Regulation, mechanisms and proposed function of ferritin translocation to cell nuclei

Identification of non-Alzheimer's disease tauopathies-related proteins by proteomic analysis

OBJECTIVES

To identify differentially expressed proteins between tauopathies cases and controls and to explore molecular mechanisms of tauopathies.

METHOD

Two-dimensional gel electrophoresis (2DE) was applied to separate the total proteins of temporal lobe obtained at autopsy from four tauopathies cases and four aged subjects without clinical or pathologic involvement of nervous system. The silver or Coomassie brilliant blue stained gels were analysed by 2-DE software Image Master 2D Elite. Selected differential protein spots were identified with MALDI-TOF/TOF tandem mass spectrometry.

RESULTS

Glyceraldehyde 3-phosphate dehydrogenase, uracil DNA glycosylase, human superoxide dismutase, isocitrate dehydrogenase subunit, synaptotagmin I, thioredoxin peroxidase 1, glial fibrillary acidic protein, P25 alpha, enoyl coenzyme A hydratase short chain 1, pyridoxine-5'-phosphate oxidase, Mn-superoxide dismutase and alpha enolase were significantly upregulated in tauopathies brains, whereas antioxidant protein 2, ferritin heavy chain, glutamate dehydrogenase precursor, peptidyl-prolyl cis-trans isomerase A, serum albumin precursor and dihydropyrimidinase-related protein 2 were lowly expressed in tauopathies brains.

CONCLUSIONS

We identified a number of tauopathy-related proteins that might be useful for discovering the molecular mechanisms of tauopathies, which could also be helpful for diagnosing and treating these disorders.

Heart protection by ischemic preconditioning: a novel pathway initiated by iron and mediated by ferritin

Ischemic preconditioning is a well-known procedure transiently protecting the heart against injury associated with prolonged ischemia, through mechanism/s only partly understood. The aim of this study was to test whether preconditioning-induced protection of the heart involves an iron-based mechanism, including the generation of an iron signal followed by accumulation of ferritin. In isolated rat hearts perfused in the Langendorff configuration, we measured heart contractility, ferritin levels, ferritin-iron content, and mRNA levels of ferritin subunits. Ischemic preconditioning caused rapid accumulation of ferritin, reaching 359% of the baseline value (set at 100%). This was accompanied by a parallel decline in ferritin-bound iron: from 2191+/-548 down to 760+/-34 Fe atoms/ferritin molecule, p<0.05. Ferritin levels remained high during the subsequent period of prolonged ischemia, and returned to nearly the baseline value during the reperfusion phase. Selective iron chelators (acetyl hydroxamate or Zn-desferrioxamine) abrogated the functional protection and suppressed ferritin accumulation, thus demonstrating the essentiality of an iron signal in the preconditioning-induced protective mechanism. Moreover, introduction of an iron-containing ternary complex, known to import iron into cells, caused a three-fold accumulation of ferritin and simulated the preconditioning-induced functional protection against prolonged myocardial ischemia. The ischemic preconditioning-and-ischemia-induced increase in ferritin levels correlated well with the accumulation of ferritin L-subunit mRNA: 5.44+/-0.47 vs 1.23+/-0.15 (units) in the baseline, p<0.05, suggesting that transcriptional control of ferritin L-subunit synthesis had been activated. Ischemic preconditioning initiates de novo synthesis of ferritin in the heart; the extra ferritin is proposed to serve a 'sink' for redox-active iron, thus protecting the heart from iron-mediated oxidative damage associated with ischemia-reperfusion injury. The present results substantiate a novel iron-based mechanism of ischemic preconditioning and could pave the way for the development of new modalities of heart protection.

Intracellular iron transport and storage: from molecular mechanisms to health implications

Maintenance of proper "labile iron" levels is a critical component in preserving homeostasis. Iron is a vital element that is a constituent of a number of important macromolecules, including those involved in energy production, respiration, DNA synthesis, and metabolism; however, excess "labile iron" is potentially detrimental to the cell or organism or both because of its propensity to participate in oxidation-reduction reactions that generate harmful free radicals. Because of this dual nature, elaborate systems tightly control the concentration of available iron. Perturbation of normal physiologic iron concentrations may be both a cause and a consequence of cellular damage and disease states. This review highlights the molecular mechanisms responsible for regulation of iron absorption, transport, and storage through the roles of key regulatory proteins, including ferroportin, hepcidin, ferritin, and frataxin. In addition, we present an overview of the relation between iron regulation and oxidative stress and we discuss the role of functional iron overload in the pathogenesis of hemochromatosis, neurodegeneration, and inflammation.

