A novel glycosylphosphatidylinositol-anchored form of ceruloplasmin is expressed by mammalian astrocytes

Ceruloplasmin, a Potential Therapeutic Agent for Alzheimer's Disease

AIMS

Ceruloplasmin (CP), a ferrous oxidase enzyme, plays an important role in regulating iron metabolism and redox reactions. Previous studies showed that CP deficiency contributes to Parkinson's disease by increasing iron accumulation and oxidative stress in the substantia nigra. However, the role of CP in Alzheimer's disease (AD) is unclear. We hypothesized that the lack of CP gene expression would affect the pathogenesis and damage of AD by promoting abnormal iron levels and oxidative stress.

RESULTS

AD mouse models were induced in CP knockout mouse either by injection of Aβ_25-35 into the lateral ventricle of the brain or transgenic APP expression. CP levels were decreased significantly in the hippocampus of AD patients, as well as Aβ-CP^+/+ and APP-CP^+/+ mice. Compared to control AD mice, CP gene deletion increased memory impairment and iron accumulation, which could be associated with elevated reactive oxygen species (ROS) levels and lead to cell apoptosis mediated through the Bcl-2/Bax and Erk/p38 signaling pathways in Aβ-CP^-/- and APP-CP^-/- mice. In contrast, the restoration of CP expression to CP^-/- mice through injection of an exogenous expression plasmid into the brain ventricle alleviated Aβ-induced neuronal damage in the hippocampus.

INNOVATION

CP alterations in iron contents were mediated through DMT1(-IRE) and changes in ROS levels, which in turn attenuated the progression of AD through the Erk/p38 and Bcl-2/Bax signaling pathways.

CONCLUSION

Our results show a protective role of CP in AD and suggest that regulating CP expression in the hippocampus may provide a new neuroprotective strategy for AD. Antioxid. Redox Signal. 28, 1323-1337.

Multi-Copper Ferroxidase-Deficient Mice Have Increased Brain Iron Concentrations and Learning and Memory Deficits

Background

The accumulation of iron occurs in the central nervous system (CNS) in several neurodegenerative diseases. Although multi-copper ferroxidases (MCFs) play an important role in cellular iron metabolism and homeostasis, the mechanism of MCFs in the CNS remains unclear.

Objective

The aim was to study the role of MCFs in CNS iron metabolism and homeostasis by using hephaestin/ceruloplasmin (Heph/Cp) double knockout (KO) mice.

Methods

Heph/Cp double KO male mice were generated by crossing both single KO mice. In Heph/Cp KO and wild-type (WT) control mice at 4 wk and 6 mo of age, iron concentrations of selected brain regions were measured by atomic absorption spectrophotometry, and gene expressions of Heph, Cp, ferroportin 1 (Fpn1) [+ iron responsive element (IRE)], L-ferritin, H-ferritin, transferrin receptor 1 (Tfrc), and divalent metal transporter 1 (Dmt1) (+IRE) were quantitated by quantitative reverse transcriptase-polymerase chain reaction. Brain region L-ferritin protein concentration, superoxide dismutase (SOD), and glutathione peroxidase (GPx) activities and malondialdehyde (MDA) concentration were also determined. Learning and memory abilities in Heph/Cp KO and WT control mice at 6 mo of age were tested by the IntelliCage system (New Behavior).

Results

Iron concentration was significantly higher in Heph/Cp KO mice than in WT control mice at 4 wk of age in the cortex (50%), hippocampus (120%), brainstem (35%), and cerebellum (220%) and at 6 mo of age in the cortex (140%), hippocampus (420%), brainstem (560%), and cerebellum (340%). L-Ferritin and MDA concentrations were significantly higher and SOD and GPx activities were significantly lower in the cortex, hippocampus, brainstem, and cerebellum of KO mice than in those of WT controls at both 4 wk and 6 mo of age. Iron-related gene expressions also differed significantly between groups. Learning and memory deficits occurred in Heph/Cp KO mice at 6 mo of age.

Conclusion

Mutation of both MCFs in mice induces iron accumulation in brain regions, oxidative damage, and learning and memory defects.

The ferroportin-ceruloplasmin system and the mammalian iron homeostasis machine: regulatory pathways and the role of lactoferrin

In the last 20 years, several new genes and proteins involved in iron metabolism in eukaryotes, particularly related to pathological states both in animal models and in humans have been identified, and we are now starting to unveil at the molecular level the mechanisms of iron absorption, the regulation of iron transport and the homeostatic balancing processes. In this review, we will briefly outline the general scheme of iron metabolism in humans and then focus our attention on the cellular iron export system formed by the permease ferroportin and the ferroxidase ceruloplasmin. We will finally summarize data on the role of the iron binding protein lactoferrin on the regulation of the ferroportin/ceruloplasmin couple and of other proteins involved in iron homeostasis in inflamed human macrophages.

Increased ceruloplasmin expression caused by infiltrated leukocytes, activated microglia, and astrocytes in injured female rat spinal cords

Ceruloplasmin (Cp), an enzyme containing six copper atoms, has important roles in iron homeostasis and antioxidant defense. After spinal cord injury (SCI), the cellular components in the local microenvironment are very complex and include functional changes of resident cells and the infiltration of leukocytes. It has been confirmed that Cp is elevated primarily in astrocytes and to a lesser extent in macrophages following SCI in mice. However, its expression in other cell types is still not very clear. In this manuscript, we provide a sensible extension of these findings by examining this system within a female Sprague-Dawley rat model and expanding the scope of inquiry to include additional cell types. Quantitative reverse transcription polymerase chain reaction and Western blot analysis revealed that the Cp mRNA and protein in SCI tissue homogenates were quite consistent with prior publications. However, we observed that Cp was expressed not only in GFAP⁺ astrocytes (consistent with prior reports) but also in CD11b⁺ microglia, CNPase⁺ oligodendrocytes, NeuN⁺ neurons, CD45⁺ leukocytes, and CD68⁺ activated microglia/macrophages. Quantitative analysis proved that infiltrated leukocytes, activated microglia/macrophages, and astrocytes should be the major sources of increased Cp.

New Progress on the Role of Glia in Iron Metabolism and Iron-Induced Degeneration of Dopamine Neurons in Parkinson's Disease

It is now increasingly appreciated that glial cells play a critical role in the regulation of iron homeostasis. Impairment of these properties might lead to dysfunction of iron metabolism and neurodegeneration of neurons. We have previously shown that dysfunction of glia could cause iron deposit and enhance iron-induced degeneration of dopamine (DA) neurons in Parkinson’s disease (PD). There also has been a substantial growth of knowledge regarding the iron metabolism of glia and their effects on iron accumulation and degeneration of DA neurons in PD in recent years. Here, we attempt to describe the role of iron metabolism of glia and the effect of glia on iron accumulation and degeneration of DA neurons in the substantia nigra of PD. This could provide evidence to reveal the mechanisms underlying nigral iron accumulation of DA neurons in PD and provide the basis for discovering new potential therapeutic targets for PD.

