Functional analysis of human/chicken transferrin receptor chimeras indicates that the carboxy-terminal region is important for ligand binding

SDF-1 chemokine signalling modulates the apoptotic responses to iron deprivation of clathrin-depleted DT40 cells

We have previously deleted both endogenous copies of the clathrin heavy-chain gene in the chicken pre B-cell-line DT40 and replaced them with clathrin under the control of a tetracycline-regulatable promoter (Tet-Off). The originally derived cell-line DKO-S underwent apoptosis when clathrin expression was repressed. We have also described a cell-line DKO-R derived from DKO-S cells that was less sensitive to clathrin-depletion. Here we show that the restriction of transferrin uptake, resulting in iron deprivation, is responsible for the lethal consequence of clathrin-depletion. We further show that the DKO-R cells have up-regulated an anti-apoptotic survival pathway based on the chemokine SDF-1 and its receptor CXCR4. Our work clarifies several puzzling features of clathrin-depleted DT40 cells and reveals an example of how SDF-1/CXCR4 signalling can abrogate pro-apoptotic pathways and increase cell survival. We propose that the phenomenon described here has implications for the therapeutic approach to a variety of cancers.

An Overview of Molecular Aspects of Iron Metabolism

Iron (Fe) is an essential, but potentially noxious, metal for almost all organisms. Its precise cellular regulation is necessary to ensure synthesis of numerous iron-containing proteins required for metabolic processes yet at the same time avoiding the build-up of potentially toxic levels of iron. In humans, iron-deficiency results in anemia, while excess iron can lead to organ damage as a result of a build-up of non-transferrin-bound iron (NTBI). In recent years, the cloning of novel proteins has clarified the mechanisms of iron uptake, storage and metabolic regulation. Our current knowledge of the molecular aspects of mammalian iron metabolism and NTBI are presented in this review.

Quantifying and comparing constitutive immunity across avian species

Studies that blend a comparative approach to immunology with an appreciation for physiological ecology are defining an important new field in biology--ecological immunology. However, a panel of assays that permits a comparative approach to immunology is not yet available. In this paper, we describe several assays of innate immunity that do not require species-specific reagents and therefore ideal for use in comparative immunology studies. We optimized the assays for use in small birds, where sample volumes are limiting. The bactericidal assay measures the capacity of whole blood to kill microorganisms, and integrates many important components of constitutive immunity. The phagocytosis assay measures the phagocytic capacity of macrophages in whole blood. Bioassays for mannan binding protein and lysozyme can be used to measure inflammation-induced levels of these acute phase proteins in the plasma. Species differences in bactericidal and phagocytic activities against Staphylococcus aureus and Escherichia coli were observed in populations of captive and in free-living birds, demonstrating the assays' utility for multi-species comparisons. However, clay-colored thrushes (Turdus grayi) that were stressed by prolonged capture and handling had diminished phagocytic and antibacterial activities, indicating the need to conduct these assays soon after capture. When birds were challenged with lipopolysaccharide (LPS), levels of mannan-binding protein, lysozyme, and haptoglobin were elevated and bactericidal and phagocytic activities of blood were altered, indicating that these measurements are sensitive to the current infection status of the animal. All assays could be done on as little as 10 microL of blood or plasma and should be useful in field studies of comparative immunity.

Molecular Evolution of the Transferrin Receptor/Glutamate Carboxypeptidase II Family

The transferrin receptor family is represented by at least seven different homologous proteins in primates. Transferrin receptor (TfR1) is a type II membrane glycoprotein that, as a cell surface homodimer, binds iron-loaded transferrin as part of the process of iron transfer and uptake. Other family members include transferrin receptor 2 (TfR2), glutamate carboxypeptidase II (GCP2 or PSMA), N-acetylated alpha-linked acidic dipeptidase-like protein (NLDL), N-acetylated alpha-linked acidic dipeptidase 2 (NAALAD2), and prostate-specific membrane antigen-like protein (PMSAL/GCPIII). We compared 86 different sequences from 24 different species, from mammals to fungi. Through this comparison, we have identified several highly conserved residues specific to each family not previously associated with clinical mutations. The evolutionary history of the TfR/GCP2 family shows repeated episodes of duplications consistent with recent theories that nondispensable, slowly evolving genes are more likely to form multiple gene families.