A comparative Mössbauer study of the mineral cores of human H-chain ferritin employing dioxygen and hydrogen peroxide as iron oxidants

Ferritins are ubiquitous iron storage and detoxification proteins distributed throughout the plant and animal kingdoms. Mammalian ferritins oxidize and accumulate iron as a ferrihydrite mineral within a shell-like protein cavity. Iron deposition utilizes both O(2) and H(2)O(2) as oxidants for Fe(2+) where oxidation can occur either at protein ferroxidase centers or directly on the surface of the growing mineral core. The present study was undertaken to determine whether the nature of the mineral core formed depends on the protein ferroxidase center versus mineral surface mechanism and on H(2)O(2) versus O(2) as the oxidant. The data reveal that similar cores are produced in all instances, suggesting that the structure of the core is thermodynamically, not kinetically controlled. Cores averaging 500 Fe/protein shell and diameter approximately 2.6 nm were prepared and exhibited superparamagnetic blocking temperatures of 19 and 22 K for the H(2)O(2) and O(2) oxidized samples, respectively. The observed blocking temperatures are consistent with the unexpectedly large effective anisotropy constant K(eff)=312 kJ/m(3) recently reported for ferrihydrite nanoparticles formed in reverse micelles [E.L. Duarte, R. Itri, E. Lima Jr., M.S. Batista, T.S. Berquó and G.F. Goya, Large Magnetic Anisotropy in ferrihydrite nanoparticles synthesized from reverse micelles, Nanotechnology 17 (2006) 5549-5555.]. All ferritin samples exhibited two magnetic phases present in nearly equal amounts and ascribed to iron spins at the surface and in the interior of the nanoparticle. At 4.2 K, the surface spins exhibit hyperfine fields, H(hf), of 436 and 445 kOe for the H(2)O(2) and O(2) samples, respectively. As expected, the spins in the interior of the core exhibit larger H(hf) values, i.e. 478 and 486 kOe for the H(2)O(2) and O(2) samples, respectively. The slightly smaller hyperfine field distribution DH(hf) for both surface (78 kOe vs. 92 kOe) and interior spins (45 kOe vs. 54 kOe) of the O(2) sample compared to the H(2)O(2) samples implies that the former is somewhat more crystalline.

Ferritin associates with marginal band microtubules

We characterized chicken erythrocyte and human platelet ferritin by biochemical studies and immunofluorescence. Erythrocyte ferritin was found to be a homopolymer of H-ferritin subunits, resistant to proteinase K digestion, heat stable, and contained iron. In mature chicken erythrocytes and human platelets, ferritin was localized at the marginal band, a ring-shaped peripheral microtubule bundle, and displayed properties of bona fide microtubule-associated proteins such as tau. Red blood cell ferritin association with the marginal band was confirmed by temperature-induced disassembly-reassembly of microtubules. During erythrocyte differentiation, ferritin co-localized with coalescing microtubules during marginal band formation. In addition, ferritin was found in the nuclei of mature erythrocytes, but was not detectable in those of bone marrow erythrocyte precursors. These results suggest that ferritin has a function in marginal band formation and possibly in protection of the marginal band from damaging effects of reactive oxygen species by sequestering iron in the mature erythrocyte. Moreover, our data suggest that ferritin and syncolin, a previously identified erythrocyte microtubule-associated protein, are identical. Nuclear ferritin might contribute to transcriptional silencing or, alternatively, constitute a ferritin reservoir.