Ceruloplasmin (2-D PAGE) Pattern and Copper Content in Serum and Brain of Alzheimer Disease Patients

A dysfunction in copper homeostasis seems to occur in Alzheimer's disease (AD). We previously evidenced that an excess of non-ceruloplasmin-copper (NCC) correlated with the main functional, anatomical as well as cerebrospinal markers of the disease. Aim of our study was to investigate ceruloplasmin isoforms as potential actors in this AD copper dysfunction. Our data show that AD patients have ceruloplasmin fragments of low molecular weight (<50 kDa) both in their serum and brain, contrary to healthy controls. Ceruloplasmin isoforms of higher molecular weight (115 and 135 kDa in serum and 135 kDa in brain), as well as copper levels in the brain, instead, do not seem to mark a difference between AD and healthy subjects. These data suggest a ceruloplasmin fragmentation in the serum of AD patients. Some clues in this direction have been found also in the AD brain.

Ceruloplasmin gene expression profile changes in the rat mammary gland during pregnancy, lactation and involution

Copper metabolism disturbances in mammary gland (MG) cells have severe consequences in newborns. The mechanism that controls the balance of copper in the MG has not been thoroughly characterized. Four primary copper homeostasis genes in mammals: (1) ceruloplasmin (Cp) encoding multifunction multicopper blue (ferr)oxidase; (2) CTR1 encoding high affinity copper importer 1; and (3 and 4) two similar genes encoding Cu(I)/Cu(II)-ATPases P1 type (ATP7A and ATP7B) responsible for copper efflux from the cells and metallation of cuproenzymes formed in the Golgi complex are expressed in MG. This study aimed to characterize expression of these genes during pregnancy, lactation and forced involution in the rat MG. We found that Cp anchored to the plasma membrane and ATP7A were expressed during pregnancy and lactation. Soluble Cp and ATP7B were highly expressed in lactating MG decreasing to its ending. CTR1 activity increased during MG growth and reached its maximum at postpartum and then it decreased until the end of lactation. During early forced MG involution, Cp gene expression persisted; while a form of Cp that lacked exon 18 appeared. We suggest that Cp gene expressional changes at the transcriptional and posttranscriptional level reflect various physiological functions of Cp proteins during MG remodeling.

Pathogenic implications of distinct patterns of iron and zinc in chronic MS lesions

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS) in which oligodendrocytes, the CNS cells that stain most robustly for iron and myelin are the targets of injury. Metals are essential for normal CNS functioning, and metal imbalances have been linked to demyelination and neurodegeneration. Using a multidisciplinary approach involving synchrotron techniques, iron histochemistry and immunohistochemistry, we compared the distribution and quantification of iron and zinc in MS lesions to the surrounding normal appearing and periplaque white matter, and assessed the involvement of these metals in MS lesion pathogenesis. We found that the distribution of iron and zinc is heterogeneous in MS plaques, and with few remarkable exceptions they do not accumulate in chronic MS lesions. We show that brain iron tends to decrease with increasing age and disease duration of MS patients; reactive astrocytes organized in large astrogliotic areas in a subset of smoldering and inactive plaques accumulate iron and safely store it in ferritin; a subset of smoldering lesions do not contain a rim of iron-loaded macrophages/microglia; and the iron content of shadow plaques varies with the stage of remyelination. Zinc in MS lesions was generally decreased, paralleling myelin loss. Iron accumulates concentrically in a subset of chronic inactive lesions suggesting that not all iron rims around MS lesions equate with smoldering plaques. Upon degeneration of iron-loaded microglia/macrophages, astrocytes may form an additional protective barrier that may prevent iron-induced oxidative damage.

Oxidation of Iron under Physiologically Relevant Conditions in Biological Fluids from Healthy and Alzheimer's Disease Subjects

Ferroxidase activity has been reported to be altered in various biological fluids in neurodegenerative disease, but the sources contributing to the altered activity are uncertain. Here we assay fractions of serum and cerebrospinal fluid with a newly validated triplex ferroxidase assay. Our data indicate that while ceruloplasmin, a multicopper ferroxidase, is the predominant source of serum activity, activity in CSF predominantly derives from a <10 kDa component, specifically from polyanions such as citrate and phosphate. We confirm that in human biological samples, ceruloplasmin activity in serum is decreased in Alzheimer's disease, but in CSF a reduction of activity in Alzheimer's disease originates from the polyanion component.

Copper chelation and interleukin-6 proinflammatory cytokine effects on expression of different proteins involved in iron metabolism in HepG2 cell line

Background

In vertebrates, there is an intimate relationship between copper and iron homeostasis. Copper deficiency, which leads to a defect in ceruloplasmin enzymatic activity, has a strong effect on iron homeostasis resulting in cellular iron retention. Much is known about the mechanisms underlying cellular iron retention under “normal” conditions, however, less is known about the effect of copper deficiency during inflammation.

Results

We show that copper deficiency and the inflammatory cytokine interleukin-6 have different effects on the expression of proteins involved in iron and copper metabolism such as the soluble and glycosylphosphtidylinositol anchored forms of ceruloplasmin, hepcidin, ferroportin1, transferrin receptor1, divalent metal transporter1 and H-ferritin subunit. We demonstrate, using the human HepG2 cell line, that in addition to ceruloplasmin isoforms, copper deficiency affects other proteins, some posttranslationally and some at the transcriptional level. The addition of interleukin-6, moreover, has different effects on expression of ferroportin1 and ceruloplasmin, in which ferroportin1 is decreased while ceruloplasmin is increased. These effects are stronger when a copper chelating agent and IL-6 are used simultaneously.

Conclusions

These results suggest that copper chelation has effects not only on ceruloplasmin but also on other proteins involved in iron metabolism, sometimes at the mRNA level and, in inflammatory conditions, the functions of ferroportin and ceruloplasmin may be independent.

Molecular and Cellular Bases of Iron Metabolism in Humans

Iron is a microelement with the most completely studied biological functions. Its wide dissemination in nature and involvement in key metabolic pathways determine the great importance of this metal for uni- and multicellular organisms. The biological role of iron is characterized by its indispensability in cell respiration and various biochemical processes providing normal functioning of cells and organs of the human body. Iron also plays an important role in the generation of free radicals, which under different conditions can be useful or damaging to biomolecules and cells. In the literature, there are many reviews devoted to iron metabolism and its regulation in pro- and eukaryotes. Significant progress has been achieved recently in understanding molecular bases of iron metabolism. The purpose of this review is to systematize available data on mechanisms of iron assimilation, distribution, and elimination from the human body, as well as on its biological importance and on the major iron-containing proteins. The review summarizes recent ideas about iron metabolism. Special attention is paid to mechanisms of iron absorption in the small intestine and to interrelationships of cellular and extracellular pools of this metal in the human body.

Orchestration of dynamic copper navigation – new and missing pieces

A general principle in all cells in the body is that an essential metal – here copper – is taken up at the plasma membrane, directed through cellular compartments for use in specific enzymes and pathways, stored in specific scavenging molecules if in surplus, and finally expelled from the cells.