Purified feline and canine transferrin receptors reveal complex interactions with the capsids of canine and feline parvoviruses that correspond to their host ranges

The cell infection processes and host ranges of canine parvovirus (CPV) and feline panleukopenia virus (FPV) are controlled by their capsid interactions with the transferrin receptors (TfR) on their host cells. Here, we expressed the ectodomains of wild-type and mutant TfR and tested those for binding to purified viral capsids and showed that different naturally variant strains of the viruses were associated with variant interactions with the receptors which likely reflect the optimization of the viral infection processes in the different hosts. While all viruses bound the feline TfR, reflecting their tissue culture host ranges, a naturally variant mutant of CPV (represented by the CPV type-2b strain) that became the dominant virus worldwide in 1979 showed significantly lower levels of binding to the feline TfR. The canine TfR ectodomain did not bind to a detectable level in the in vitro assays, but this appears to reflect the naturally low affinity of that interaction, as only low levels of binding were seen when the receptor was expressed on mammalian cells; however, that was sufficient to allow endocytosis and infection. The apical domain of the canine TfR controls the specific interaction with CPV capsids, as a canine TfR mutant altering a glycosylation site in that domain bound FPV, CPV-2, and CPV-2b capsids efficiently. Enzymatic removal of the N-linked glycans did not allow FPV binding to the canine TfR, suggesting that the protein sequence difference is itself important. The purified feline TfR inhibited FPV and CPV-2 binding and infection of feline cells but not CPV-2b, indicating that the receptor binding may be able to prevent the attachment to the same receptor on cells.

Release of the Soluble Transferrin Receptor Is Directly Regulated by Binding of Its Ligand Ferritransferrin

The human transferrin receptor (TfR) is shed by an integral metalloprotease releasing a soluble form (sTfR) into serum. The sTfR reflects the iron demand of the body and is postulated as a regulator of iron homeostasis via binding to the hereditary hemochromatosis protein HFE. To study the role of transferrin in this process, we investigated TfR shedding in HL60 cells and TfR-deficient Chinese hamster ovary cells transfected with human TfR. Independent of TfR expression, sTfR release decreases with increasing ferritransferrin concentrations, whereas apo-transferrin exhibits no inhibitory effect. To investigate the underlying mechanism, we generated several TfR mutants with different binding affinities for transferrin. Shedding of TfR mutants in transfected cells correlates exactly with their binding affinity, implying that the effect of ferritransferrin on TfR shedding is mediated by a direct molecular interaction. Analysis of sTfR release from purified microsomal membranes revealed that the regulation is independent from intracellular trafficking or cellular signaling events. Our results clearly demonstrated that sTfR does not only reflect the iron demand of the cells but also the iron availability in the bloodstream, mirrored by iron saturation of transferrin, corroborating the important potential function of sTfR as a regulator of iron homeostasis.

Identification of the epitope of a monoclonal antibody that disrupts binding of human transferrin to the human transferrin receptor

The molecular basis of the transferrin (TF)-transferrin receptor (TFR) interaction is not known. The C-lobe of TF is required to facilitate binding to the TFR and both the N- and C-lobes are necessary for maximal binding. Several mAb have been raised against human transferrin (hTF). One of these, designated F11, is specific to the C-lobe of hTF and does not recognize mouse or pig TF. Furthermore, mAb F11 inhibits the binding of TF to TFR on HeLa cells. To map the epitope for mAb F11, constructs spanning various regions of hTF were expressed as glutathione S-transferase (GST) fusion proteins in Escherichia coli. The recombinant fusion proteins were analysed in an iterative fashion by immunoblotting using mAb F11 as the probe. This process resulted in the localization of the F11 epitope to the C1 domain (residues 365-401) of hTF. Subsequent computer modelling suggested that the epitope is probably restricted to a surface patch of hTF consisting of residues 365-385. Mutagenesis of the F11 epitope of hTF to the sequence of either mouse or pig TF confirmed the identity of the epitope as immunoreactivity was diminished or lost. In agreement with other studies, these epitope mapping studies support a role for residues in the C1 domain of hTF in receptor binding.

