Evolutionary significance of the renal excretion of transferrin half-molecule fragments

Lactoferrin: from the structure to the functional orchestration of iron homeostasis

Iron is by far the most widespread and essential transition metal, possessing crucial biological functions for living systems. Despite chemical advantages, iron biology has forced organisms to face with some issues: ferric iron insolubility and ferrous-driven formation of toxic radicals. For these reasons, acquisition and transport of iron constitutes a formidable challenge for cells and organisms, which need to maintain adequate iron concentrations within a narrow range, allowing biological processes without triggering toxic effects. Higher organisms have evolved extracellular carrier proteins to acquire, transport and manage iron. In recent years, a renewed interest in iron biology has highlighted the role of iron-proteins dysregulation in the onset and/or exacerbation of different pathological conditions. However, to date, no resolutive therapy for iron disorders has been found. In this review, we outline the efficacy of Lactoferrin, a member of the transferrin family mainly secreted by exocrine glands and neutrophils, as a new emerging orchestrator of iron metabolism and homeostasis, able to counteract iron disorders associated to different pathologies, including iron deficiency and anemia of inflammation in blood, Parkinson and Alzheimer diseases in the brain and cystic fibrosis in the lung.

Phylogenomic analysis of transferrin family from animals and plants

Transferrins have been identified in animals and green algae, and they consist of a family of evolutionarily related proteins that play a central role in iron transport, immunity, growth and differentiation. This study assessed the transferrin genes among 100 genomes from a wide range of animal and plant kingdoms. The results showed that putative transferrins were widespread in animals, but their gene quantity and type differ greatly between animal groups. Generally, Mammalia possess abundant transferrin genes, whereas Trematoda contain few ones. Melanotransferrin and serotransferrin are widely distributed in vertebrates, while melanotransferrin-like and transferrin-like 1 are frequent in invertebrates. However, only a few plant species detected putative transferrins, and a novel transferrin member was first uncovered in Angiospermae and Pteridophyta. The structural comparison among transferrin family members revealed seven very well-repeated and conserved characteristic motifs, despite a considerable variation in the overall sequences. The phylogenetic analysis suggested that gene duplication, gene loss and horizontal transfer contributed to the diversification of transferrin family members, and their inferred evolutionary scenario was proposed. These findings help to the understanding of transferrin distribution, characteristic motifs and residues, and evolutionary process.

Polymorphism, selection and tandem duplication of transferrin genes in Atlantic cod (Gadus morhua)--conserved synteny between fish monolobal and tetrapod bilobal transferrin loci

BACKGROUND

The two homologous iron-binding lobes of transferrins are thought to have evolved by gene duplication of an ancestral monolobal form, but any conserved synteny between bilobal and monolobal transferrin loci remains unexplored. The important role played by transferrin in the resistance to invading pathogens makes this polymorphic gene a highly valuable candidate for studying adaptive divergence among local populations.

RESULTS

The Atlantic cod genome was shown to harbour two tandem duplicated serum transferrin genes (Tf1, Tf2), a melanotransferrin gene (MTf), and a monolobal transferrin gene (Omp). Whereas Tf1 and Tf2 were differentially expressed in liver and brain, the Omp transcript was restricted to the otoliths. Fish, chicken and mammals showed highly conserved syntenic regions in which monolobal and bilobal transferrins reside, but contrasting with tetrapods, the fish transferrin genes are positioned on three different linkage groups. Sequence alignment of cod Tf1 cDNAs from Northeast (NE) and Northwest (NW) Atlantic populations revealed 22 single nucleotide polymorphisms (SNP) causing the replacement of 16 amino acids, including eight surface residues revealed by the modelled 3D-structures, that might influence the binding of pathogens for removal of iron. SNP analysis of a total of 375 individuals from 14 trans-Atlantic populations showed that the Tf1-NE variant was almost fixed in the Baltic cod and predominated in the other NE Atlantic populations, whereas the NW Atlantic populations were more heterozygous and showed high frequencies of the Tf-NW SNP alleles.

CONCLUSIONS

The highly conserved synteny between fish and tetrapod transferrin loci infers that the fusion of tandem duplicated Omp-like genes gave rise to the modern transferrins. The multiple nonsynonymous substitutions in cod Tf1 with putative structural effects, together with highly divergent allele frequencies among different cod populations, strongly suggest evidence for positive selection and local adaptation in trans-Atlantic cod populations.

