The ferritin content of human red blood cells during the replacement of embryonic cells by fetal cells

Coordinating responses to iron and oxygen stress with DNA and mRNA promoters: The ferritin story

Combinations of DNA antioxidant response element and mRNA iron responsive element regulate ferritin expression in animals in response to oxidant and iron stress, or normal developmental signals. Ferritins are protein nanocages, found in animals, plants, bacteria, and archaea, that convert iron and oxygen to ferric oxy biominerals in the protein central cavity; the mineral traps potentially toxic reactants and concentrates iron for the future synthesis of other iron/heme proteins. Regulatory signals and the nanocage gene products are the same throughout biology, but the genetic mechanisms, DNA versus DNA + mRNA, vary. The number of genes, temporal regulation, tissue distribution in multi-cellular organisms, and gene product size (maxi-ferritins have 24 subunits and mini-ferritins, or Dps proteins, have 12 subunits and are restricted to bacteria and archaea) suggest an overwhelming diversity and variability. However, common themes of regulation and function are described which indicate not only that the three-dimensional protein structure and the functions of the ferritins are conserved, but also that broad features of genetic regulation are conserved relative to organismal and/or community needs. The analysis illustrates the centrality of the ferritins to life with iron and oxygen and models how Nature harnesses potentially dangerous chemistry for biology.

Early embryonic lethality of H ferritin gene deletion in mice

Ferritin molecules play an important role in the control of intracellular iron distribution and in the constitution of long term iron stores. In vitro studies on recombinant ferritin subunits have shown that the ferroxidase activity associated with the H subunit is necessary for iron uptake by the ferritin molecule, whereas the L subunit facilitates iron core formation inside the protein shell. However, plant and bacterial ferritins have only a single type of subunit which probably fulfills both functions. To assess the biological significance of the ferroxidase activity associated with the H subunit, we disrupted the H ferritin gene (Fth) in mice by homologous recombination. Fth(+/-) mice are healthy, fertile, and do not differ significantly from their control littermates. However, Fth(-/-) embryos die between 3.5 and 9.5 days of development, suggesting that there is no functional redundancy between the two ferritin subunits and that, in the absence of H subunits, L ferritin homopolymers are not able to maintain iron in a bioavailable and nontoxic form. The pattern of expression of the wild type Fth gene in 9.5-day embryos is suggestive of an important function of the H ferritin gene in the heart.

The ferritins: molecular properties, iron storage function and cellular regulation

The iron storage protein, ferritin, plays a key role in iron metabolism. Its ability to sequester the element gives ferritin the dual functions of iron detoxification and iron reserve. The importance of these functions is emphasised by ferritin's ubiquitous distribution among living species. Ferritin's three-dimensional structure is highly conserved. All ferritins have 24 protein subunits arranged in 432 symmetry to give a hollow shell with an 80 A diameter cavity capable of storing up to 4500 Fe(III) atoms as an inorganic complex. Subunits are folded as 4-helix bundles each having a fifth short helix at roughly 60 degrees to the bundle axis. Structural features of ferritins from humans, horse, bullfrog and bacteria are described: all have essentially the same architecture in spite of large variations in primary structure (amino acid sequence identities can be as low as 14%) and the presence in some bacterial ferritins of haem groups. Ferritin molecules isolated from vertebrates are composed of two types of subunit (H and L), whereas those from plants and bacteria contain only H-type chains, where 'H-type' is associated with the presence of centres catalysing the oxidation of two Fe(II) atoms. The similarity between the dinuclear iron centres of ferritin H-chains and those of ribonucleotide reductase and other proteins suggests a possible wider evolutionary linkage. A great deal of research effort is now concentrated on two aspects of ferritin: its functional mechanisms and its regulation. These form the major part of the review. Steps in iron storage within ferritin molecules consist of Fe(II) oxidation, Fe(III) migration and the nucleation and growth of the iron core mineral. H-chains are important for Fe(II) oxidation and L-chains assist in core formation. Iron mobilisation, relevant to ferritin's role as iron reserve, is also discussed. Translational regulation of mammalian ferritin synthesis in response to iron and the apparent links between iron and citrate metabolism through a single molecule with dual function are described. The molecule, when binding a [4Fe-4S] cluster, is a functioning (cytoplasmic) aconitase. When cellular iron is low, loss of the [4Fe-4S] cluster allows the molecule to bind to the 5'-untranslated region (5'-UTR) of the ferritin m-RNA and thus to repress translation. In this form it is known as the iron regulatory protein (IRP) and the stem-loop RNA structure to which it binds is the iron regulatory element (IRE). IREs are found in the 3'-UTR of the transferrin receptor and in the 5'-UTR of erythroid aminolaevulinic acid synthase, enabling tight co-ordination between cellular iron uptake and the synthesis of ferritin and haem. Degradation of ferritin could potentially lead to an increase in toxicity due to uncontrolled release of iron. Degradation within membrane-encapsulated "secondary lysosomes' may avoid this problem and this seems to be the origin of another form of storage iron known as haemosiderin. However, in certain pathological states, massive deposits of "haemosiderin' are found which do not arise directly from ferritin breakdown. Understanding the numerous inter-relationships between the various intracellular iron complexes presents a major challenge.