Involvement of Hif-1 in desferrioxamine-induced invasion of glioblastoma cells

Glioblastoma multiforme are highly invasive brain tumors. Experimental approaches focus on unravelling the mechanisms of invasion, this being a major reason for the poor prognosis of these tumors. Our previous results hinted towards involvement of the iron metabolism in invasion. In this study, we examined the effect of iron depletion on the invasive phenotype of glioblastoma cells. Transwell Matrigel invasion assays were used to monitor iron-dependent invasion of human glioblastoma cell lines U373MG and DBTRG05MG. Intracellular iron concentrations were modulated by applying desferrioxamine (DFO) and ferric ammonium citrate (FAC). We detected enhanced invasion of glioblastoma cells upon DFO-induced iron depletion. Treatment of cells with FAC strongly inhibited invasion. DFO treatment resulted in hypoxia-inducible factor 1 (Hif-1)-mediated induction of urokinase plasminogen activator receptor and matrix metalloproteinase 2. Further, RNA interference-mediated repression of urokinase plasminogen activator receptor inhibited DFO-induced invasion. Our data demonstrate a direct effect of DFO on Hif-1 expression resulting in activation of factors associated with ECM degradation and invasion of glioma cells. These findings caution on utilization of DFO and other iron chelators in the treatment of tumors with invasive potential.

Abnormalities of the nucleus and nuclear inclusions in neurodegenerative disease: a work in progress

Neurodegenerative diseases are characterized pathologically by the abnormal accumulation of pathogenic protein species within the cell. Several neurodegenerative diseases feature intranuclear protein aggregation in the form of intranuclear inclusion bodies. Studies of these intranuclear inclusions are providing important clues regarding the cellular pathophysiology of these diseases, as exemplified by recent progress in defining the genetic basis of a subset of frontotemporal dementia cases. The precise role of intranuclear inclusion bodies in disease pathogenesis is currently a focus of debate. The present review provides an overview of the diverse family of neurodegenerative diseases in which nuclear inclusions form part of the neuropathological spectrum. In addition, current pathogenetic concepts relevant to these diseases will be reviewed and arguments for and against a protective role for intranuclear inclusions will be presented. The relationship of pathological intranuclear inclusions to functional intranuclear bodies will also be discussed. Finally, by analogy with pathological intranuclear inclusions, I will speculate on the possibility that intranuclear protein aggregation may represent a constitutive cellular protective mechanism occurring in neurons under physiological conditions.

Ferritin and ferritin isoforms I: Structure-function relationships, synthesis, degradation and secretion

Ferritin is the intracellular protein responsible for the sequestration, storage and release of iron. Ferritin can accumulate up to 4500 iron atoms as a ferrihydrite mineral in a protein shell and releases these iron atoms when there is an increase in the cell's need for bioavailable iron. The ferritin protein shell consists of 24 protein subunits of two types, the H-subunit and the L-subunit. These ferritin subunits perform different functions in the mineralization process of iron. The ferritin protein shell can exist as various combinations of these two subunit types, giving rise to heteropolymers or isoferritins. Isoferritins are functionally distinct and characteristic populations of isoferritins are found depending on the type of cell, the proliferation status of the cell and the presence of disease. The synthesis of ferritin is regulated both transcriptionally and translationally. Translation of ferritin subunit mRNA is increased or decreased, depending on the labile iron pool and is controlled by an iron-responsive element present in the 5'-untranslated region of the ferritin subunit mRNA. The transcription of the genes for the ferritin subunits is controlled by hormones and cytokines, which can result in a change in the pool of translatable mRNA. The levels of intracellular ferritin are determined by the balance between synthesis and degradation. Degradation of ferritin in the cytosol results in complete release of iron, while degradation in secondary lysosomes results in the formation of haemosiderin and protection against iron toxicity. The majority of ferritin is found in the cytosol. However, ferritin with slightly different properties can also be found in organelles such as nuclei and mitochondria. Most of the ferritin produced intracellularly is harnessed for the regulation of iron bioavailability; however, some of the ferritin is secreted and internalized by other cells. In addition to the regulation of iron bioavailability ferritin may contribute to the control of myelopoiesis and immunological responses.