Evolution reversed: the ability to bind iron restored to the N-lobe of the murine inhibitor of carbonic anhydrase by strategic mutagenesis

The murine inhibitor of carbonic anhydrase (mICA) is a member of the superfamily related to the bilobal iron transport protein transferrin (TF), which binds a ferric ion within a cleft in each lobe. Although the gene encoding ICA in humans is classified as a pseudogene, an apparently functional ICA gene has been annotated in mice, rats, cows, pigs, and dogs. All ICAs lack one (or more) of the amino acid ligands in each lobe essential for high-affinity coordination of iron and the requisite synergistic anion, carbonate. The reason why ICA family members have lost the ability to bind iron is potentially related to acquiring a new function(s), one of which is inhibition of certain carbonic anhydrase (CA) isoforms. A recombinant mutant of the mICA (W124R/S188Y) was created with the goal of restoring the ligands required for both anion (Arg124) and iron (Tyr188) binding in the N-lobe. Absorption and fluorescence spectra definitively show that the mutant binds ferric iron in the N-lobe. Electrospray ionization mass spectrometry confirms the presence of both ferric iron and carbonate. At the putative endosomal pH of 5.6, iron is released by two slow processes indicative of high-affinity coordination. Induction of specific iron binding implies that (1) the structure of mICA resembles those of other TF family members and (2) the N-lobe can adopt a conformation in which the cleft closes when iron binds. Because the conformational change in the N-lobe indicated by metal binding does not impact the inhibitory activity of mICA, inhibition of CA was tentatively assigned to the C-lobe. Proof of this assignment is provided by limited trypsin proteolysis of porcine ICA.

On the evolutionary significance and metal-binding characteristics of a monolobal transferrin from Ciona intestinalis

Transferrins are a family of proteins that bind and transport Fe(III). Modern transferrins are typically bilobal and are believed to have evolved from an ancient gene duplication of a monolobal form. A novel monolobal transferrin, nicatransferrin (nicaTf), was identified in the primitive ascidian species Ciona intestinalis that possesses the characteristic features of the proposed ancestral Tf protein. In this work, nicaTf was expressed in Pichia pastoris. Extensive solution studies were performed on nicaTf, including UV-vis, fluorescence, CD, EPR and NMR spectroscopies, and electrospray time-of-flight mass spectrometry. The expressed protein is nonglycosylated, unlike the protein isolated from the organism. This property does not affect its ability to bind Fe(III). However, Fe(III)-bound nicaTf displays important spectral differences from other Fe(III)-bound transferrins, which are likely the consequence of differences in metal coordination. Coordination differences could also account for the weaker affinity of nicaTf for Fe(III) (log K = 18.5) compared with bilobal human serum transferrin (HsTf) (log K = 22.5 and 21.4). The Fe-nicaTf complex is not labile, as indicated by slow metal removal kinetics by the high-affinity chelator tiron at pH 7.4. The protein alternatively binds up to one equivalent of Ti(IV) or V(V), which suggests that it may transport nonferric metals. These solution studies provide insight into the structure and function of the primitive monolobal transferrin of C. intestinalis for comparison with higher order bilobal transferrins. They suggest that a major advantage for the evolution of modern transferrins, dominantly of bilobal form, is stronger Fe(III) affinity because of cooperativity.

Transferrin as a muscle trophic factor

Muscle cells grow by proliferation and protein accumulation. During the initial stages of development the participation of nerves is not always required. Myoblasts and satellite cells proliferate, fusing to form myotubes which further differentiate to muscle fibers. Myotubes and muscle fibers grow by protein accumulation and fusion with other myogenic cells. Muscle fibers finally reach a quasi-steady state which is then maintained for a long period. The mechanism of maintenance is not well understood. However, it is clear that protein metabolism plays a paramount role. The role played by satellite cells in the maintenance of muscle fibers is not known. Growth and maintenance of muscle cells are under the influence of various tissues and substances. Among them are Tf and the motor nerve, the former being the main object of this review and essential for both DNA and protein synthesis. Two sources of Tf have been proposed, i.e., the motor nerve and the tissue fluid. The first proposal is that the nervous trophic influence on muscle cells is mediated by Tf which is released from the nerve terminals. In this model, the sole source of Tf which is donated to muscle cells should be the nerve, and Tf should not be provided for muscle fiber at sites other than the synaptic region; otherwise, denervation atrophy would not occur, since Tf provided from TfR located at another site would cancel the effect of denervation. The second proposal is that Tf is provided from tissue fluid. This implies that an adequate amount of Tf is transferred from serum to tissue fluid; in this case TfR may be distributed over the entire surface of the cells. The trophic effects of the motor neuron have been studied in vivo, but its effects of myoblast proliferation have not been determined. There are few experiments on its effects on myotubes. Most work has been made on muscle fibers, where innervation is absolutely required for their maintenance. Without it, muscle fibers atrophy, although they do not degenerate. In contrast, almost all the work on Tf has been performed in vitro. Its effects on myoblast proliferation and myotube growth and maintenance have been established; myotubes degenerate following Tf removal. But its effects on mature muscle fibers in vivo are not well understood. Muscle fibers possess TfR all over on their cell surface and contain a variety of Fe-binding proteins, such as myoglobin. It is entirely plausible that muscle fibers require an amount of Tf, and that this is provided by TfR scattered on the cell surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Evolution of duplications in the transferrin family of proteins

