Biochemical analysis of a missense mutation in aceruloplasminemia

Trace Elements Levels in Major Depressive Disorder-Evaluation of Potential Threats and Possible Therapeutic Approaches

The multifactorial etiology of major depressive disorder (MDD) includes biological, environmental, genetic, and psychological aspects. Recently, there has been an increasing interest in metallomic studies in psychiatry, aiming to evaluate the role of chosen trace elements in the MDD etiology as well as the progression of symptoms. This narrative review aims to summarize the available literature on the relationship between the concentration of chosen elements in the serum of patients with MDD and the onset and progression of this psychiatric condition. The authors reviewed PubMed, Web of Science, and Scopus databases searching for elements that had been investigated so far and further evaluated them in this paper. Ultimately, 15 elements were evaluated, namely, zinc, magnesium, selenium, iron, copper, aluminium, cadmium, lead, mercury, arsenic, calcium, manganese, chromium, nickel, and phosphorus. The association between metallomic studies and psychiatry has been developing dynamically recently. According to the results of current research, metallomics might act as a potential screening tool for patients with MDD while at the same time providing an assessment of the severity of symptoms. Either deficiencies or excessive amounts of chosen elements might be associated with the progression of depressive symptoms or even the onset of the disease among people predisposed to MDD.

Zinc transport via ZNT5-6 and ZNT7 is critical for cell surface glycosylphosphatidylinositol-anchored protein expression

Glycosylphosphatidylinositol (GPI)-anchored proteins play crucial roles in various enzyme activities, cell signaling and adhesion, and immune responses. While the molecular mechanism underlying GPI-anchored protein biosynthesis has been well studied, the role of zinc transport in this process has not yet been elucidated. Zn transporter (ZNT) proteins mobilize cytosolic zinc to the extracellular space and to intracellular compartments. Here, we report that the early secretory pathway ZNTs (ZNT5–ZNT6 heterodimers [ZNT5-6] and ZNT7–ZNT7 homodimers [ZNT7]), which supply zinc to the lumen of the early secretory pathway compartments are essential for GPI-anchored protein expression on the cell surface. We show, using overexpression and gene disruption/re-expression strategies in cultured human cells, that loss of ZNT5-6 and ZNT7 zinc transport functions results in significant reduction in GPI-anchored protein levels similar to that in mutant cells lacking phosphatidylinositol glycan anchor biosynthesis (PIG) genes. Furthermore, medaka fish with disrupted Znt5 and Znt7 genes show touch-insensitive phenotypes similar to zebrafish Pig mutants. These findings provide a previously unappreciated insight into the regulation of GPI-anchored protein expression and protein quality control in the early secretory pathway.

Molecular Functions of Ceruloplasmin in Metabolic Disease Pathology

Ceruloplasmin (CP) is a multicopper oxidase and antioxidant that is mainly produced in the liver. CP not only plays a crucial role in the metabolic balance of copper and iron through its oxidase function but also exhibits antioxidant activity. In addition, CP is an acute-phase protein. In addition to being associated with aceruloplasminemia and neurodegenerative diseases such as Wilson's disease, Alzheimer's disease, and Parkinson's disease, CP also plays an important role in metabolic diseases, which are caused by metabolic disorders and vigorous metabolism, mainly including diabetes, obesity, hyperlipidemia, etc. Based on the physiological functions of CP, we provide an overview of the association of type 2 diabetes, obesity, hyperlipidemia, coronary heart disease, CP oxidative stress, inflammation, and metabolism of copper and iron. Studies have shown that metabolic diseases are closely related to systemic inflammation, oxidative stress, and disorders of copper and iron metabolism. Therefore, we conclude that CP, which can reduce the formation of free radicals in tissues, can be induced during inflammation and infection, and can correct the metabolic disorder of copper and iron, has protective and diagnostic effects on metabolic diseases.