Hephaestin and ceruloplasmin facilitate iron metabolism in the mouse kidney

Multicopper ferroxidases (MCFs) play an important role in cellular iron homeostasis. However, the role of MCFs in renal metabolism remains unclear. We used Hephaestin (Heph) and Ceruloplasmin (Cp) single or double (Heph/Cp) knockout (KO) mice to study the roles of MCFs in the kidney. Renal iron levels and the expression of iron metabolism genes were examined. The non-heme iron content both in the renal cortex and medulla of Heph/Cp KO mice was significantly increased. Perls' Prussian blue staining showed iron accumulation on the apical side of renal tubular cells in Heph/Cp KO mice. A significant increase in ferritin protein expression was also observed in the renal medulla and cortex of Heph/Cp KO mice. Both DMT1 and TfR1 protein expression were significantly decreased in the renal medulla of Heph/Cp KO mice, while the expression of DMT1 protein was significantly increased in the renal cortex of these animals. Significant increase in proteinuria and total urinary iron was observed in the double knockout mice, and this was associated with compromised structural integrity. These results suggest that KO of both the HEPH and CP genes leads to kidney iron deposition and toxicity, MCFs could protect kidney against a damage from iron excess.

Iron uptake and transport across physiological barriers

Iron is an essential element for human development. It is a major requirement for cellular processes such as oxygen transport, energy metabolism, neurotransmitter synthesis, and myelin synthesis. Despite its crucial role in these processes, iron in the ferric form can also produce toxic reactive oxygen species. The duality of iron’s function highlights the importance of maintaining a strict balance of iron levels in the body. As a result, organisms have developed elegant mechanisms of iron uptake, transport, and storage. This review will focus on the mechanisms that have evolved at physiological barriers, such as the intestine, the placenta, and the blood–brain barrier (BBB), where iron must be transported. Much has been written about the processes for iron transport across the intestine and the placenta, but less is known about iron transport mechanisms at the BBB. In this review, we compare the established pathways at the intestine and the placenta as well as describe what is currently known about iron transport at the BBB and how brain iron uptake correlates with processes at these other physiological barriers.

Ceruloplasmin is regulated by copper and lactational hormones in PMC42-LA mammary epithelial cell culture models

Ceruloplasmin (Cp) is a multicopper ferroxidase that is considered to be an important source of copper in milk for normal neonatal development. We investigated the expression, subcellular localization and secretion of Cp in PMC42-LA cell culture models representative of resting, lactating and suckled human mammary epithelia. Both secreted Cp (sCp) and plasma membrane associated glycosylphosphatidylinositol-linked Cp (GPI-Cp) were expressed in PMC42-LA cells. In all three epithelial models (resting, lactating and suckled), the expression and secretion of copper-bound, ferroxidase active, Cp (holo-Cp) was dependent on media copper concentration. In low copper (bathocuproinedisulphonic acid/d-penicillamine treated models) there was greater than a 2-fold decrease in holo-Cp expression and secretion, which was mirrored by a 2-fold increase in the expression and secretion of copper-free Cp protein (apo-Cp). Cell surface biotinylation studies revealed that the state of PMC42-LA cell differentiation (functionality), and the level of extracellular copper, had no significant effect on the level of plasma membrane bound GPI-Cp. Quantitative real time PCR analyses determined that there was no significant (P > 0.05) difference in Cp mRNA levels across all copper conditions investigated (0, 5, 50 μM). However, there was a significant (P < 0.05) increase (∼2-fold) in Cp mRNA in both the lactating and suckled models in comparison to the resting model. Furthermore, the Cp mRNA increase in response to PMC42-LA differentiation corresponded with more secreted Cp protein, both apo and holo forms, indicating a link between function and Cp requirement. Our results provide significant insight on the regulation of Cp expression and secretion in lactation and copper incorporation into milk.

Ceruloplasmin and other copper binding components of blood plasma and their functions: an update

We know that blood plasma contains many proteins and also other components that bind copper. The largest contributor to copper in the plasma is ceruloplasmin, which accounts for 40-70 percent. Apart from ceruloplasmin and albumin, most of these components have not been studied extensively, and even for ceruloplasmin and albumin, much remains to be discovered. New components with new functions, and new functions of known components are emerging, some warranting reconsideration of earlier findings. The author's laboratory has been actively involved in research on this topic. This review summarizes and updates our knowledge of the nature and functions of ceruloplasmin and the other known and emerging copper-containing molecules (principally proteins) in this fluid, to better understand how they contribute to copper homeostasis and consider their potential significance to health and disease.

Iron in the Brain: An Important Contributor in Normal and Diseased States

Iron is essential for normal neurological function because of its role in oxidative metabolism and because it is a cofactor in the synthesis of neurotransmitters and myelin. In the past several years, there has been increased attention to the importance of oxidative stress in the central nervous system. Iron is the most important inducer of reactive oxygen species, therefore, the relation of iron to neurodegenerative processes is more appreciated today than it was a few years ago. Nevertheless, despite this increased attention and awareness, our knowledge of iron metabolism in the brain at the cellular and molecular levels is still limited. Iron is distributed in a heterogeneous fashion among the different regions and cells of the brain. This regional and cellular heterogeneity is preserved across many species. Brain iron concentrations are not static; they increase with age and in many diseases and decrease when iron is deficient in the diet. In infants and children, insufficient iron in the diet is associated with decreased brain iron and with changes in behavior and cognitive functioning. Abnormal iron accumulation in the diseased brain areas and, in some cases, alterations in iron-related proteins have been reported in many neurodegenerative diseases, including Hallervorden-Spatz syndrome, Alzheimer’s disease, Parkinson’s disease, and Friedreich’s ataxia. There is strong evidence for iron-mediated oxidative damage as a primary contributor to cell death in these disorders. Demyelinating diseases, such as multiple sclerosis, especially warrant study in relation to iron availability. Myelin synthesis and maintenance have a high iron requirement, thus, oligodendrocytes must have a relatively high and constant supply of iron. However, the high oxygen utilization, high density of lipids, and high iron content of white matter all combine to increase the risk of oxidative damage. We review here the current knowledge of the normal metabolism of iron in the brain and the suspected role of iron in neuropathology.

Copper Homeostasis as a Therapeutic Target in Amyotrophic Lateral Sclerosis with SOD1 Mutations

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disease affecting both upper and lower motor neurons, and currently, there is no cure or effective treatment. Mutations in a gene encoding a ubiquitous antioxidant enzyme, Cu,Zn-superoxide dismutase (SOD1), have been first identified as a cause of familial forms of ALS. It is widely accepted that mutant SOD1 proteins cause the disease through a gain in toxicity but not through a loss of its physiological function. SOD1 is a major copper-binding protein and regulates copper homeostasis in the cell; therefore, a toxicity of mutant SOD1 could arise from the disruption of copper homeostasis. In this review, we will briefly review recent studies implying roles of copper homeostasis in the pathogenesis of SOD1-ALS and highlight the therapeutic interventions focusing on pharmacological as well as genetic regulations of copper homeostasis to modify the pathological process in SOD1-ALS.