The chianti zebrafish mutant provides a model for erythroid-specific disruption of transferrin receptor 1

Iron is a crucial metal for normal development, being required for the production of heme, which is incorporated into cytochromes and hemoglobin. The zebrafish chianti (cia) mutant manifests a hypochromic, microcytic anemia after the onset of embryonic circulation, indicative of a perturbation in red blood cell hemoglobin production. We show that cia encodes tfr1a, which is specifically expressed in the developing blood and requisite only for iron uptake in erythroid precursors. In the process of isolating zebrafish tfr1, we discovered two tfr1-like genes (tfr1a and tfr1b) and a single tfr2 ortholog. Abrogation of tfr1b function using antisense morpholinos revealed that this paralog was dispensable for hemoglobin production in red cells. tfr1b morphants exhibited growth retardation and brain necrosis, similar to the central nervous system defects observed in the Tfr1 null mouse, indicating that tfr1b is probably used by non-erythroid tissues for iron acquisition. Overexpression of mouse Tfr1, mouse Tfr2, and zebrafish tfr1b partially rescued hypochromia in cia embryos, establishing that each of these transferrin receptors are capable of supporting iron uptake for hemoglobin production in vivo. Taken together, these data show that zebrafish tfr1a and tfr1b share biochemical function but have restricted domains of tissue expression, and establish a genetic model to study the specific function of Tfr1 in erythroid cells.

Parvovirus infection of cells by using variants of the feline transferrin receptor altering clathrin-mediated endocytosis, membrane domain localization, and capsid-binding domains

The feline and canine transferrin receptors (TfRs) bind canine parvovirus to host cells and mediate rapid capsid uptake and infection. The TfR and its ligand transferrin have well-described pathways of endocytosis and recycling. Here we tested several receptor-dependent steps in infection for their role in virus infection of cells. Deletions of cytoplasmic sequences or mutations of the Tyr-Thr-Arg-Phe internalization motif reduced the rate of receptor uptake from the cell surface, while polar residues introduced into the transmembrane sequence resulted in increased degradation of transferrin. However, the mutant receptors still mediated efficient virus infection. In contrast, replacing the cytoplasmic and transmembrane sequences of the feline TfR with those of the influenza virus neuraminidase (NA) resulted in a receptor that bound and endocytosed the capsid but did not mediate viral infection. This chimeric receptor became localized to detergent-insoluble membrane domains. To test the effect of structural virus receptor interaction on infection, two chimeric receptors were prepared which contained antibody-variable domains that bound the capsid in place of the TfR ectodomain. These chimeric receptors bound CPV capsids and mediated uptake but did not result in cell infection. Adding soluble feline TfR ectodomain to the virus during that uptake did not allow infection.

Structure of the human transferrin receptor-transferrin complex

Iron, insoluble as free Fe(3+) and toxic as free Fe(2+), is distributed through the body as Fe(3+) bound to transferrin (Tf) for delivery to cells by endocytosis of its complex with transferrin receptor (TfR). Although much is understood of the transferrin endocytotic cycle, little has been uncovered of the molecular details underlying the formation of the receptor-transferrin complex. Using cryo-electron microscopy, we have produced a density map of the TfR-Tf complex at subnanometer resolution. An atomic model, obtained by fitting crystal structures of diferric Tf and the receptor ectodomain into the map, shows that the Tf N-lobe is sandwiched between the membrane and the TfR ectodomain and that the C-lobe abuts the receptor helical domain. When Tf binds receptor, its N-lobe moves by about 9 A with respect to its C-lobe. The structure of TfR-Tf complex helps account for known differences in the iron-release properties of free and receptor bound Tf.