Production of Recombinant Human Ceruloplasmin: Improvements and Perspectives

The ferroxidase ceruloplasmin (CP) plays a crucial role in iron homeostasis in vertebrates together with the iron exporter ferroportin. Mutations in the CP gene give rise to aceruloplasminemia, a rare neurodegenerative disease for which no cure is available. Many aspects of the (patho)physiology of CP are still unclear and would benefit from the availability of recombinant protein for structural and functional studies. Furthermore, recombinant CP could be evaluated for enzyme replacement therapy for the treatment of aceruloplasminemia. We report the production and preliminary characterization of high-quality recombinant human CP in glycoengineered Pichia pastoris SuperMan5. A modified yeast strain lacking the endogenous ferroxidase has been generated and employed as host for heterologous expression of the secreted isoform of human CP. Highly pure biologically active protein has been obtained by an improved two-step purification procedure. Glycan analysis indicates that predominant glycoforms HexNAc2Hex8 and HexNAc2Hex11 are found at Asn119, Asn378, and Asn743, three of the canonical four N-glycosylation sites of human CP. The availability of high-quality recombinant human CP represents a significant advancement in the field of CP biology. However, productivity needs to be increased and further careful glycoengineering of the SM5 strain is mandatory in order to evaluate the possible therapeutic use of the recombinant protein for enzyme replacement therapy of aceruloplasminemia patients.

Clinical relevance of heterozygosis for aceruloplasminemia

Aceruloplasminemia is a rare form of brain iron overload of autosomal recessive inheritance that results from mutations in the CP gene, encoding the iron oxidase ceruloplasmin. Homozygous aceruloplasminemia causes progressive neurodegenerative disease, anemia, and diabetes, and is usually diagnosed late in life upon investigation of anemia, high ferritin, or movement disorders, but its heterozygous state is less characterized and believed to be silent. Here we report two heterozygotes for new mutations causing aceruloplasminemia from whom peripheral blood samples were collected for complete blood counts, iron studies, and genotyping by automated sequencing. We then performed a systematic review of preview reports of heterozygotes with data on genotype and clinical findings. Heterozygosity for aceruloplasminemia invariably causes reduced ceruloplasmin levels, and similarly to previews reports in the literature, our cases did not present with anemia. Mild hyperferritinemia was found only in two reports. Nevertheless, 5 out of 11 variants have been associated with significant neurological symptoms despite the presence of one wild-type alelle. This review contributes to better genetic counseling of heterozygotes for CP gene variants and supports that measuring ceruloplasmin levels may be useful when investigating patients with movement disorders or rare cases of unexplained high ferritin.

Inborn errors of metabolism associated with hyperglycaemic ketoacidosis and diabetes mellitus: narrative review

Inborn errors of metabolism (IEM) are heterogeneous group of disorders that might present in the clinics or emergency departments in different phenotypes, and one of these is a diabetes scenario. Diabetes is the most common endocrine disorder among children. The mechanism of how IEM could lead to diabetes is unclear; however, the postulated pathogenesis consists of three mechanisms: 1) accumulation of toxic substance in the gland, ruining structure and normal functionality, 2) disturbing energy availability required for hormone synthesis and 3) defect of complex molecules. The differential diagnosis of IEM associated with hyperglycaemic ketoacidosis and diabetes include: organic acidemias specifically propionic acidemia, methylmalonic acidemia, isovaleric acidemia, hereditary hemochromatosis, aceruloplasminemia, holocarboxylase synthetase deficiency, β-ketothiolase deficiency and finally, cystinosis, Rogers syndrome (thiamine-responsive megaloblastic anaemia) and congenital disorders of glycosylation type Ia. Clinical approach will help in ready diagnosis and treatment for IEM disorders in early detection of diabetes. In this review, we will discuss the differential diagnosis, clinical features and diagnostic approaches of IEM presenting as hyperglycaemic ketoacidosis and diabetes.