Chemokine CXCL12 induces binding of ferritin heavy chain to the chemokine receptor CXCR4, alters CXCR4 signaling, and induces phosphorylation and nuclear translocation of ferritin heavy chain

Chemokine receptor-initiated signaling plays critical roles in cell differentiation, proliferation, and migration. However, the regulation of chemokine receptor signaling under physiological and pathological conditions is not fully understood. In the present study, we demonstrate that the CXC chemokine receptor 4 (CXCR4) formed a complex with ferritin heavy chain (FHC) in a ligand-dependent manner. Our in vitro binding assays revealed that purified FHC associated with both the glutathione S-transferase-conjugated N-terminal and C-terminal domains of CXCR4, thereby suggesting the presence of more than one FHC binding site in the protein sequence of CXCR4. Using confocal microscopy, we observed that stimulation with CXCL12, the receptor ligand, induced colocalization of the internalized CXCR4 with FHC into internal vesicles. Furthermore, after CXCL12 treatment, FHC underwent time-dependent nuclear translocation and phosphorylation at serine residues. By contrast, a mutant form of FHC in which serine 178 was replaced by alanine (S178A) failed to undergo phosphorylation, suggesting that serine 178 is the major phosphorylation site. Compared with the wild type FHC, the FHC-S178A mutant exhibited reduced association with CXCR4 and constitutive nuclear translocation. We also found that CXCR4-mediated extracellular signal-regulated kinase 1/2 (ERK1/2) activation and chemotaxis were inhibited by overexpression of wild type FHC but not FHC-S178A mutant, and were prolonged by FHC knockdown. In addition to CXCR4, other chemokine receptor-initiated signaling appeared to be similarly regulated by FHC, because CXCR2-mediated ERK1/2 activation was also inhibited by FHC overexpression and prolonged by FHC knockdown. Altogether, our data provide strong evidence for an important role of FHC in chemokine receptor signaling and receptor-mediated cell migration.

Ferritins: iron/oxygen biominerals in protein nanocages

Ferritin protein nanocages that form iron oxy biominerals in the central nanometer cavity are nature's answer to managing iron and oxygen; gene deletions are lethal in mammals and render bacteria more vulnerable to host release of antipathogen oxidants. The multifunctional, multisubunit proteins couple iron with oxygen (maxi-ferritins) or hydrogen peroxide (mini-ferritins) at catalytic sites that are related to di-iron sites oxidases, ribonucleotide reductase, methane monooxygenase and fatty acid desaturases, and synthesize mineral precursors. Gated pores, distributed symmetrically around the ferritin cages, control removal of iron by reductants and chelators. Gene regulation of ferritin, long known to depend on iron and, in animals, on a noncoding messenger RNA (mRNA) structure linked in a combinatorial array to functionally related mRNA of iron transport, has recently been shown to be linked to an array of proteins for antioxidant responses such as thioredoxin and quinone reductases. Ferritin DNA responds more to oxygen signals, and ferritin mRNA responds more to iron signals. Ferritin genes (DNA and RNA) and protein function at the intersection of iron and oxygen chemistry in biology.

Iron(II) and hydrogen peroxide detoxification by human H-chain ferritin. An EPR spin-trapping study

Overexpression of human H-chain ferritin (HuHF) is known to impart a degree of protection to cells against oxidative stress and the associated damage to DNA and other cellular components. However, whether this protective activity resides in the protein's ability to inhibit Fenton chemistry as found for Dps proteins has never been established. Such inhibition does not occur with the related mitochondrial ferritin which displays much of the same iron chemistry as HuHF, including an Fe(II)/H(2)O(2) oxidation stoichiometry of approximately 2:1. In the present study, the ability of HuHF to attenuate hydroxyl radical production by the Fenton reaction (Fe(2+) + H(2)O(2) --> Fe(3+) + OH(-) + *OH) was examined by electron paramagnetic resonance (EPR) spin-trapping methods. The data demonstrate that the presence of wild-type HuHF during Fe(2+) oxidation by H(2)O(2) greatly decreases the amount of .OH radical produced from Fenton chemistry whereas the ferroxidase site mutant 222 (H62K + H65G) and human L-chain ferritin (HuLF) lack this activity. HuHF catalyzes the pairwise oxidation of Fe(2+) by the detoxification reaction [2Fe(2+) + H(2)O(2) + 2H(2)O --> 2Fe(O)OH(core) + 4H(+)] that occurs at the ferroxidase site of the protein, thereby preventing the production of hydroxyl radical. The small amount of *OH radical that is produced in the presence of ferritin (<or=1% of the iron oxidized) appears to arise from the reaction of H(2)O(2) with Fe(III) in the protein rather than from simple Fenton chemistry. The results are discussed in terms of recent experiments reporting both protective and degradative effects of ferritin iron on the integrity of nuclear DNA.