Mechanism for multiple ligand recognition by the human transferrin receptor

Transferrin receptor 1 (TfR) plays a critical role in cellular iron import for most higher organisms. Cell surface TfR binds to circulating iron-loaded transferrin (Fe-Tf) and transports it to acidic endosomes, where low pH promotes iron to dissociate from transferrin (Tf) in a TfR-assisted process. The iron-free form of Tf (apo-Tf) remains bound to TfR and is recycled to the cell surface, where the complex dissociates upon exposure to the slightly basic pH of the blood. Fe-Tf competes for binding to TfR with HFE, the protein mutated in the iron-overload disease hereditary hemochromatosis. We used a quantitative surface plasmon resonance assay to determine the binding affinities of an extensive set of site-directed TfR mutants to HFE and Fe-Tf at pH 7.4 and to apo-Tf at pH 6.3. These results confirm the previous finding that Fe-Tf and HFE compete for the receptor by binding to an overlapping site on the TfR helical domain. Spatially distant mutations in the TfR protease-like domain affect binding of Fe-Tf, but not iron-loaded Tf C-lobe, apo-Tf, or HFE, and mutations at the edge of the TfR helical domain affect binding of apo-Tf, but not Fe-Tf or HFE. The binding data presented here reveal the binding footprints on TfR for Fe-Tf and apo-Tf. These data support a model in which the Tf C-lobe contacts the TfR helical domain and the Tf N-lobe contacts the base of the TfR protease-like domain. The differential effects of some TfR mutations on binding to Fe-Tf and apo-Tf suggest differences in the contact points between TfR and the two forms of Tf that could be caused by pH-dependent conformational changes in Tf, TfR, or both. From these data, we propose a structure-based model for the mechanism of TfR-assisted iron release from Fe-Tf.

Differences in the contact points between the transferrin receptor and the two forms of transferrin (with or without iron) are consistent with pH-dependent conformational changes in transferrin, its receptor, or both

Structural Reorganization of the Transferrin C-Lobe and Transferrin Receptor upon Complex Formation: The C-Lobe Binds to the Receptor Helical Domain

Human transferrin, a bilobal protein, with each lobe bearing a single iron-binding site, functions to transport iron into cells. While the N-terminal lobe alone does not measurably bind cellular transferrin receptors or serve as an iron donor for cells, the C-lobe is capable of both functions. We used hydroxyl radical-mediated protein footprinting and mass spectrometry to reveal the conformational changes that occur upon complex formation for the human transferrin C-lobe (residues 334-679) bound to the ectodomain of human transferrin receptor 1 (residues 121-760). Oxidation rates for proteolytic peptides in the C-lobe, the receptor, and their complex have been measured by mass spectrometry; upon formation of the complex, a dramatic decrease in modification rates, indicating protection of specific side chain groups, can be seen in C-lobe sequences corresponding to residues 381-401, 415-433, and 457-470. Peptide sequences experiencing modification rate decreases in the transferrin receptor upon C-lobe binding include residues 232-240, 365-371, 496-508, 580 and 581, 614-623, 634-646, 647-681, and 733-760. In addition, several peptides in the receptor exhibit enhancements in the rate of modification consistent with allosteric effects of complex formation. Using tandem mass spectrometry, the sites of modification with altered reactivity in the complex include Met382, Met389, Trp460, Met464, and Phe427 in the C-lobe and Tyr503, Pro581, Tyr611, Leu619, Met635, Phe650, Trp740, Trp754, and Phe760 within the transferrin receptor. Using available genetic, biochemical, and structural data, we confirm that the conserved RGD sequence (residues 646-648) in the helical domain of the transferrin receptor, including residues from Leu619 to Phe650, is a primary binding site for the transferrin C-lobe.