Role of copper in depression. Relationship with ketamine treatment

Depression is one of the most common psychiatric issues with a proportion of adults with major depressive disorder who fail to achieve remission with index pharmacological treatment. There are unmet needs in ADT focus on non-monoaminergic agents. Accumulating evidence suggests that the N-Methyl-d-aspartate receptor (NMDAR) plays an important role in the neurobiology and treatment of major depressive disorder. The role of copper ions in pathogenesis and treatment of depression is not fully clarified, however interaction between copper and NMDAR is of prime importance. Release of copper ions inhibits NMDAR and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor function thus protects neurons from glutamatergic excitotoxity. Abnormalities in glutamatergic transmission are the key of glutamate hypothesis of depression. Some authors revealed that NMDARs are also regulated by cellular prion protein (PrP^C) and indicated that interactions of copper, glycine and NMDARs subunits are vital for the regulation of the receptor. As NMDAR antagonist ketamine is known to produce rapid antidepressive effect, observation of copper serum levels in patients treated with ketamine may provide important information about connections between NMDAR antagonistic agents and trace elements antagonistic to that receptor. It is necessary to carry out further studies related to copper and ketamine in depression treatment.

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.

Decrease in APP and CP mRNA expression supports impairment of iron export in Alzheimer's disease patients

Alzheimer's disease (AD) is a neurodegenerative disorder of still unknown etiology and the leading cause of dementia worldwide. Besides its main neuropathological hallmarks, a dysfunctional homeostasis of transition metals has been reported to play a pivotal role in the pathogenesis of this disease. Dysregulation of iron (Fe) metabolism in AD has been suggested, particularly at the level of cellular iron efflux. Herein, we intended to further clarify the molecular mechanisms underlying Fe homeostasis in AD. In order to achieve this goal, the expression of specific Fe metabolism-related genes directly involved in Fe regulation and export was assessed in peripheral blood mononuclear cells (PBMCs) from 73AD patients and 74 controls by quantitative PCR. The results obtained showed a significant decrease in the expression of aconitase 1 (ACO1; P=0.007); ceruloplasmin (CP; P<0.001) and amyloid-beta precursor protein (APP; P=0.006) genes in AD patients compared with healthy volunteers. These observations point out to a significant downregulation in the expression of genes associated with ferroportin-mediated cellular Fe export in PBMCs from AD patients, when compared to controls. Taken together, these findings support previous studies suggesting impairment of Fe homeostasis in AD, which may lead to cellular Fe retention and oxidative stress, a typical feature of this disease.

Oxidative stress and major depression

BACKGROUND

Major causative factor for major depression is inflammation, autoimmune tissue damage and prolonged psychological stress, which leads to oxidative stress. The aim of this study was to know the association of free radicals and antioxidant status in subjects suffering from major depression.

MATERIALS AND METHODS

Sixty patients diagnosed as a case of unipolar depression as per DSM IV, fulfilling the inclusion and exclusion criteria were compared with 40 healthy age and sex matched controls. The sera of both the groups were collected taking aseptic precautions and were evaluated for the markers of oxidative stress and for the antioxidants. The age group of the sample and the controls was between 18-60 y, both males and females were equally represented in the groups.

RESULTS

A significantly high level of malondialdehyde (MDA) was found in the patients with major depression (1.95 ± 1.04 mmol/L) as compared to healthy controls (0.366 ± 0.175 mmol/L) (p < 0.0001). The serum level of nitrite was found to be lower in cases (23.18 ± 12.08 μmol/L) in comparison to controls (26.18 ± 8.68 μmol/L) (p = 0.1789). Similarly the serum level of ascorbic acid and superoxide dismutase (SOD) were significantly below as compared to healthy controls (all p < 0.0001). Ceruloplasmin levels were also depressed in cases (p = 0.3943).

CONCLUSION

The study concluded that in the absence of known oxidative injury causative agents, the lowered levels of antioxidants and higher levels of MDA implicate the high degree of oxidative stress in unipolar depression.