The transferrin family is a group of proteins, defined by conserved amino acid motifs and putative function, found in both vertebrates and invertebrates. Included in this group are molecules known to bind iron, including serum transferrin, ovotransferrin, lactotransferrin, and melanotransferrin (MTF). Additional members of this family include inhibitor of carbonic anhydrase (ICA; mammals), major yolk protein (sea urchins), saxiphilin (frog), pacifastin (crayfish), and TTF-1 (algae). Most family members contain two lobes (N and C) of around 340 amino acids, the result of an ancient duplication event. In this article, we review the known functions of these proteins and speculate as to when the different homologs arose. From multiple-sequence alignments and neighbor-joining trees using 71 transferrin family sequences from 51 different species, including several novel sequences found in the Takifugu and Ciona genome databases, we conclude that melanotransferrins are much older (>670 MY) and more pervasive than previously thought, and the serum transferrin/melanotransferrin split may have occurred not long after lobe duplication. All subsequent duplication events diverged from the serum transferrin gene. The creation of such a large multiple-sequence alignment provides important information and could, in the future, highlight the role of specific residues in protein function.

Large cooperativity in the removal of iron from transferrin at physiological temperature and chloride ion concentration

Iron removal from serum transferrin by various chelators has been studied by gel electrophoresis, which allows direct quantitation of all four forms of transferrin (diferric, C-monoferric, N-monoferric, and apotransferrin). Large cooperativity between the two lobes of serum transferrin is found for iron removal by several different chelators near physiological conditions (pH 7.4, 37 degrees C, 150 mM NaCl, 20 mM NaHCO(3)). This cooperativity is manifested in a dramatic decrease in the rate of iron removal from the N-monoferric transferrin as compared with iron removal from the other forms of ferric transferrin. Cooperativity is diminished as the pH is decreased; it is also very sensitive to changes in chloride ion concentration, with a maximum cooperativity at 150 mM NaCl. A mechanism is proposed that requires closure of the C-lobe before iron removal from the N-lobe can be effected; the "open" conformation of the C-lobe blocks a kinetically significant anion-binding site of the N-lobe, preventing its opening. Physiological implications of this cooperativity are discussed.

A poly-His tag method for obtaining the C-terminal lobe of human transferrin

Human serum transferrin is an essential bilobal protein that transports iron in the circulation for delivery to iron-requiring cells. Obtaining the C-terminal lobe of human transferrin in verified native conformation has been problematic, possibly because its 11 disulfide bonds lead to misfolding when the lobe is expressed without its accompanying N-lobe. A recently reported method for preparing the C-lobe free of extraneous residues, with normal iron-binding properties and capable of delivering iron to cells, makes use of a Factor Xa cleavage site inserted into the interlobal connecting strand of the full-length protein. An inefficient step in this method requires the use of ConA chromatography to separate the cleaved lobes from each other, since only the C-lobe is glycosylated. Inserting a 6-His sequence near the start of the N-lobe enhances recovery of the recombinant transferrin from other proteins in the culture medium of the BHK21 cells expressing the mutant transferrin. The new procedure is more economical in time and effort than its predecessor, and offers the additional advantage of isolating C-lobe expressed with or without its glycan chains.

Alteration in the organ distribution of iron by truncated transferrin: implications for iron chelation therapy

The ability of the partial molecule of transferrin, truncated transferrin (t-Tf), to act as an excretable biologic iron chelator was examined. We confirmed the observations of Zak and Aisen (Zak O, Aisen P. Biochem Biophys Acta 1985;1952:24-8) that thermolysin treatment of human transferrin produces half molecules that retain iron-binding capacity. These molecules are poorly recognized by surface receptors on either human or murine cells. Although the plasma half-life of human transferrin in mice is moderately long (40 hours), injection of t-Tf into mice results in its rapid clearance (half-life = 10 minutes). Injection of iron 59-labeled transferrin results in the deposition of iron in the major hematopoetic organs of mice such as the spleen, bone marrow, and liver. Injection of 59Fe-labeled t-Tf results in the quantitative recovery of iron in the kidneys: 59Fe is retained in the kidney for substantial periods of time with little evidence of its excretion into urine. Injection of iodine 125-labeled t-Tf also results in the deposition of radioactivity in the kidneys, but 125I is rapidly excreted into the urine, where it is detected as free iodine. These results indicate that although t-Tf is directed to the kidney and filtered by the glomerulus, the molecule is reabsorbed and degraded, and iron is retained. These results have implications in the design of iron chelators.