Hemin-mediated regulation of an antioxidant-responsive element of the human ferritin H gene and role of Ref-1 during erythroid differentiation of K562 cells

An effective utilization of intracellular iron is a prerequisite for erythroid differentiation and hemoglobinization. Ferritin, consisting of 24 subunits of H and L, plays a crucial role in iron homeostasis. Here, we have found that the H subunit of the ferritin gene is activated at the transcriptional level during hemin-induced differentiation of K562 human erythroleukemic cells. Transfection of various 5' regions of the human ferritin H gene fused to a luciferase reporter into K562 cells demonstrated that hemin activates ferritin H transcription through an antioxidant-responsive element (ARE) that is responsible for induction of a battery of phase II detoxification genes by oxidative stress. Gel retardation and chromatin immunoprecipitation assays demonstrated that hemin induced binding of cJun, JunD, FosB, and Nrf2 b-zip transcription factors to AP1 motifs of the ferritin H ARE, despite no significant change in expression levels or nuclear localization of these transcription factors. A Gal4-luciferase reporter assay did not show activation of these b-zip transcription factors after hemin treatment; however, redox factor 1 (Ref-1), which increases DNA binding of Jun/Fos family members via reduction of a conserved cysteine in their DNA binding domains, showed induced nuclear translocation after hemin treatment in K562 cells. Consistently, Ref-1 enhanced Nrf2 binding to the ARE and ferritin H transcription. Hemin also activated ARE sequences of other phase II genes, such as GSTpi and NQO1. Collectively, these results suggest that hemin activates the transcription of the ferritin H gene during K562 erythroid differentiation by Ref-1-mediated activation of these b-zip transcription factors to the ARE.

Study of the localization of iron, ferritin, and hemosiderin in Alzheimer’s disease hippocampus by analytical microscopy at the subcellular level

Previous studies of the structure of core nanocrystals of ferritin (Ft) in the brains of patients with Alzheimer's disease (AD) have shown differences in the mineral compound in comparison with physiological Ft. Both Ft cores have a polyphasic composition but whereas the major phase in physiological Ft is hexagonal ferric iron oxide (ferrihydrite), the major phases in brain AD Ft are two cubic mixed ferric-ferrous iron oxides (magnetite and wüstite). One of these (wüstite) is similar to what is detected in hemosiderin (Hm) cores in primary hemochromatosis (Quintana, C., Cowley, J.M, Marhic, C., 2004. Electron nanodiffraction and high resolution electron microscopy studies of the structure and composition of physiological and pathological ferritin. J. Struct. Biol. 147, 166-178). We have studied, herein, the distribution of iron, Ft, and Hm in sections of AD hippocampus using analytical microscopy. Iron present in Ft cores was directly mapped in a nanoSIMS microscope and the iron distribution has been correlated with the constituent elements N, P, and S. Ft and Hm cores were visualized at an ultrastructural level in an analytical transmission electron microscope. In senile plaques, Ft was observed in the coronal region associated with a non-beta-amyloid component and in the periphery of plaques, together with Hm, in sulfur-rich dense bodies of dystrophic neurites. Hm was also found in lysosomes and siderosomes of glial cells. Ft was observed in the cytoplasm and nucleus of oligodendrocytes. Ft was particularly abundant in myelinated axons in association with oligodendrocyte processes. These findings provide new arguments to support the hypothesis of a dysfunction of Ft (with eventual degradation to Hm) in AD resulting in an increase of toxic brain ferrous ions that may contribute to the production of free radicals that induce both cellular oxidative stress and aged-related myelin breakdown associated with cognitive decline and AD (Bartzokis, G., 2004. Age-related myelin breakdown: a developmental model of cognitive decline and Alzheimer's disease. Neurobiol. Aging 25, 5-18).