Mottled Mice and Non-Mammalian Models of Menkes Disease

Menkes disease is a multi-systemic copper metabolism disorder caused by mutations in the X-linked ATP7A gene and characterized by progressive neurodegeneration and severe connective tissue defects. The ATP7A protein is a copper (Cu)-transporting ATPase expressed in all tissues and plays a critical role in the maintenance of copper homeostasis in cells of the whole body. ATP7A participates in copper absorption in the small intestine and in copper transport to the central nervous system (CNS) across the blood-brain-barrier (BBB) and blood–cerebrospinal fluid barrier (BCSFB). Cu is essential for synaptogenesis and axonal development. In cells, ATP7A participates in the incorporation of copper into Cu-dependent enzymes during the course of its maturation in the secretory pathway. There is a high degree of homology (>80%) between the human ATP7A and murine Atp7a genes. Mice with mutations in the Atp7a gene, called mottled mutants, are well-established and excellent models of Menkes disease. Mottled mutants closely recapitulate the Menkes phenotype and are invaluable for studying Cu-metabolism. They provide useful models for exploring and testing new forms of therapy in Menkes disease. Recently, non-mammalian models of Menkes disease, Drosophila melanogaster and Danio rerio mutants were used in experiments which would be technically difficult to carry out in mammals.

Altered metal metabolism in patients with HCV-related cirrhosis and hepatic encephalopathy

Dysfunctional metal homeostasis contributes to oxidative stress and neuronal damage. These have been implicated in hepatic encephalopathy pathogenesis. To investigate whether altered metal metabolism is associated with hepatic encephalopathy. Twenty-one controls and 34 HCV-cirrhotic patients (ENC/NEC patients according to presence/absence of previous overt episodes of hepatic encephalopathy) and a control group were studied. Serum iron, copper, ceruloplasmin, ceruloplasmin activity, transferrin, and ceruloplasmin/transferrin ratio were determined. Neuropsychological tests were performed by the repeatable battery of neuropsychological status. Magnetic resonance assessed basal ganglia volumes and metal deposition (pallidal index and T2*). Cirrhotic patients performed worse than controls at cognitive tests, especially ENC patients,. At biochemical analysis copper concentrations, ceruloplasmin activity and transferrin levels were lower in ENC than in NEC patients and controls (p < 0.05 and p < 0.01, respectively). Ceruloplasmin/transferrin ratio was higher in ENC compared to NEC patients (p < 0.05), and controls (p < 0.01). By brain magnetic resonance, ENC patients showed reduced caudate and globus pallidus volumes compared to controls (p < 0.05), and ENC and NEC patients an increased pallidal index compared to controls (p < 0.01). In ENC patients, ceruloplasmin activity correlated with caudate volume and pallidal index (ρ = 0.773 and ρ = -0.683, p < 0.05). Altered metal metabolism likely contributes to cirrhotic hepatic encephalopathy.

Cp/Heph mutant mice have iron-induced neurodegeneration diminished by deferiprone

Brain iron accumulates in several neurodegenerative diseases and can cause oxidative damage, but mechanisms of brain iron homeostasis are incompletely understood. Patients with mutations in the cellular iron-exporting ferroxidase ceruloplasmin (Cp) have brain iron accumulation causing neurodegeneration. Here, we assessed the brains of mice with combined mutation of Cp and its homolog hephaestin. Compared to single mutants, brain iron accumulation was accelerated in double mutants in the cerebellum, substantia nigra, and hippocampus. Iron accumulated within glia, while neurons were iron deficient. There was loss of both neurons and glia. Mice developed ataxia and tremor, and most died by 9 months. Treatment with the oral iron chelator deferiprone diminished brain iron levels, protected against neuron loss, and extended lifespan. Ferroxidases play important, partially overlapping roles in brain iron homeostasis by facilitating iron export from glia, making iron available to neurons. Above: Iron (Fe) normally moves from capillaries to glia to neurons. It is exported from the glia by ferroportin (Fpn) with ferroxidases ceruloplasmin (Cp) and/or Hephaestin (Heph). Below: In mice with mutation of Cp and Heph, iron accumulates in glia, while neurons have low iron levels. Both neurons and glia degenerate and mice become ataxic unless given an iron chelator.

Ceruloplasmin is Involved in the Nigral Iron Accumulation of 6-OHDA-Lesioned Rats

Elevated iron levels in the substantia nigra (SN) participate in neuronal death in Parkinson's disease (PD), while the mechanisms underlying the increased iron are still unknown. Ceruloplasmin (CP), a ferroxidase, converts highly toxic ferrous iron to its non-toxic ferric form, which cooperated with ferroportin1 (FP1) facilitating the export of iron from cells. To elucidate if the abnormal expression of CP is involved in the nigral iron accumulation, here, we investigated CP expression in the SN of rats lesioned by 6-hydroxydopamine (6-OHDA). We showed that FP1 and CP colocalized in the rat SN. One day after 6-OHDA lesion, when there was a half reduction in the number of dopaminergic neurons, the iron level was increased compared with the normal rats; both the mRNA and protein expressions of CP decreased compared with the control. When rats began showing rotation behavior induced by apomorphine, usually after 6 weeks since 6-OHDA lesion, they are considered PD models. In these PD models, almost no dopaminergic neurons can be detected in the lesioned SN and nigral iron level was further increased. At this time point, a further decrease of CP was observed. These results show that FP1 and CP colocalize in the rat brain, indicating the coordinated actions of the two proteins in the cellular iron export, and suggest that decreased expression of CP in the SN is involved in the nigral iron accumulation of 6-OHDA-lesioned rats.

Copper at the Fungal Pathogen-Host Axis

Fungal infections are responsible for millions of human deaths annually. Copper, an essential but toxic trace element, plays an important role at the host-pathogen axis during infection. In this review, we describe how the host uses either Cu compartmentalization within innate immune cells or Cu sequestration in other infected host niches such as in the brain to combat fungal infections. We explore Cu toxicity mechanisms and the Cu homeostasis machinery that fungal pathogens bring into play to succeed in establishing an infection. Finally, we address recent approaches that manipulate Cu-dependent processes at the host-pathogen axis for antifungal drug development.

Hephaestin and ceruloplasmin play distinct but interrelated roles in iron homeostasis in mouse brain

BACKGROUND

Iron accumulation in the central nervous system (CNS) is a common feature of many neurodegenerative diseases. Multicopper ferroxidases (MCFs) play an important role in cellular iron metabolism. However, the role of MCFs in the CNS in health and disease remains poorly characterized.

OBJECTIVE

The aim was to study the role of hephaestin (HEPH) and ceruloplasmin (CP) in CNS iron metabolism and homeostasis.

METHODS

Iron concentrations and L-ferritin protein levels of selected brain regions were determined in global hephaestin knockout (Heph KO), global ceruloplasmin knockout (Cp KO), and wild-type (WT) male mice at 6-7 mo of age. Gene expression of divalent metal transporter 1 (Dmt1), ferroportin 1 (Fpn1), Heph, Cp, and transferrin receptor 1 (Tfrc) and HEPH protein level was quantitated in the same brain regions.