Protein misfolding, congophilia, oligomerization, and defective amyloid processing in preeclampsia

Preeclampsia is a pregnancy-specific disorder of unknown etiology and a leading contributor to maternal and perinatal morbidity and mortality worldwide. Because there is no cure other than delivery, preeclampsia is the leading cause of iatrogenic preterm birth. We show that preeclampsia shares pathophysiologic features with recognized protein misfolding disorders. These features include urine congophilia (affinity for the amyloidophilic dye Congo red), affinity for conformational state–dependent antibodies, and dysregulation of prototype proteolytic enzymes involved in amyloid precursor protein (APP) processing. Assessment of global protein misfolding load in pregnancy based on urine congophilia (Congo red dot test) carries diagnostic and prognostic potential for preeclampsia. We used conformational state–dependent antibodies to demonstrate the presence of generic supramolecular assemblies (prefibrillar oligomers and annular protofibrils), which vary in quantitative and qualitative representation with preeclampsia severity. In the first attempt to characterize the preeclampsia misfoldome, we report that the urine congophilic material includes proteoforms of ceruloplasmin, immunoglobulin free light chains, SERPINA1, albumin, interferon-inducible protein 6-16, and Alzheimer’s β-amyloid. The human placenta abundantly expresses APP along with prototype APP-processing enzymes, of which the α-secretase ADAM10, the β-secretases BACE1 and BACE2, and the γ-secretase presenilin-1 were all up-regulated in preeclampsia. The presence of β-amyloid aggregates in placentas of women with preeclampsia and fetal growth restriction further supports the notion that this condition should join the growing list of protein conformational disorders. If these aggregates play a pathophysiologic role, our findings may lead to treatment for preeclampsia.

The effects of silver ions on copper metabolism in rats

The influence of short and prolonged diet containing silver ions (Ag-diet) on copper metabolism was studied. Two groups of animals were used: one group of adult rats received a Ag-diet for one month (Ag-A1) and another group received a Ag-diet for 6 months from birth (Ag-N6). In Ag-A1 rats, the Ag-diet caused a dramatic decrease of copper status indexes that was manifested as ceruloplasmin-associated copper deficiency. In Ag-N6 rats, copper status indexes decreased only 2-fold as compared to control rats. In rats of both groups, silver entered the bloodstream and accumulated in the liver. Silver was incorporated into ceruloplasmin (Cp), but not SOD1. In the liver, a prolonged Ag-diet caused a decrease of the expression level of genes, associated with copper metabolism. Comparative spectrophotometric analysis of partially purified Cp fractions has shown that Cp from Ag-N6 rats was closer to holo-Cp by specific enzymatic activities and tertiary structure than Cp from Ag-A1 rats. However, Cp of Ag-N6 differs from control holo-Cp and Cp of Ag-A1 in its affinity to DEAE-Sepharose and in its binding properties to lectins. In the bloodstream of Ag-N6, two Cp forms are present as shown in pulse-experiments on rats with the liver isolated from circulation. One of the Cp isoforms is of hepatic origin, and the other is of extrahepatic origin; the latter is characterized by a faster rate of secretion than hepatic Cp. These data allowed us to suggest that the disturbance of holo-Cp formation in the liver was compensated by induction of extrahepatic Cp synthesis. The possible biological importance of these effects is discussed.

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.

Neurodegeneration with brain iron accumulation: update on pathogenic mechanisms

Perturbation of iron distribution is observed in many neurodegenerative disorders, including Alzheimer’s and Parkinson’s disease, but the comprehension of the metal role in the development and progression of such disorders is still very limited. The combination of more powerful brain imaging techniques and faster genomic DNA sequencing procedures has allowed the description of a set of genetic disorders characterized by a constant and often early accumulation of iron in specific brain regions and the identification of the associated genes; these disorders are now collectively included in the category of neurodegeneration with brain iron accumulation (NBIA). So far 10 different genetic forms have been described but this number is likely to increase in short time. Two forms are linked to mutations in genes directly involved in iron metabolism: neuroferritinopathy, associated to mutations in the FTL gene and aceruloplasminemia, where the ceruloplasmin gene product is defective. In the other forms the connection with iron metabolism is not evident at all and the genetic data let infer the involvement of other pathways: Pank2, Pla2G6, C19orf12, COASY, and FA2H genes seem to be related to lipid metabolism and to mitochondria functioning, WDR45 and ATP13A2 genes are implicated in lysosomal and autophagosome activity, while the C2orf37 gene encodes a nucleolar protein of unknown function. There is much hope in the scientific community that the study of the NBIA forms may provide important insight as to the link between brain iron metabolism and neurodegenerative mechanisms and eventually pave the way for new therapeutic avenues also for the more common neurodegenerative disorders. In this work, we will review the most recent findings in the molecular mechanisms underlining the most common forms of NBIA and analyze their possible link with brain iron metabolism.