Residues in the apical domain of the feline and canine transferrin receptors control host-specific binding and cell infection of canine and feline parvoviruses

Canine parvovirus (CPV) and feline panleukopenia virus (FPV) capsids bind to the transferrin receptors (TfRs) of their hosts and use these receptors to infect cells. The binding is partially host specific, as FPV binds only to the feline TfR, while CPV binds to both the canine and feline TfRs. The host-specific binding is controlled by a combination of residues within a raised region of the capsid. To define the TfR structures that interact with the virus, we altered the apical domain of the feline or canine TfR or prepared chimeras of these receptors and tested the altered receptors for binding to FPV or CPV capsids. Most changes in the apical domain of the feline TfR did not affect binding, but replacing Leu221 with Ser or Asp prevented receptor binding to either FPV or CPV capsids, while replacing Leu221 with Lys resulted in a receptor that bound only to CPV but not to FPV. Analysis of recombinants of the feline and canine TfRs showed that sequences controlling CPV-specific binding were within the apical domain and that more than one difference between these receptors determined the CPV-specific binding of the canine TfR. Single changes within the canine TfR which removed a single amino acid insertion or which eliminated a glycosylation site gave that receptor the expanded ability to bind to FPV and CPV. In some cases, binding of capsids to mutant receptors did not result in infection, suggesting a structural role for the receptor in cell infection by the viruses.

Enhancement of MHC class II-restricted responses by receptor-mediated uptake of peptide antigens

Peptides, either as altered peptide ligands, competitors, or vaccines, offer an outstanding potential for regulating immune responses because of their exquisite specificity. However, a major problem associated with peptide therapies is that they are poorly taken up by APCs. Because of poor bioavailability, high concentrations and repeated treatments are required for peptide therapies in vivo. To circumvent this problem, we tested whether covalently coupling a peptide T cell determinant, OVA(323-339), to transferrin (Tf) enhances APC uptake and presentation as monitored by Th cell activation. Functional analysis of the Tf-peptide conjugates revealed that the conjugates were presented 10,000- and 100-fold more effectively by B cells than intact Ag and free peptide, respectively. Furthermore, we demonstrate that the Tf-peptide conjugates are taken up by B cells through a receptor-mediated process and subsequently delivered to the lysosomal compartment. Using an adoptive transfer assay, we show that that the Tf-peptide complexes are 100-fold more effective in vivo than the free peptide in activating CD4(+) T cells by following an early activation marker, CD69. Our results demonstrate that coupling peptides to Tf enhances peptide presentation, thereby making it possible to take full advantage of peptide-specific therapies in modulating T cell responses.

Transferrin/transferrin receptor-mediated drug delivery

Since transferrin was discovered more than half a century ago, a considerable effort has been made towards understanding tranferrin-mediated iron uptake. However, it was not until recently with the identification and characterization of several new genes related to iron homeostasis, such as the hemochromatosis protein HFE and the iron transporter DMT1, that our knowledge has been advanced dramatically. A major pathway for cellular iron uptake is through internalization of the complex of iron-bound transferrin and the transferrin receptor, which is negatively modulated by HFE, a protein related to hereditary hemochromatosis. Iron is released from transferrin as the result of the acidic pH in endosome and then is transported to the cytosol by DMT1. The iron is then utilized as a cofactor by heme and ribonucleotide reductase or stored in ferritin. Apart from iron, many other metal ions of therapeutic and diagnostic interests can also bind to transferrin at the iron sites and their transferrin complexes can be recognized by many cells. Therefore, transferrin has been thought as a "delivery system" for many beneficial and harmful metal ions into the cells. Transferrin has also be widely applied as a targeting ligand in the active targeting of anticancer agents, proteins, and genes to primary proliferating malignant cells that overexpress transferrin receptors. This is achieved by conjugation of transferrin with drugs, proteins, hybride systems with marcomolecules and as liposomal-coated systems. Conjugates of anticancer drugs with transferrin can significantly improve the selectivity and toxicity and overcome drug resistance, thereby leading to a better treatment. The coupling of DNA to transferrin via a polycation such as polylysine or via cationic liposomes can target and transfer of the extrogenous DNA particularly into proliferating cells through receptor-mediated endocytosis. These kinds of non-viral vectors are potential alternatives to viral vectors for gene therapy, if the transfection efficiency can be improved. Moreover, transferrin receptors have shown potentials in delivery of therapeutic drugs or genes into the brain across blood-brain barrier.