RESULTS

Iron and L-ferritin protein levels were significantly increased in Heph KO mouse brain cortex (iron: 30%, P < 0.05; L-ferritin: 200%, P < 0.05), hippocampus (iron: 80%, P < 0.05; L-ferritin: 300%, P < 0.05), brainstem (iron: 20%, P < 0.05; L-ferritin: 150%, P < 0.05), and cerebellum (iron: 20%, P < 0.05; L-ferritin: 100%, P < 0.05) regions than in WT and Cp KO mouse brain regions at 6 mo of age. Expression of the Heph gene was significantly increased in the Cp KO mouse cortex (100%; P < 0.01), hippocampus (350%; P < 0.001), brainstem (30%; P < 0.01), and cerebellum (150%; P < 0.001) than in WT controls, and Cp gene expression was significantly decreased in the Heph KO mouse hippocampus (20%; P < 0.05) than in WT control mice at 6 mo of age.

CONCLUSIONS

Ablation of HEPH or CP results in disordered brain iron homeostasis in mice. Heph KO may provide a novel model for neurodegenerative disorders.

Inductively coupled mass spectrometry analysis of biometals in conditional Hamp1 and Hamp1 and Hamp2 transgenic mouse models

Hepcidin, a circulatory antimicrobial peptide, is involved in iron homeostasis, inflammation, infection and metabolic signals. Humans express one hepcidin gene, HAMP but mice express two hepcidin genes, Hamp1 and Hamp2. Consecutive gene targeting events were performed to produce transgenic mice expressing conditional alleles of either Hamp1 or both Hamp1 and Hamp2 (Hamp1/2). The deletion of Hamp1 alleles elevated Hamp2 expression, particularly in males, which was reduced by endotoxin treatment. The tissue levels of iron and other biometals were quantified by inductively coupled mass spectrometry. The ubiquitous or liver-specific deletion of Hamp1 alleles yielded similar quantitative changes in iron levels in the liver, duodenum, spleen, kidney, heart and brain. The introduction of Hamp2 null allele did not exacerbate the iron-related phenotype of Hamp1 null allele. Besides iron, Hamp1 null allele significantly elevated the levels of selenium in the liver, manganese in the liver and duodenum, and copper in the brain. Mice with conditional Hamp alleles will be useful to determine the tissue-specific regulation and functions of Hamp1 and Hamp2 in biometal homeostasis and other biological processes.

Iron transport across the blood-brain barrier: development, neurovascular regulation and cerebral amyloid angiopathy

There are two barriers for iron entry into the brain: (1) the brain-cerebrospinal fluid (CSF) barrier and (2) the blood-brain barrier (BBB). Here, we review the literature on developmental iron accumulation by the brain, focusing on the transport of iron through the brain microvascular endothelial cells (BMVEC) of the BBB. We review the iron trafficking proteins which may be involved in the iron flux across BMVEC and discuss the plausible mechanisms of BMVEC iron uptake and efflux. We suggest a model for how BMVEC iron uptake and efflux are regulated and a mechanism by which the majority of iron is trafficked across the developing BBB under the direct guidance of neighboring astrocytes. Thus, we place brain iron uptake in the context of the neurovascular unit of the adult brain. Last, we propose that BMVEC iron is involved in the aggregation of amyloid-β peptides leading to the progression of cerebral amyloid angiopathy which often occurs prior to dementia and the onset of Alzheimer's disease.

Haemolysis and perturbations in the systemic iron metabolism of suckling, copper-deficient mosaic mutant mice - an animal model of Menkes disease

The biological interaction between copper and iron is best exemplified by the decreased activity of multicopper ferroxidases under conditions of copper deficiency that limits the availability of iron for erythropoiesis. However, little is known about how copper deficiency affects iron homeostasis through alteration of the activity of other copper-containing proteins, not directly connected with iron metabolism, such as superoxide dismutase 1 (SOD1). This antioxidant enzyme scavenges the superoxide anion, a reactive oxygen species contributing to the toxicity of iron via the Fenton reaction. Here, we analyzed changes in the systemic iron metabolism using an animal model of Menkes disease: copper-deficient mosaic mutant mice with dysfunction of the ATP7A copper transporter. We found that the erythrocytes of these mutants are copper-deficient, display decreased SOD1 activity/expression and have cell membrane abnormalities. In consequence, the mosaic mice show evidence of haemolysis accompanied by haptoglobin-dependent elimination of haemoglobin (Hb) from the circulation, as well as the induction of haem oxygenase 1 (HO1) in the liver and kidney. Moreover, the hepcidin-ferroportin regulatory axis is strongly affected in mosaic mice. These findings indicate that haemolysis is an additional pathogenic factor in a mouse model of Menkes diseases and provides evidence of a new indirect connection between copper deficiency and iron metabolism.

Ceruloplasmin potentiates nitric oxide synthase activity and cytokine secretion in activated microglia

BACKGROUND

Ceruloplasmin is a ferroxidase expressed in the central nervous system both as soluble form in the cerebrospinal fluid (CSF) and as membrane-bound GPI-anchored isoform on astrocytes, where it plays a role in iron homeostasis and antioxidant defense. It has been proposed that ceruloplasmin is also able to activate microglial cells with ensuing nitric oxide (NO) production, thereby contributing to neuroinflammatory conditions. In light of the possible role of ceruloplasmin in neurodegenerative diseases, we were prompted to investigate how this protein could contribute to microglial activation in either its native form, as well as in its oxidized form, recently found generated in the CSF of patients with Parkinson's and Alzheimer's diseases.

METHODS

Primary rat microglial-enriched cultures were treated with either ceruloplasmin or oxidized-ceruloplasmin, alone or in combination with lipopolysaccharide (LPS). Production of NO and expression of inducible nitric oxide synthase (iNOS) were evaluated by Griess assay and Western blot analysis, respectively. The productions of the pro-inflammatory cytokine IL-6 and the chemokine MIP-1α were assessed by quantitative RT-PCR and ELISA.

RESULTS

Regardless of its oxidative status, ceruloplasmin by itself was not able to activate primary rat microglia. However, ceruloplasmin reinforced the LPS-induced microglial activation, promoting an increase of NO production, as well as the induction of IL-6 and MIP-1α. Interestingly, the ceruloplasmin-mediated effects were observed in the absence of an additional induction of iNOS expression. The evaluation of iNOS activity in primary glial cultures and in vitro suggested that the increased NO production induced by the combined LPS and ceruloplasmin treatment is mediated by a potentiation of the enzymatic activity.

CONCLUSIONS

Ceruloplasmin potentiates iNOS activity in microglial cells activated by a pro-inflammatory stimulus, without affecting iNOS expression levels. This action might be mediated by the activation of a yet unknown Cp receptor that triggers intracellular signaling that cross-talks with the response elicited by LPS or other pro-inflammatory stimuli. Therefore, ceruloplasmin might contribute to pathological conditions in the central nervous system by exacerbating neuroinflammation.

Role of iron in ischemia-induced neurodegeneration: mechanisms and insights

Iron is an important micronutrient for neuronal function and survival. It plays an essential role in DNA and protein synthesis, neurotransmission and electron transport chain due to its dual redox states. On the contrary, iron also catalyses the production of free radicals and hence, causes oxidative stress. Therefore, maintenance of iron homeostasis is very crucial and it involves a number of proteins in iron metabolism and transport that maintain the balance. In ischemic conditions large amount of iron is released and this free iron catalyzes production of more free radicals and hence, causing more damage. In this review we have focused on the iron transport and maintenance of iron homeostasis at large and also the effect of imbalance in iron homeostasis on retinal and brain tissue under ischemic conditions. The understanding of the proteins involved in the homeostasis imbalance will help in developing therapeutic strategies for cerebral as well retinal ischemia.