Comparative nutrition of iron and copper

The suggestion from nutritional studies with mammals of a link between iron and copper metabolism has been reinforced by recent investigations with yeast cells. Iron must be in the reduced ferrous (FeII) state for uptake by yeast cells, and reoxidation to ferric (FeIII) by a copper oxidase is part of the transport process. Thus, yeast cells deficient in copper are unable to absorb iron. In an analogous way, animals deficient in copper appear to be unable to move FeII out of cells, probably because it cannot be oxidized to FeIII. Invertebrate animals use copper and iron in ways very similar to vertebrates, with some notable exceptions. In the cases where vertebrates and invertebrates are similar, the latter may be useful models for vertebrate metabolism. In cases where they differ (e.g. predominance of serum ferritin in insects, oxygen transport by a copper protein in many arthropods, central importance of phenoloxidase, a copper enzyme in arthropods), the differences may represent processes that are exaggerated in invertebrates and thus more amenable to study in these organisms. On the other hand, they may represent processes unique to invertebrates, thus providing novel information on species diversity.

Cockroach transferrin closely resembles vertebrate transferrins in its metal ion-binding properties: a spectroscopic study

The optical and electron paramagnetic resonance (EPR) spectroscopic properties of a transferrin from the cockroach Blaberus discoidalis have been investigated to determine the relation of this protein to vertebrate transferrins. Difference spectrophotometry substantiates the involvement of tyrosyl residues in iron binding, and confirms the specific binding of two equivalents of iron per molecule. The far-UV CD spectrum also indicates a secondary structure with marked similarity to those of vertebrate transferrins. EPR studies show a dependence of iron binding on (bi)carbonate, consistent with the absolute requirement of transferrins for a synergistic anion in binding iron. Continuous wave (CW) and pulsed EPR studies of the cupric complex of the protein implicate a histidyl nitrogen ligand in metal coordination, as in human transferrin. Additional studies establish that the pH-dependent release of iron is similar to that of human serum transferrin. The present data confirm cockroach transferrin as an authentic member of the transferrin superfamily, thereby suggesting an ancestral relationship of insect to vertebrate transferrins.

Coordination of iron by the ferric iron-binding protein of pathogenic Neisseria is homologous to the transferrins

The ferric iron-binding protein (Fbp) functions as a periplasmic-binding protein in the high-affinity active transport of growth-essential iron by pathogenic Neisseria. Fbp reversibly binds a single ferric ion per molecule of protein with high affinity. Similarly, the transferrins are a highly conserved family of bilobed vertebrate proteins that reversibly bind a single molecule of iron on each of the N- and C-terminal lobes. While evolutionarily divergent, iron binding by all described transferrin lobes is accomplished by a remarkably similar repertoire of residues, including two Tyr, one His, and one Asp, as well as a synergestic bicarbonate anion. With a molecular mass of ca. 34 kDa, Fbp approximates the size of a transferrin lobe. Given the similarities in iron-binding properties, it was investigated whether Fbp bound iron by a similar molecular strategy as the transferrins. The studies reported here demonstrate that the spectral properties of purified Fbp and human transferrin are similar in the visible range. Chemical modification of purified Fbp in the presence and absence of iron using the Tyr-specific modifier tetranitromethane demonstrates that between two and three Tyr residues are implicated in iron binding. A similar experiment using the His-specific reagent diethyl pyrocarbonate indicates that one of the six Fbp-encoded His residues is protected by iron. In addition, like the transferrins, a bicarbonate anion is required for the efficient coordination of iron by Fbp. The range of metals bound by Fbp and human transferrin, including the luminescent lanthanide terbium, is identical. Finally, terbium derivatives of Fbp and human transferrin yield virtually identical luminescence excitation spectra, implying a highly similar binding site environment. These studies suggest that the prokaryotic Fbp is a mono-sited analog for iron binding by the eukaryotic transferrins.