A new method for obtaining human transferrin C-lobe in the native conformation: preparation and properties

Eukaryotic transferrins comprise a class of bilobal iron-binding proteins in which each lobe carries a single binding site. Although expression of full-length transferrins and their N-terminal lobes, in wild-type and mutated forms, has been successfully accomplished by several laboratories, expression of C-lobes has been much less satisfactory. A possible explanation of the difficulty is that proper folding of the C-lobe, with its 11 disulfide bonds, depends on prior synthesis and proper folding of the N-lobe. We have therefore developed a new strategy, introducing a specific factor Xa cleavage site in the interlobe-connecting strand to permit separation of the lobes after expression of the full-length protein. The resulting protein was expressed in satisfactory yield, >20 mg/L, and could be easily and completely cleaved to yield two distinguishable fragments representing N- and C-lobes, respectively. Retaining the glycosylation sites, found only in the C-lobe, made it possible to separate the fragments from each other by ConA affinity chromatography. The isolated C-lobe so obtained displayed spectroscopic and kinetic features of the C-lobe in native transferrin and was competent as an iron donor for K562 cells to which it bound in saturable fashion inhibitable by native diferric transferrin. Since the N-lobe by itself will neither bind nor donate iron to cells, the primary receptor-recognition site of transferrin resides in its C-lobe.

Mutational analysis of the transferrin receptor reveals overlapping HFE and transferrin binding sites

The transferrin receptor (TfR) binds two proteins critical for iron metabolism: transferrin (Tf) and HFE, the protein mutated in hereditary hemochromatosis. Previous results demonstrated that Tf and HFE compete for binding to TfR, suggesting that Tf and HFE bind to the same or an overlapping site on TfR. TfR is a homodimer that binds one Tf per polypeptide chain (2:2, TfR/Tf stoichiometry), whereas both 2:1 and 2:2 TfR/HFE stoichiometries have been observed. In order to more fully characterize the interaction between HFE and TfR, we determined the binding stoichiometry using equilibrium gel-filtration and analytical ultracentrifugation. Both techniques indicate that a 2:2 TfR/HFE complex can form at submicromolar concentrations in solution, consistent with the hypothesis that HFE competes for Tf binding to TfR by blocking the Tf binding site rather than by exerting an allosteric effect. To determine whether the Tf and HFE binding sites on TfR overlap, residues at the HFE binding site on TfR were identified from the 2.8 A resolution HFE-TfR co-crystal structure, then mutated and tested for their effects on HFE and Tf binding. The binding affinities of soluble TfR mutants for HFE and Tf were determined using a surface plasmon resonance assay. Substitutions of five TfR residues at the HFE binding site (L619A, R629A, Y643A, G647A and F650Q) resulted in significant reductions in Tf binding affinity. The findings that both HFE and Tf form 2:2 complexes with TfR and that mutations at the HFE binding site affect Tf binding support a model in which HFE and Tf compete for overlapping binding sites on TfR.

The roles of iron in health and disease

Iron is vital for almost all living organisms by participating in a wide variety of metabolic processes, including oxygen transport, DNA synthesis, and electron transport. However, iron concentrations in body tissues must be tightly regulated because excessive iron leads to tissue damage, as a result of formation of free radicals. Disorders of iron metabolism are among the most common diseases of humans and encompass a broad spectrum of diseases with diverse clinical manifestations, ranging from anemia to iron overload and, possibly, to neurodegenerative diseases. The molecular understanding of iron regulation in the body is critical in identifying the underlying causes for each disease and in providing proper diagnosis and treatments. Recent advances in genetics, molecular biology and biochemistry of iron metabolism have assisted in elucidating the molecular mechanisms of iron homeostasis. The coordinate control of iron uptake and storage is tightly regulated by the feedback system of iron responsive element-containing gene products and iron regulatory proteins that modulate the expression levels of the genes involved in iron metabolism. Recent identification and characterization of the hemochromatosis protein HFE, the iron importer Nramp2, the iron exporter ferroportin1, and the second transferrin-binding and -transport protein transferrin receptor 2, have demonstrated their important roles in maintaining body's iron homeostasis. Functional studies of these gene products have expanded our knowledge at the molecular level about the pathways of iron metabolism and have provided valuable insight into the defects of iron metabolism disorders. In addition, a variety of animal models have implemented the identification of many genetic defects that lead to abnormal iron homeostasis and have provided crucial clinical information about the pathophysiology of iron disorders. In this review, we discuss the latest progress in studies of iron metabolism and our current understanding of the molecular mechanisms of iron absorption, transport, utilization, and storage. Finally, we will discuss the clinical presentations of iron metabolism disorders, including secondary iron disorders that are either associated with or the result of abnormal iron accumulation.