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.

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.

Immune cells and hepatocytes express glycosylphosphatidylinositol-anchored ceruloplasmin at their cell surface

BACKGROUND

Ceruloplasmin is a positive acute-phase protein with both anti- and pro-oxidant activities, thus having still unclear physiological functions in inflammatory processes. Importantly, ceruloplasmin has been implicated in iron metabolism due to its ferroxidase activity, assisting ferroportin on cellular iron efflux. Ceruloplasmin can be expressed as a secreted or as a membrane glycosylphosphatidylinositol-anchored protein (GPI-ceruloplasmin), this latter one being reported as expressed mostly in the brain.

DESIGN AND METHODS

We studied the expression of both ceruloplasmin isoforms in human peripheral blood lymphocytes, monocytes, mouse macrophages and human hepatocarcinoma cell line HepG2, using immunofluorescence and immunoblotting techniques. Co-localization of ceruloplasmin and ferroportin was also investigated by immunofluorescence in mouse macrophages.

RESULTS

Ceruloplasmin was detected by immunoblotting and immunofluorescence in membrane and cytosol of all cell types. The cell surface ceruloplasmin was identified as the GPI-isoform and localized in lipid rafts from monocytes, macrophages and HepG2 cells. In macrophages, increased expression levels and co-localization of ferroportin and GPI-ceruloplasmin in cell surface lipid rafts were observed after iron treatment. Such iron upregulation of ceruloplasmin was not observed in HepG2.

CONCLUSIONS

Our results revealed an unexpected ubiquitous expression of the GPI-ceruloplasmin isoform in immune and hepatic cells. Different patterns of regulation of ceruloplasmin in these cells may reflect distinct physiologic functions of this oxidase. In macrophages, GPI-ceruloplasmin and ferroportin likely interact in lipid rafts to export iron from cells. Precise knowledge about ceruloplasmin isoforms expression and function in various cell types will help to clarify the role of ceruloplasmin in many diseases related to iron metabolism, inflammation and oxidative biology.

Experimental switching of copper status in laboratory rodents

There is an emerging link between copper metabolism, tumor growth and efficiency of antitumor treatment with platinum drugs or copper chelators. So there is an urgent need for well-defined and reproduced animal models with different states of copper metabolism. In the present study an animal model (rats and mice) with switching copper status in blood serum (copper concentration, oxidase activity and ceruloplasmin (Cp) protein content) is characterized. The drop of copper status is caused by addition of AgCl to fodder (Ag-animals). In rats and mice, the influence of silver ions on oxidase and ferroxidase activity of blood serum is similar, but copper concentration is reduced by 90% in rats, and by 60% in mice. The absorbed silver ions are transported to liver cells and included to Cp polypeptides, which are secreted to blood serum then. Cp, which circulates in bloodstream of Ag-animals contains silver atoms, and is misfolded, as judged by circular dichroism spectroscopy and differential scanning calorimetry. Single intraperitoneal or per oral injection of Cu(II) salt to Ag-animals causes recovery of oxidase and ferroxidase activity of blood serum within 4 hours in both rodent species, presumably by rapid metabolic insertion of copper into forming Cp in liver. The recovered copper status persists for 3 days under the continuing Ag-diet. The possibilities of use of Ag-rodents with switching copper status in investigation of influence of copper status on tissue-specific intracellular copper metabolism and role of copper in tumor genesis, bone metabolism and neurodegenerative diseases are discussed.