The biology of transferrin

The chemistry and molecular biology of transferrin is discussed. The discussion covers the genetic control of transferrin synthesis, its intracellular synthesis, intra- and extracellular transport, and its interaction with transferrin receptors. The role of transferrin in iron metabolism is evaluated, both with regard to iron uptake by transferrin as to iron uptake from transferrin by different cells. The knowledge on the biochemical mechanisms involved in iron uptake is presented, with special reference to the triple role of the acidification of endocytotic vesicles. Apart from its traditional role in iron metabolism, transferrin acts as a growth factor. A distinction of two groups of growth-stimulating properties of transferrin has been made. As an early effect, membranous and intracellular changes are initiated, possibly based on electrochemical effects on the cell. The late effects seem to relate to its role in iron transport. Interestingly, the early growth stimulating effects can be segregated from the former function of transferrin and strictly speaking neither depend on iron nor on the transferrin molecule itself. Also the trophic effect of transferrin on several cell types has been described. Hypotheses concerning the biochemical basis of this effect are presented and within this context a new hypothesis on the differential occupation of iron binding sites of serum transferrin is forwarded. Examples of the applicability of present understanding of the biology of transferrin in clinical settings are presented.

Monoclonal antibodies to the amino- and carboxyl-terminal domains of ovotransferrin

Monoclonal antibodies to the iron transport protein ovotransferrin were produced by immunizing mice simultaneously with ovotransferrin and with the proteolytically derived amino- and carboxyl-terminal half-molecule domains of ovotransferrin. Two isolated hybridoma clones (designated alpha OT + N1 and alpha OT + N2) produced antibodies (IgG1) to determinants located on holo-ovotransferrin and the amino-terminal domain; two hybridoma clones (designated alpha OT + C1 and alpha OT + C2) produced antibodies (IgG1) to determinants on holo-ovotransferrin and the carboxyl-terminal domain. One hybridoma clone (designated alpha OT-N1) produced an antibody (IgG1) that bound only the amino-terminal domain and did not bind holo-ovotransferrin. Both alpha OT + N1, and alpha OT-N1 bound to antigen less tightly after removal of iron; antibodies alpha OT + N2, alpha OT + Cl, and alpha OT + C2 were unaffected by removal of iron from holo-ovotransferrin or the isolated domains. Intact disulfide bonds in the antigens were required for binding by the antibodies. These antibodies should prove useful as probes for discrete regions of the ovotransferrin molecule, in particular, those regions involved in binding to the transferrin receptor.

The dimerization of half-molecule fragments of transferrin

Partial proteolysis was used to prepare half-molecule fragments of hen ovotransferrin. N-Terminal and C-terminal fragments associate to form an N-terminal fragment-C-terminal fragment dimer. Variant forms of the N- and C-terminal fragments can be prepared in which a few amino acid residues are lacking from the C-terminal ends of the fragments. These variant fragments are partially or completely unable to associate; the suggestion that the molecular recognition sites are located in these C-terminal stretches of the N-terminal half-molecule (320-332) and of the C-terminal half-molecule (683-686) is in agreement with X-ray-crystallography data for human lactotransferrin [Anderson, Baker, Dodson, Norris, Rumball, Waters & Baker (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 1769-1773].

Preparation and properties of a single-sited fragment from the C-terminal domain of human transferrin

A single-sited iron-binding fragment of human transferrin has been obtained by thermolysin cleavage of the protein, selectively loaded with iron in the C-terminal binding site, in a urea-containing buffer. The fragment contains carbohydrate, and hence derives from the C-terminal half of transferrin. Its metal-binding site accepts Fe3+ and Cu2+ with bicarbonate as accompanying anion, but only Fe3+ with oxalate as anion. EPR spectroscopic properties of the fragment are similar to those of the corresponding site in the intact protein. However, iron-binding by the fragment is weaker than by the C-terminal site of the intact protein, particularly at low pH, suggesting that overall as well as local protein conformation influences the metal-binding functions of the site.

Occupancy of the iron binding sites of human transferrin

The in vivo distribution of iron between the binding sites of transferrin was examined. Plasma was obtained from normal subjects under basal conditions and after in vitro and in vivo iron loading. Independent methods, including measurement of the transferrin profile after isoelectric focusing and cross immunoelectrophoresis, and determination of the iron content in the separated fractions were in agreement that there was a random distribution of iron on binding sites. This held true with in vitro loading, when iron was increased by intestinal absorption and with loading from the reticuloendothelial system. The data indicate that the distribution of apo-, monoferric, and diferric transferrins is predictable on the basis of the plasma transferrin saturation and negate the concept that iron loading of transferrin in vitro is a selective process with possible functional consequences in tissue iron delivery.