Crystal structure of the hereditary haemochromatosis protein HFE complexed with transferrin receptor

HFE is related to major histocompatibility complex (MHC) class I proteins and is mutated in the iron-overload disease hereditary haemochromatosis. HFE binds to the transferrin receptor (TfR), a receptor by which cells acquire iron-loaded transferrin. The 2.8 A crystal structure of a complex between the extracellular portions of HFE and TfR shows two HFE molecules which grasp each side of a twofold symmetric TfR dimer. On a cell membrane containing both proteins, HFE would 'lie down' parallel to the membrane, such that the HFE helices that delineate the counterpart of the MHC peptide-binding groove make extensive contacts with helices in the TfR dimerization domain. The structures of TfR alone and complexed with HFE differ in their domain arrangement and dimer interfaces, providing a mechanism for communicating binding events between TfR chains. The HFE-TfR complex suggests a binding site for transferrin on TfR and sheds light upon the function of HFE in regulating iron homeostasis.

The transferrin receptor of Trypanosoma brucei

Bloodstream forms of Trypanosoma brucei, the causative agent of sleeping sickness in humans, require transferrin for growth. Uptake of host transferrin is mediated by a heterodimeric glycosylphosphatidylinositol-anchored receptor. The trypanosomal transferrin receptor is homologous to the N-terminal domain of the variant surface glycoprotein (VSG) and bears no structural similarity with the human transferrin receptor. In this review, the structure, biochemical properties and function of the transferrin receptor of T. brucei are summarized and compared to the transferrin receptor of mammalian cells.

Crystal structure of the ectodomain of human transferrin receptor

The transferrin receptor (TfR) undergoes multiple rounds of clathrin-mediated endocytosis and reemergence at the cell surface, importing iron-loaded transferrin (Tf) and recycling apotransferrin after discharge of iron in the endosome. The crystal structure of the dimeric ectodomain of the human TfR, determined here to 3.2 angstroms resolution, reveals a three-domain subunit. One domain closely resembles carboxy- and aminopeptidases, and features of membrane glutamate carboxypeptidase can be deduced from the TfR structure. A model is proposed for Tf binding to the receptor.

The transferrin receptor: role in health and disease

The transferrin receptor is a membrane glycoprotein whose only clearly defined function is to mediate cellular uptake of iron from a plasma glycoprotein, transferrin. Iron uptake from transferrin involves the binding of transferrin to the transferrin receptor, internalization of transferrin within an endocytic vesicle by receptor-mediated endocytosis and the release of iron from the protein by a decrease in endosomal pH. With the exception of highly differentiated cells, transferrin receptors are probably expressed on all cells but their levels vary greatly. Transferrin receptors are highly expressed on immature erythroid cells, placental tissue, and rapidly dividing cells, both normal and malignant. In proliferating nonerythroid cells the expression of transferrin receptors is negatively regulated post-transcriptionally by intracellular iron through iron responsive elements (IREs) in the 3' untranslated region of transferrin receptor mRNA. IREs are recognized by specific cytoplasmic proteins (IRPs; iron regulatory proteins) that, in the absence of iron in the labile pool, bind to the IREs of transferrin receptor mRNA, preventing its degradation. On the other hand, the expansion of the labile iron pool leads to a rapid degradation of transferrin receptor mRNA that is not protected since IRPs are not bound to it. However, some cells and tissues with specific requirements for iron probably evolved mechanisms that can override the IRE/IRP-dependent control of transferrin receptor expression. Erythroid cells, which are the most avid consumers of iron in the organism, use a transcriptional mechanism to maintain very high transferrin receptor levels. Transcriptional regulation is also involved in the receptor expression during T and B lymphocyte activation. Macrophages are another example of a cell type that shows 'unorthodox' responses in terms of IRE/IRP paradigm since in these cells elevated iron levels increase (rather than decrease) transferrin receptor mRNA and protein levels. Erythroid cells contain the highest mass of the total organismal transferrin receptors which are released from reticulocytes during their maturation to erythrocytes. Hence, plasma contains small amounts of transferrin receptors which represent a soluble fragment of the extracellular receptor domain. Measurements of serum transferrin receptor concentrations are clinically useful since their levels correlate with the total mass of immature erythroid cells.

