for CsFerM in not only iron storage but also innate immunity. These results also indicate the importance of the conserved iron uptake and mineralization sites to the function of CsFerM.

Ferritin structure from Mycobacterium tuberculosis: comparative study with homologues identifies extended C-terminus involved in ferroxidase activity

Ferritins are recognized as key players in the iron storage and detoxification processes. Iron acquisition in the case of pathogenic bacteria has long been established as an important virulence mechanism. Here, we report a 3.0 Å crystal structure of a ferritin, annotated as Bacterioferritin B (BfrB), from Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis that continues to be one of the world's deadliest diseases. Similar to the other members of ferritin family, the Mtb BfrB subunit exhibits the characteristic fold of a four-helical bundle that possesses the ferroxidase catalytic centre. We compare the structure of Mtb BfrB with representatives of the ferritin family belonging to the archaea, eubacteria and eukarya. Unlike most other ferritins, Mtb BfrB has an extended C-terminus. To dissect the role of this extended C-terminus, truncated Mtb BfrB was purified and biochemical studies implicate this region in ferroxidase activity and iron release in addition to providing stability to the protein. Functionally important regions in a protein of known 3D-structure can be determined by estimating the degree of conservation of the amino-acid sites with its close homologues. Based on the comparative studies, we identify the slowly evolving conserved sites as well as the rapidly evolving variable sites and analyze their role in relation to structure and function of Mtb BfrB. Further, electrostatic computations demonstrate that although the electrostatic environment of catalytic residues is preserved within the family, extensive variability is exhibited by residues defining the channels and pores, in all likelihood keeping up with the diverse functions executed by these ferritins in varied environments.

pH-dependent structures of ferritin and apoferritin in solution: disassembly and reassembly

The pH-dependent structures of the ferritin shell (apoferritin, 24-mer) and the ferrihydrite core, under physiological conditions that permit enzymatic activity, were investigated by synchrotron small-angle X-ray scattering (SAXS). The solution structure of apoferritin was found to be nearly identical to the crystal structure. The shell thickness and hollow core volumes were estimated. The intact hollow spherical apoferritin was stable over a wide pH range, 3.40-10.0, and the ferrihydrite core was stable over the pH range 2.10-10.0. The apoferritin subunits underwent aggregation below pH 0.80, whereas the ferrihydrite cores aggregated below pH 2.10 as a result of the disassembly of the ferritin shell under the strongly acidic conditions. As the pH decreased from 3.40 to 0.80, apoferritin underwent stepwise disassembly by first forming a hollow sphere with two holes, then a headset-shaped structure, and, finally, rodlike oligomers. As the pH was increased from pH 1.96, the disassembled rodlike oligomers recovered only to the headset-shaped structure, and the disassembled headset-shaped intermediates recovered only to the hollow spherical structure with two hole defects. The apoferritin hole defects that formed during the disassembly process did not heal as the pH was increased to neutral or slightly basic conditions. The pH-induced apoferritin disassembly and reassembly processes were not fully reversible, although they were pseudoreversible over a limited pH range, between 10.0 and 2.66.

Change of Ferritin-binding Activity in the Serum of Foal after Birth

In mammal circulation, various ferritin-binding proteins (FBPs) are thought to be involved in the clearance of circulating ferritin after complex formation with it. However, horse FBPs are known to cause inhibitory effects on ferritin immunoassay due to the concealment of the ferritin molecule to anti-ferritin antibodies used in the ferritin immunoassay. These inhibitory effects are eliminated by heat treatment of horse serum at 75°C for 15 min. The inhibitory effects on ferritin immunoassay in the sera of ten foal sera (5 females and 5 males) from 1 to 18 months were detected during all periods, and ferritin concentrations of the foal sera increased 20–100% as compared with those of untreated sera by same heat treatment. Ferritin concentrations of heat-treated foal sera increased after birth, reaching to ferritin levels of adult horse at 9 months of age. Thereafter, although serum ferritin concentrations fell down at 12 months of age, these concentrations increased to adult levels at 15 months of age again. The ratio of ferritin concentration of heat-treated serum to that of the untreated serum was regarded as an apparent ferritin-binding activity. Ferritin-binding activities in the sera of foals showed peak at 2 and 4 months of age in females and males, respectively. These results suggested that horse FBPs were heat unstable, and FBPs may play an important role in iron metabolism at early developmental stage.

Iron loading-induced aggregation and reduction of iron incorporation in heteropolymeric ferritin containing a mutant light chain that causes neurodegeneration

Hereditary ferritinopathy (HF) is a neurodegenerative disease characterized by intracellular ferritin inclusion bodies (IBs) and iron accumulation throughout the central nervous system. Ferritin IBs are composed of mutant ferritin light chain as well as wild-type light (Wt-FTL) and heavy chain (FTH1) polypeptides. In vitro studies have shown that the mutant light chain polypeptide p.Phe167SerfsX26 (Mt-FTL) forms soluble ferritin 24-mer homopolymers having a specific structural disruption that explains its functional problems of reduced ability to incorporate iron and aggregation during iron loading. However, because ferritins are usually 24-mer heteropolymers and all three polypeptides are found in IBs, we investigated the properties of Mt-FTL/FTH1 and Mt-FTL/Wt-FTL heteropolymeric ferritins. We show here the facile assembly of Mt-FTL and FTH1 subunits into soluble ferritin heteropolymers, but their ability to incorporate iron was significantly reduced relative to Wt-FTL/FTH1 heteropolymers. In addition, Mt-FTL/FTH1 heteropolymers formed aggregates during iron loading, contrasting Wt-FTL/FTH1 heteropolymers and similar to what was seen for Mt-FTL homopolymers. The resulting precipitate contained both Mt-FTL and FTH1 polypeptides as do ferritin IBs in patients with HF. The presence of Mt-FTL subunits in Mt-FTL/Wt-FTL heteropolymers also caused iron loading-induced aggregation relative to Wt-FTL homopolymers, with the precipitate containing Mt- and Wt-FTL polypeptides again paralleling HF. Our data demonstrate that co-assembly with wild-type subunits does not circumvent the functional problems caused by mutant subunits. Furthermore, the functional problems characterized here in heteropolymers that contain mutant subunits parallel those problems previously reported in homopolymers composed exclusively of mutant subunits, which strongly suggests that the structural disruption characterized previously in Mt-FTL homopolymers occurs in a similar manner and to a significant extent in both Mt-FTL/FTH1 and Mt-FTL/Wt-FTL heteropolymers.