Iron in Eukarya

Iron is essential for the normal physiological function of all organisms. In humans it is required for a plethora of biochemical roles including the transport of oxygen in the blood and energy production in the mitochondria. However, iron is also highly cytotoxic when present at high levels as it readily participates in oxidation-reduction reactions that lead to the generation of reactive oxygen species. One unique feature of iron biology is the lack of excretory mechanisms to remove excess iron from the body. Therefore, the concerted action of several genes and proteins working together to regulate the movement of iron across cell membranes, its storage in peripheral tissues and its physiological utilization in the body is essential for maintaining iron homeostasis. Humans are exposed to iron in a number of chemical forms (haem or non-haem; ferric or ferrous). This chapter will describe how humans acquire iron from their diet; the subsequent delivery of iron to its sites of utilization and storage; and how iron is recycled from effete erythrocytes for re-use in metabolism. Mutations in a number of the genes controlling iron metabolism have been identified and study of the pathological consequences of these mutations has allowed us to gain a greater understanding of how the body senses changes in iron status and coordinates its transport, storage and utilization to maintain homeostasis.

Discovery of novel Schistosoma japonicum antigens using a targeted protein microarray approach

Background

Novel vaccine candidates against Schistosoma japonicum are required, and antigens present in the vulnerable larval developmental stage are attractive targets. Post-genomic technologies are now available which can contribute to such antigen discovery.

Methods

A schistosome-specific protein microarray was probed using the local antibody response against migrating larvae. Antigens were assessed for their novelty and predicted larval expression and host-exposed features. One antigen was further characterised and its sequence and structure were analysed in silico. Real-time polymerase chain reaction was used to analyse transcript expression throughout development, and immunoblotting and enzyme-linked immunosorbent assays employed to determine antigen recognition by antibody samples.

Results

Several known and novel antigens were discovered, two of which showed up-regulated transcription in schistosomula. One novel antigen, termed S. japonicum Ly-6-like protein 1 (Sj-L6L-1), was further characterised and shown to share structural and sequence features with the Ly-6 protein family. It was found to be present in the worm tegument and expressed in both the larval and adult worms, but was found to be antigenic only in the lungs that the larvae migrate to and traverse.

Conclusions

This study represents a novel approach to vaccine antigen discovery and may contribute to schistosome vaccine development against this important group of human and veterinary pathogens.

Molecular mediators governing iron-copper interactions

Given their similar physiochemical properties, it is a logical postulate that iron and copper metabolism are intertwined. Indeed, iron-copper interactions were first documented over a century ago, but the homeostatic effects of one on the other has not been elucidated at a molecular level to date. Recent experimental work has, however, begun to provide mechanistic insight into how copper influences iron metabolism. During iron deficiency, elevated copper levels are observed in the intestinal mucosa, liver, and blood. Copper accumulation and/or redistribution within enterocytes may influence iron transport, and high hepatic copper may enhance biosynthesis of a circulating ferroxidase, which potentiates iron release from stores. Moreover, emerging evidence has documented direct effects of copper on the expression and activity of the iron-regulatory hormone hepcidin. This review summarizes current experimental work in this field, with a focus on molecular aspects of iron-copper interplay and how these interactions relate to various disease states.

sAPP modulates iron efflux from brain microvascular endothelial cells by stabilizing the ferrous iron exporter ferroportin

A sequence within the E2 domain of soluble amyloid precursor protein (sAPP) stimulates iron efflux. This activity has been attributed to a ferroxidase activity suggested for this motif. We demonstrate that the stimulation of efflux supported by this peptide and by sAPPα is due to their stabilization of the ferrous iron exporter, ferroportin (Fpn), in the plasma membrane of human brain microvascular endothelial cells (hBMVEC). The peptide does not bind ferric iron explaining why it does not and thermodynamically cannot promote ferrous iron autoxidation. This peptide specifically pulls Fpn down from the plasma membrane of hBMVEC; based on these results, FTP, for ferroportin-targeting peptide, correctly identifies the function of this peptide. The data suggest that in stabilizing Fpn via the targeting due to the FTP sequence, sAPP will increase the flux of iron into the cerebral interstitium. This inference correlates with the observation of significant iron deposition in the amyloid plaques characteristic of Alzheimer's disease.

Ceruloplasmin-ferroportin system of iron traffic in vertebrates

Safe trafficking of iron across the cell membrane is a delicate process that requires specific protein carriers. While many proteins involved in iron uptake by cells are known, only one cellular iron export protein has been identified in mammals: ferroportin (SLC40A1). Ceruloplasmin is a multicopper enzyme endowed with ferroxidase activity that is found as a soluble isoform in plasma or as a membrane-associated isoform in specific cell types. According to the currently accepted view, ferrous iron transported out of the cell by ferroportin would be safely oxidized by ceruloplasmin to facilitate loading on transferrin. Therefore, the ceruloplasmin-ferroportin system represents the main pathway for cellular iron egress and it is responsible for physiological regulation of cellular iron levels. The most recent findings regarding the structural and functional features of ceruloplasmin and ferroportin and their relationship will be described in this review.

Ceruloplasmin and β-amyloid precursor protein confer neuroprotection in traumatic brain injury and lower neuronal iron

Traumatic brain injury (TBI) is in part complicated by pro-oxidant iron elevation independent of brain hemorrhage. Ceruloplasmin (CP) and β-amyloid protein precursor (APP) are known neuroprotective proteins that reduce oxidative damage through iron regulation. We surveyed iron, CP, and APP in brain tissue from control and TBI-affected patients who were stratified according to time of death following injury. We observed CP and APP induction after TBI accompanying iron accumulation. Elevated APP and CP expression was also observed in a mouse model of focal cortical contusion injury concomitant with iron elevation. To determine if changes in APP or CP were neuroprotective we employed the same TBI model on APP(-/-) and CP(-/-) mice and found that both exhibited exaggerated infarct volume and iron accumulation postinjury. Evidence supports a regulatory role of both proteins in defence against iron-induced oxidative damage after TBI, which presents as a tractable therapeutic target.