Ferritin gene organization: differences between plants and animals suggest possible kingdom-specific selective constraints

Ferritin, a protein widespread in nature, concentrates iron approximately 10(11)-10(12)-fold above the solubility within a spherical shell of 24 subunits; it derives in plants and animals from a common ancestor (based on sequence) but displays a cytoplasmic location in animals compared to the plastid in contemporary plants. Ferritin gene regulation in plants and animals is altered by development, hormones, and excess iron; iron signals target DNA in plants but mRNA in animals. Evolution has thus conserved the two end points of ferritin gene expression, the physiological signals and the protein structure, while allowing some divergence of the genetic mechanisms. Comparison of ferritin gene organization in plants and animals, made possible by the cloning of a dicot (soybean) ferritin gene presented here and the recent cloning of two monocot (maize) ferritin genes, shows evolutionary divergence in ferritin gene organization between plants and animals but conservation among plants or among animals; divergence in the genetic mechanism for iron regulation is reflected by the absence in all three plant genes of the IRE, a highly conserved, noncoding sequence in vertebrate animal ferritin mRNA. In plant ferritin genes, the number of introns (n = 7) is higher than in animals (n = 3). Second, no intron positions are conserved when ferritin genes of plants and animals are compared, although all ferritin gene introns are in the coding region; within kingdoms, the intron positions in ferritin genes are conserved. Finally, secondary protein structure has no apparent relationship to intron/exon boundaries in plant ferritin genes, whereas in animal ferritin genes the correspondence is high. The structural differences in introns/exons among phylogenetically related ferritin coding sequences and the high conservation of the gene structure within plant or animal kingdoms of the gene structure within plant or animal kingdoms suggest that kingdom-specific functional constraints may exist to maintain a particular intron/exon pattern within ferritin genes. In the case of plants, where ferritin gene intron placement is unrelated to triplet codons or protein structure, and where ferritin is targeted to the plastid, the selection pressure on gene organization may relate to RNA function and plastid/nuclear signaling.

Changes in the amount of nonhaem iron in the plasma, whole body, and selected organs during the postlarval life of the lamprey Geotria australis

The concentration of plasma nonhaem iron and the concentration and weight of all nonhaem iron in the whole body and selected organs, together with its partitioning into ferritin and haemosiderin iron, have been measured during the metamorphosis and upstream spawning migration of the Southern Hemisphere lamprey Geotria australis. Some nonhaem iron was lost from the animal during metamorphosis. However, the concentration and weight of nonhaem iron in the liver rose sharply at this time, following its release from important storage sites in adipose tissue and the degradation of larval haemoglobins. The nephric fold of larval and metamorphosing stages contained over 40% of all nonhaem iron in the body at the commencement of metamorphosis. This was predominantly in the form of haemosiderin. While the rise in liver iron during the transition from larva to adult primarily reflected an increase in the weight of ferritin iron, the amount of iron stored as haemosiderin rose conspicuously towards the end of metamorphosis. The rise in ferritin iron in the liver was accompanied by a decrease in ferritin iron in the plasma, which implies that changes in the liver during metamorphosis result in a greater filtering of circulating ferritin. Such a process would account for the very much lower plasma nonhaem iron concentrations which characterise later adult stages. The weight of nonhaem iron increased markedly in the liver and adult opisthonephros and in the whole animal during the nontrophic upstream spawning migration. This was primarily due to a marked rise in ferritin which in turn could be related to the degradation of adult haemoglobins.(ABSTRACT TRUNCATED AT 250 WORDS)