Characterization of nuclear ferritin and mechanism of translocation

Ferritin, normally considered a cytoplasmic iron-storage protein, is also found in cell nuclei. It is an established fact that H-ferritin is the major form of nuclear ferritin, but little is known about the roles of ferritin in nuclei or about the mechanisms that control its appearance within the nuclear volume. In the present study, we show that, for human SW1088 astrocytoma cells, the nuclear and cytoplasmic forms of H-ferritin are products of the same mRNA. Histochemical and biochemical evidence is presented showing that ferritin is distributed non-randomly within the nuclear volume and that it preferentially associates with heterochromatin. Both cytoplasmic and nuclear populations of H-ferritin contain mixtures of non- and O-glycosylated forms, but the nuclear population is enriched in O-glycosylated forms. Cells treated with alloxan, a potent inhibitor of O-glycosylation, contained significantly less nuclear ferritin compared with cells grown in control media. Alloxan inhibited the reappearance of H-ferritin in nuclei of cells released from conditions of iron depletion, but did not prevent its disappearance from nuclei of cells undergoing iron depletion. These results suggest that O-glycosylation accompanies the transfer of ferritin from the cytoplasm to the nucleus, but does not influence the reverse process. The picture that emerges is one in which ferritin translocation between the cytoplasm and the nucleus is post-translationally regulated and responds to environmental and nutritional cues.

Brain slice invasion model reveals genes differentially regulated in glioma invasion

Invasion of tumor cells into adjacent brain areas is one of the major problems in treatment of glioma patients. To identify genes that might contribute to invasion, fluorescent F98 glioma cells were allowed to invade an organotypic brain slice. Gene expression analysis revealed 5 up-regulated and 14 down-regulated genes in invasive glioma cells as compared to non-invasive glioma cells. Two gene products, ferritin and cyclin B1, were verified in human gliomas by immunohistochemistry. Ferritin exhibited high mRNA levels in migratory F98 cells and also showed higher protein expression in the infiltrating edge of human gliomas. Cyclin B1 with high mRNA expression levels in stationary F98 cells showed marked protein expression in the central portions of gliomas. These findings are compatible with the concept of tumor cells either proliferating or migrating. Our study is the first to apply brain slice cultures for the identification of differentially regulated genes in glioma invasion.

Nuclear iron deposits in hepatocytes of iron-loaded HFE-knock-out mice: a morphometric and immunocytochemical analysis

Nuclear deposits of stainable iron in hepatocytes are a sign of liver iron overload in mice. Animals with no, partial or total knock-out of the HFE alleles, the deletion of which is responsible for hereditary haemochromatosis, were given different forms of dietary iron to measure nuclear iron deposits which were then related to cytoplasmic iron load. Wild type and heterozygous HFE-knock-out mice kept for 52 weeks on a standard diet showed no such deposits. These were, however, demonstrated in low numbers and with small diameters in homozygous HFE-knock-out mice kept on this diet. Nuclear iron deposits were most abundant in all type of mice fed carbonyl iron (2.5% w/w) for 52 weeks almost irrespective of their genetic background. The diameter of these deposits increased with the genetically conditioned extent of hepatocellular iron overload. Mice that were fed a diet containing TMH-ferrocene for 4 weeks showed amounts of hepatic iron that were comparable to those in the carbonyl iron-fed group but nuclear deposits were small and present in only 0.3% of the hepatocytes. While surrounding karyoplasm was immunostained for H- and L-ferritin, the nuclear iron deposits were not. As the nuclear iron deposits corresponded electron microscopically to aggregated ferritin molecules, they represent a non-immunoreactive form of presumably denatured ferritin.