Glial cell ceruloplasmin and hepcidin differentially regulate iron efflux from brain microvascular endothelial cells

We have used an in vitro model system to probe the iron transport pathway across the brain microvascular endothelial cells (BMVEC) of the blood-brain barrier (BBB). This model consists of human BMVEC (hBMVEC) and C6 glioma cells (as an astrocytic cell line) grown in a transwell, a cell culture system commonly used to quantify metabolite flux across a cell-derived barrier. We found that iron efflux from hBMVEC through the ferrous iron permease ferroportin (Fpn) was stimulated by secretion of the soluble form of the multi-copper ferroxidase, ceruloplasmin (sCp) from the co-cultured C6 cells. Reciprocally, expression of sCp mRNA in the C6 cells was increased by neighboring hBMVEC. In addition, data indicate that C6 cell-secreted hepcidin stimulates internalization of hBMVEC Fpn but only when the end-feet projections characteristic of this glia-derived cell line are proximal to the endothelial cells. This hepcidin-dependent loss of Fpn correlated with knock-down of iron efflux from the hBMVEC; this result was consistent with the mechanism by which hepcidin regulates iron efflux in mammalian cells. In summary, the data support a model of iron trafficking across the BBB in which the capillary endothelium induce the underlying astrocytes to produce the ferroxidase activity needed to support Fpn-mediated iron efflux. Reciprocally, astrocyte proximity modulates the effective concentration of hepcidin at the endothelial cell membrane and thus the surface expression of hBMVEC Fpn. These results are independent of the source of hBMVEC iron (transferrin or non-transferrin bound) indicating that the model developed here is broadly applicable to brain iron homeostasis.

Copper and copper proteins in Parkinson's disease

Copper is a transition metal that has been linked to pathological and beneficial effects in neurodegenerative diseases. In Parkinson's disease, free copper is related to increased oxidative stress, alpha-synuclein oligomerization, and Lewy body formation. Decreased copper along with increased iron has been found in substantia nigra and caudate nucleus of Parkinson's disease patients. Copper influences iron content in the brain through ferroxidase ceruloplasmin activity; therefore decreased protein-bound copper in brain may enhance iron accumulation and the associated oxidative stress. The function of other copper-binding proteins such as Cu/Zn-SOD and metallothioneins is also beneficial to prevent neurodegeneration. Copper may regulate neurotransmission since it is released after neuronal stimulus and the metal is able to modulate the function of NMDA and GABA A receptors. Some of the proteins involved in copper transport are the transporters CTR1, ATP7A, and ATP7B and the chaperone ATOX1. There is limited information about the role of those biomolecules in the pathophysiology of Parkinson's disease; for instance, it is known that CTR1 is decreased in substantia nigra pars compacta in Parkinson's disease and that a mutation in ATP7B could be associated with Parkinson's disease. Regarding copper-related therapies, copper supplementation can represent a plausible alternative, while copper chelation may even aggravate the pathology.

Iron deficiency during pregnancy: the consequences for placental function and fetal outcome

This review examines the importance of the placenta in iron metabolism during development and the effect of iron deficiency on maternal and fetal physiology. Iron is an essential micronutrient, required for a wide variety of biological processes. During pregnancy, the mother has to deplete her iron stores in order to provide the baby with adequate amounts. Trans-placental iron transfer involves binding transferrin (Tf)-bound iron to the Tf receptor, uptake into an endosome, acidification, release of iron through divalent metal transporter 1, efflux across the basolateral membrane through ferroportin and oxidation of Fe(II) by zyklopen. An additional haem transport system has been hypothesised, which may explain why certain gene knockouts are not lethal for the developing fetus. Iron deficiency is a common phenomenon during pregnancy, and the placenta adapts by up-regulating its transfer systems, maintaining iron at the expense of the mother. Despite these adaptations, deficiency cannot be completely prevented, and the offspring suffers both short- and long-term consequences. Some of these, at least, may arise from decreased expression of genes involved in the cell cycle and altered expression of transcription factors, such as c-myc, which in turn can produce, for example, kidneys with reduced numbers of nephrons. The mechanism whereby these changes are induced is not certain, but may simply be as a result of the reduced availability of iron resulting in decreased enzyme activity. Since these changes are so significant, and because some of the changes are irreversible, we believe that iron prophylaxis should be considered in all pregnancies.

Mechanisms for cellular NO oxidation and nitrite formation in lung epithelial cells

Airway lining fluid contains relatively high concentrations of nitrite, and arterial blood levels of nitrite are higher than venous levels, suggesting the lung epithelium may represent an important source of nitrite in vivo. To investigate whether lung epithelial cells possess the ability to convert NO to nitrite by oxidation, and the effect of oxygen reactions on nitrite formation, the NO donor DETA NONOate was incubated with or without A549 cells or primary human bronchial epithelial (HBE) cells for 24 h under normoxic (21% O2) and hypoxic (1% O2) conditions. Nitrite production was significantly increased under all conditions in the presence of A549 or HBE cells, suggesting that both A549 and HBE cells have the capacity to oxidize NO to nitrite even under low-oxygen conditions. The addition of oxyhemoglobin to the A549 cell medium decreased the production of nitrite, consistent with NO scavenging limiting nitrite formation. Heat-denatured A549 cells produced much lower nitrite and nitrate, suggesting an enzymatic activity is required. This NO oxidation activity was highest in membrane-bound proteins with molecular size <100kDa. In addition, 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one and cyanide inhibited formation of nitrite in A549 cells. It has been shown that ceruloplasmin (Cp) possesses an NO oxidase and nitrite synthase activity in plasma based on NO oxidation to nitrosonium cation. We observed that Cp is expressed intracellularly in lung epithelial A549 cells and secreted into the medium under basal conditions and during cytokine stimulation. However, an analysis of Cp expression level and activity measured via p-phenylenediamine oxidase activity assay revealed very low activity compared with plasma, suggesting that there is insufficient Cp to contribute to detectable NO oxidation to nitrite in A549 cells. Additionally, Cp levels were knocked down using siRNA by more than 75% in A549 cells, with no significant change in either nitrite or cellular S-nitrosothiol formation compared to scrambled siRNA control under basal conditions or cytokine stimulation. These data suggest that lung epithelial cells possess NO oxidase activity, which is enhanced in cell-membrane-associated proteins and not regulated by intracellular or secreted Cp, indicating that alternative NO oxidases determine hypoxic and normoxic nitrite formation from NO in human lung epithelial cells.

Metal ions in macrophage antimicrobial pathways: emerging roles for zinc and copper

The immunomodulatory and antimicrobial properties of zinc and copper have long been appreciated. In addition, these metal ions are also essential for microbial growth and survival. This presents opportunities for the host to either harness their antimicrobial properties or limit their availability as defence strategies. Recent studies have shed some light on mechanisms by which copper and zinc regulation contribute to host defence, but there remain many unanswered questions at the cellular and molecular levels. Here we review the roles of these two metal ions in providing protection against infectious diseases in vivo, and in regulating innate immune responses. In particular, we focus on studies implicating zinc and copper in macrophage antimicrobial pathways, as well as the specific host genes encoding zinc transporters (SLC30A, SLC39A family members) and CTRs (copper transporters, ATP7 family members) that may contribute to pathogen control by these cells.