Characterization of a novel cellular defect in patients with phenotypic homozygous familial hypercholesterolemia

Familial hypercholesterolemia (FH) is characterized by a raised concentration of LDL in plasma that results in a significantly increased risk of premature atherosclerosis. In FH, impaired removal of LDL from the circulation results from inherited mutations in the LDL receptor gene or, more rarely, in the gene for apo B, the ligand for the LDL receptor. We have identified two unrelated clinically homozygous FH patients whose cells exhibit no measurable degradation of LDL in culture. Extensive analysis of DNA and mRNA revealed no defect in the LDL receptor, and alleles of the LDL receptor or apo B genes do not cosegregate with hypercholesterolemia in these families. FACS((R)) analysis of binding and uptake of fluorescent LDL or anti-LDL receptor antibodies showed that LDL receptors are on the cell surface and bind LDL normally, but fail to be internalized, suggesting that some component of endocytosis through clathrin-coated pits is defective. Internalization of the transferrin receptor occurs normally, suggesting that the defective gene product may interact specifically with the LDL receptor internalization signal. Identification of the defective gene will aid genetic diagnosis of other hypercholesterolemic patients and elucidate the mechanism by which LDL receptors are internalized.

Iron metabolism

The understanding of iron metabolism at the molecular level has been enormously expanded in recent years by new findings about the functioning of transferrin, the transferrin receptor and ferritin. Other recent developments include the discovery of the hemochromatosis gene HFE, identification of previously unknown proteins involved in iron transport, divalent metal transporter 1 and stimulator of Fe transport, and expanded insights into the regulation and expression of proteins involved in iron metabolism. Interactions among principal participants in iron transport have been uncovered, although the complexity of such interactions is still incompletely understood. Correlated efforts involving techniques and concepts of crystallography, spectroscopy and molecular biology applied to cellular processes have been, and should continue to be, particularly revealing.

Human transferrin receptor is active and plasma membrane-targeted in yeast

The human transferrin receptor, a type II plasma membrane protein which mediates iron transport in human cells, was expressed in the yeast Saccharomyces cerevisiae. The transferrin receptor synthesized by yeast cells was posttranslationally modified comparable to the native receptor with respect to glycosylation and dimer formation. The location of the expressed receptor in the yeast plasma membrane indicates that the targeting of this type II membrane protein shares similar mechanisms in yeast and mammalian cells. The yeast-expressed transferrin receptor showed binding activity towards its natural ligand, transferrin in an ELISA binding assay.

Exon/intron structure of the human transferrin receptor gene

A PCR-based intron jumping strategy has been utilized to investigate the exon/intron structure of the human transferrin receptor gene and determine the sequences of exon/intron junctions. There are 18 exons and introns 5' to a large exon encoding the last translated segment and a sizable 3' untranslated segment. All of the translated segments are encoded by exons 2-19. The tight turn motif, which is critical to the process of endocytosis, is encoded by exon 3. Based on recent studies of human/chicken receptor chimeras, it appears that the residues most likely to be involved in transferrin binding are encoded by exons 17-19. Exon 12 exhibits the greatest degree of homology with the gene for the prostate specific membrane antigen. A polymorphism has been tentatively identified at nucleotide position 519 in exon 4; the presence of either adenine or guanine should result in either serine or glycine, respectively, at position 142 of the amino acid sequence. This analysis of genomic structure will permit further detailed studies of the regulation, expression and evolution of the human transferrin receptor gene.