Neurodegeneration Caused by Proteins with an Aberrant Carboxyl-Terminus

In recent years, 2 groups of hereditary neurodegenerative diseases have been recognized in which different genetic defects lead to the accumulation of proteins that contain a carboxyl-terminus that is abnormal in length and primary sequence. In this paper, we review the current knowledge on the molecular basis of diseases from these 2 groups. The first group includes familial British and Danish dementias, in which the molecular genetic defect resides in the BRI2 gene located on chromosome 13. In this group, carboxyl-terminal proteolytic products of the mutant BRI2 proteins aggregate in the extracellular space of the brain and in blood vessels. The second group includes 2 recently described ferritinopathies, in which the molecular genetic defect resides in the ferritin light polypeptide gene located on chromosome 19. In this group, full-length ferritin polypeptides aggregate intracellularly. The study of these conditions has led to the discovery of the BRI2 gene and to the finding of an unsuspected role for ferritin in neurodegeneration. These diseases provide new models in which to study the relationship between abnormal protein aggregation, neuronal cell death, and dementia.

Computer-identified nuclear localization signal in exon 1A of the transporter DMT1 is essentially ineffective in nuclear targeting

Divalent metal transporter 1 (DMT1; also called DCT1, Nramp2, or SLC11A2) has multiple isoforms that localize differently in many cell types. DMT1 +IRE species (encoded by mRNA with an iron-responsive element) are limited to the plasma membrane and cytosolic vesicles. In neural cells, -IRE isoforms of DMT1 (encoded by mRNA lacking an IRE) localize to the nucleus, plasma membrane, and cytosolic vesicles. In considering nuclear compartmentalization of -IRE isoforms, we hypothesized that the newly identified exon 1A in the N-terminus of this transporter might contain a nuclear localization signal. DNA constructs starting with exon 1A and ending with exons encoding alternative isoforms were made and transiently transfected into HEK293T and PC12 cells as well as rat sympathetic neurons. None of the constructs appeared in the nucleus despite the presence of exon 1A. Antibody specific for exon 1A was also used in both immunostaining and Western blots to investigate localization of exon 1A expressed both endogenously and ectopically in cells. Again, nuclear localization of DMT1 containing exon 1A was not observed. Our data suggest that exon 1A is neither sufficient nor necessary for DMT1 to appear in the nucleus.

Intracellular ferritin accumulation in neural and extraneural tissue characterizes a neurodegenerative disease associated with a mutation in the ferritin light polypeptide gene

Abnormal accumulation of ferritin was found to be associated with an autosomal dominant slowly progressing neurodegenerative disease clinically characterized by tremor, cerebellar ataxia, parkinsonism and pyramidal signs, behavioral disturbances, and cognitive decline. These symptoms may appear sequentially over a period of 4 decades. Pathologically, intranuclear and intracytoplasmic bodies were found in glia and subsets of neurons in the central nervous system as well as in extraneural tissue. Biochemical analyses of these bodies isolated from the striatum and cerebellar cortex revealed that ferritin light polypeptide (FTL) and ferritin heavy polypeptide (FTH1) were the main constituents. Molecular genetic studies revealed a 2-bp insertion mutation in exon 4 of the FTL gene. The resulting mutant polypeptide is predicted to have a carboxy terminus that is altered in amino-acid sequence and length. In tissue sections, the bodies were immunolabeled by anti-ferritin and anti-ubiquitin antibodies and were stained by Perls' method for ferric iron. Synthetic peptides homologous to the altered and wild-type carboxy termini were used to raise polyclonal antibodies. These novel antibodies as well as an antibody recognizing FTH1 immunolabeled the bodies. This study of this disorder has provided additional knowledge and insights in the growing area of ferritin-related neurodegeneration.

Nup88 mRNA overexpression is associated with high aggressiveness of breast cancer

The nuclear pore complex protein Nup88 is overexpressed in tumor cells. Immunohistochemical studies have shown that this overexpression is linked to higher aggressiveness of colorectal carcinoma and to enhanced metastatic potential of melanoma cells. However, the antibodies so far developed against Nup88 have the drawback of recognizing a number of other, up to now unspecified antigens besides Nup88. For this reason, we devised the present study on Nup88 expression at the mRNA level. RNA was extracted from fresh tumor tissue corresponding to 122 breast cancer patients. Nup88 mRNA expression was measured by means of differential RT-PCR, standardizing against a constitutive internal control gene (beta-actin). The results were dichotomized into "high" and "low" expression levels, using the median value as cut-off. High Nup88 mRNA expression levels correlated significantly with ductal and tubular histology (p = 0.012), histologic and nuclear grade 3 of tumors (p < 0.001), absence of hormone receptor expression (p < 0.001), expression of the c-erb-B2 oncogene (p < 0.001), expression of mutant p53 protein (p < 0.001), high proliferation (defined by Ki67 labeling index >20%, p < 0.001), DNA aneuploidy (p < 0.001) as well as the most important ominous clinical prognostic factor, axillary node invasion (p < 0.001). We also found an inverse correlation (p < 0.001) with expression of the H-MAM (mammaglobin) gene, a marker of low biologic and clinical aggressiveness of breast cancer. All of these factors, without exception, define a highly aggressive tumor phenotype. These findings appear to be specific to Nup88 and not to nuclear pore proteins in general. Indeed, analysis of Nup107 (which is a limiting component of the nuclear pore complex) under the same conditions in the same tumors did not yield comparable results.