Retinal iron homeostasis in health and disease

Iron is essential for life, but excess iron can be toxic. As a potent free radical creator, iron generates hydroxyl radicals leading to significant oxidative stress. Since iron is not excreted from the body, it accumulates with age in tissues, including the retina, predisposing to age-related oxidative insult. Both hereditary and acquired retinal diseases are associated with increased iron levels. For example, retinal degenerations have been found in hereditary iron overload disorders, like aceruloplasminemia, Friedreich's ataxia, and pantothenate kinase-associated neurodegeneration. Similarly, mice with targeted mutation of the iron exporter ceruloplasmin and its homolog hephaestin showed age-related retinal iron accumulation and retinal degeneration with features resembling human age-related macular degeneration (AMD). Post mortem AMD eyes have increased levels of iron in retina compared to age-matched healthy donors. Iron accumulation in AMD is likely to result, in part, from inflammation, hypoxia, and oxidative stress, all of which can cause iron dysregulation. Fortunately, it has been demonstrated by in vitro and in vivo studies that iron in the retinal pigment epithelium (RPE) and retina is chelatable. Iron chelation protects photoreceptors and retinal pigment epithelial cells (RPE) in a variety of mouse models. This has therapeutic potential for diminishing iron-induced oxidative damage to prevent or treat AMD.

Multi-copper oxidases and human iron metabolism

Multi-copper oxidases (MCOs) are a small group of enzymes that oxidize their substrate with the concomitant reduction of dioxygen to two water molecules. Generally, multi-copper oxidases are promiscuous with regards to their reducing substrates and are capable of performing various functions in different species. To date, three multi-copper oxidases have been detected in humans—ceruloplasmin, hephaestin and zyklopen. Each of these enzymes has a high specificity towards iron with the resulting ferroxidase activity being associated with ferroportin, the only known iron exporter protein in humans. Ferroportin exports iron as Fe²⁺, but transferrin, the major iron transporter protein of blood, can bind only Fe³⁺ effectively. Iron oxidation in enterocytes is mediated mainly by hephaestin thus allowing dietary iron to enter the bloodstream. Zyklopen is involved in iron efflux from placental trophoblasts during iron transfer from mother to fetus. Release of iron from the liver relies on ferroportin and the ferroxidase activity of ceruloplasmin which is found in blood in a soluble form. Ceruloplasmin, hephaestin and zyklopen show distinctive expression patterns and have unique mechanisms for regulating their expression. These features of human multi-copper ferroxidases can serve as a basis for the precise control of iron efflux in different tissues. In this manuscript, we review the biochemical and biological properties of the three human MCOs and discuss their potential roles in human iron homeostasis.

Ferroportin and exocytoplasmic ferroxidase activity are required for brain microvascular endothelial cell iron efflux

The mechanism(s) of iron flux across the brain microvasculature endothelial cells (BMVEC) of the blood-brain barrier remains unknown. Although both hephaestin (Hp) and the ferrous iron permease ferroportin (Fpn) have been identified in BMVEC, their roles in iron efflux have not been examined. Using a human BMVEC line (hBMVEC), we have demonstrated that these proteins are required for iron efflux from these cells. Expression of both Hp and Fpn protein was confirmed in hBMVEC by immunoblot and indirect immunofluorescence; we show that hBMVEC express soluble ceruloplasmin (Cp) transcript as well. Depletion of endogenous Hp and Cp via copper chelation leads to the reduction of hBMVEC Fpn protein levels as well as a complete inhibition of (59)Fe efflux. Both hBMVEC Fpn protein and (59)Fe efflux activity are restored upon incubation with 6.6 nm soluble plasma Cp. These results are independent of the source of cell iron, whether delivered as transferrin- or non-transferrin-bound (59)Fe. Our results demonstrate that iron efflux from hBMVEC Fpn requires the action of an exocytoplasmic ferroxidase, which can be either endogenous Hp or extracellular Cp.

Aceruloplasminemia: an update

Aceruloplasminemia is an inherited neurodegenerative disorder involving "neurodegeneration with brain iron accumulation," which is caused by genetic defects in the ceruloplasmin gene. Ceruloplasmin is a multicopper oxidase with ferroxidase activity that oxidizes ferrous iron following its transfer to extracellular transferrin. In the central nervous system, a glycosylphosphatidylinositol-linked ceruloplasmin bound to the cell membranes was found to be the major isoform of this protein. Aceruloplasminemia is characterized by diabetes, retinal degeneration, and progressive neurological symptoms, including extrapyramidal symptoms, ataxia, and dementia. Clinical and pathological studies and investigations of cell culture and murine models revealed that there is an iron-mediated cellular radical injury caused by a marked accumulation of iron in the affected parenchymal tissues. The aim of this chapter is to provide an overview of not only the clinical features, genetic and molecular pathogenesis, and treatment of aceruloplasminemia but also the biological and physiological features of iron metabolism.

Copper deficiency has minimal impact on ferroportin expression or function

Interactions between copper and iron homeostasis have been known since the nineteenth century when anemia in humans was first described due to copper limitation. However, the mechanism remains unknown. Intestinal and liver iron concentrations are usually higher following copper deficiency (CuD). This may be due to impaired function of the multicopper oxidases hephaestin or ceruloplasmin (Cp), respectively. However, iron retention could be due to altered ferroportin (Fpn), the essential iron efflux transporter in enterocytes and macrophages. Fpn mRNA is controlled partially by intracellular iron and IRE dependence. CuD should augment Fpn based on iron level. Some argue that Fpn stability is controlled partially by membrane ferroxidase (GPI-Cp). CuD should result in lower Fpn since GPI-Cp expression and function is reduced. Fpn turnover is controlled by hepcidin. CuD results in variable Hamp (hepcidin) expression. Fpn mRNA and protein level were evaluated following dietary CuD in rats and mice. To correlate with Fpn expression, measurements of tissue iron were conducted in several rodent models. Following CuD there was little change in Fpn mRNA. Previous work indicated that under certain circumstances Fpn protein was augmented in liver and spleen following CuD. Fpn levels in CuD did not correlate with either total iron or non-heme iron (NHI), as iron levels in CuD liver were higher and in spleen lower than copper adequate controls. Fpn steady state levels appear to be regulated by a complex set of factors. Changes in Fpn do not explain the anemia of CuD.

Impairment of interrelated iron- and copper homeostatic mechanisms in brain contributes to the pathogenesis of neurodegenerative disorders

Iron and copper are important co-factors for a number of enzymes in the brain, including enzymes involved in neurotransmitter synthesis and myelin formation. Both shortage and an excess of iron or copper will affect the brain. The transport of iron and copper into the brain from the circulation is strictly regulated, and concordantly protective barriers, i.e., the blood-brain barrier (BBB) and the blood-cerebrospinal fluid (CSF) barrier (BCB) have evolved to separate the brain environment from the circulation. The uptake mechanisms of the two metals interact. Both iron deficiency and overload lead to altered copper homeostasis in the brain. Similarly, changes in dietary copper affect the brain iron homeostasis. Moreover, the uptake routes of iron and copper overlap each other which affect the interplay between the concentrations of the two metals in the brain. The divalent metal transporter-1 (DMT1) is involved in the uptake of both iron and copper. Furthermore, copper is an essential co-factor in numerous proteins that are vital for iron homeostasis and affects the binding of iron-response proteins to iron-response elements in the mRNA of the transferrin receptor, DMT1, and ferroportin, all highly involved in iron transport. Iron and copper are mainly taken up at the BBB, but the BCB also plays a vital role in the homeostasis of the two metals, in terms of sequestering, uptake, and efflux of iron and copper from the brain. Inside the brain, iron and copper are taken up by neurons and glia cells that express various transporters.