Interactions and Reactions of Ferritin with DNA

Ferritin, normally considered a cytoplasmic iron-storage protein, is also found in the nuclei of some cells. There is no current agreement about its function(s) in this environment. Proposals include DNA protection, provision of iron to nuclear enzymes, and regulation of transcription initiation, but evidence for these functions is scanty. We have shown previously that H-ferritin subunits can be cross-linked to chromosomal DNA in vivo (Thompson, K. J., Fried, M. G., Ye, Z., Boyer, P., and Connor, J. R. (2002) J. Cell Sci. 115, 2165-2177). Here we describe systematic analyses of DNA binding and the covalent stability of DNA in the presence of ferritins from several different sources. Our data show that the H-subunit of human ferritin binds DNA, whereas neither the L-subunit nor the ferroxidase-deficient 222-mutant of the H-subunit has detectable binding activity. DNA binding is without significant preference for base composition, sequence, or the nature of DNA ends. H- and L-ferritins and ferritins of mixed subunit composition stimulate the conversion of superhelical plasmid DNA to the relaxed form. The sensitivity of this conversion to glycerol suggests that DNA is nicked by a free radical mechanism. The rate of nicking correlates with the iron content of the ferritin and is strongly inhibited by chelators. Ferritin-dependent nicking is characterized by a kinetic lag that is not seen in control reactions containing free iron species. These results suggest that the release of iron from ferritin is an important part of the nicking mechanism. The potential role of ferritin as a protector of the genome is discussed in the context of these results.

Structure of frataxin iron cores: an X-ray absorption spectroscopic study

X-ray absorption spectroscopy at the iron K-edge indicates that the iron cores of human and yeast frataxin polymers assembled in vitro are identical to each other and are similar but not identical to ferritin cores. Both frataxin polymers contain ferrihydrite, a biomineral composed of ferric oxide/hydroxide octahedra. The ferrihydrite in frataxin is less ordered than iron cores of horse spleen ferritin, having fewer face-sharing Fe-Fe interactions but similar double corner-sharing interactions. The extended X-ray absorption fine structure (EXAFS) analysis agrees with previous electron microscopy data showing that frataxin cores are composed of very small ferrihydrite crystallites.

Formation of nuclear matrix filaments by p27(BBP)/eIF6

p27(BBP)/eIF6 is an evolutionarily conserved protein necessary for ribosome biogenesis which was cloned in mammals for its ability to bind the cytodomain of beta 4 integrin. In cultured cells, a conspicuous fraction of p27(BBP)/eIF6 is associated with the intermediate filaments/nuclear matrix (IF/NM) cytoskeleton. The mechanism of this association is not known. Here we show that in epidermis p27(BBP)/eIF6 is naturally associated with IF/NM. To analyze the intrinsic capability of p27(BBP)/eIF6 to generate cytoskeletal networks, the properties of the pure, recombinant, untagged protein were studied. Recombinant p27(BBP)/eIF6 binds beta 4 integrin. Upon dialysis against IF buffer, p27(BBP)/eIF6 forms polymers which, strikingly, have a morphology identical to NM filaments. Cross-linking experiments suggested that polymerization is favored by the formation of disulphide bridges. These data suggest that p27(BBP)/eIF6 is associated with the cytoskeleton, and contributes to formation of NM filaments. These findings help to settle the controversy on nuclear matrix.