The effect of diet on total antioxidant status, ceruloplasmin, transferrin and ferritin serum levels in phenylketonuric children

OBJECTIVES

To investigate the effect of diet on total antioxidative status (TAS), transferrin, ferritin and ceruloplasmin serum levels in phenylketonuric (PKU) children.

PATIENTS AND METHODS

Seventeen poorly controlled PKU children underwent clinical and laboratory examinations before, 'off diet', and 60 days after adhering to their special diet 'on diet', whereas controls (N = 24) were examined once. Blood chemistry was performed with the appropriate methodologies.

RESULTS

Phenylalanine levels differed significantly among the examined groups. Lipids and lipoproteins were higher in 'off diet' than in 'on diet' group, except of high density lipoprotein and apolipoprotein AI that remained unaffected. Total antioxidative status (386 ± 30 vs 204 ± 23 μmol/L, p < 0.001), ferritin (48.2 ± 2.3 vs 33.0 ± 2.8 μg/L, p < 0.001) and ceruloplasmin (40.02 ± 2.5 vs 25.5 ± 2.8 mg/dL, p < 0.001) levels were significantly higher in 'on diet' patients' group compared to 'off diet' one. The low lipoprotein and the high TAS and ferritin levels in patients with PKU 'on diet' may be related to the vegetarian diet and the rich in iron formula supplementation.

CONCLUSIONS

The low ferritin levels found in 'off diet' patients with PKU may be attributed to a decreased liver production of ceruloplasmin, which evaluation may be a useful tool for the follow-up of patients with PKU.

Biological effects of mutant ceruloplasmin on hepcidin-mediated internalization of ferroportin

Ceruloplasmin plays an essential role in cellular iron efflux by oxidizing ferrous iron exported from ferroportin. Ferroportin is posttranslationally regulated through internalization triggered by hepcidin binding. Aceruloplasminemia is an autosomal recessive disorder of iron homeostasis resulting from mutations in the ceruloplasmin gene. The present study investigated the biological effects of glycosylphosphatidylinositol (GPI)-linked ceruloplasmin on the hepcidin-mediated internalization of ferroportin. The prevention of hepcidin-mediated ferroportin internalization was observed in the glioma cells lines expressing endogenous ceruloplasmin as well as in the cells transfected with GPI-linked ceruloplasmin under low levels of hepcidin. A decrease in the extracellular ferrous iron by an iron chelator and incubation with purified ceruloplasmin in the culture medium prevented hepcidin-mediated ferroportin internalization, while the reconstitution of apo-ceruloplasmin was not able to prevent ferroportin internalization. The effect of ceruloplasmin on the ferroportin stability was impaired due to three distinct properties of the mutant ceruloplasmin: namely, a decreased ferroxidase activity, the mislocalization in the endoplasmic reticulum, and the failure of copper incorporation into apo-ceruloplasmin. Patients with aceruloplasminemia exhibited low serum hepcidin levels and a decreased ferroportin protein expression in the liver. The in vivo findings supported the notion that under low levels of hepcidin, mutant ceruloplasmin cannot stabilize ferroportin because of a loss-of-function in the ferroxidase activity, which has been reported to play an important role in the stability of ferroportin. The properties of mutant ceruloplasmin regarding the regulation of ferroportin may therefore provide a therapeutic strategy for aceruloplasminemia patients.

Dominant mutants of ceruloplasmin impair the copper loading machinery in aceruloplasminemia

The multicopper oxidase ceruloplasmin plays a key role in iron homeostasis, and its ferroxidase activity is required to stabilize cell surface ferroportin, the only known mammalian iron exporter. Missense mutations causing the rare autosomal neurodegenerative disease aceruloplasminemia were investigated by testing their ability to prevent ferroportin degradation in rat glioma C6 cells silenced for endogenous ceruloplasmin. Most of the mutants did not complement (i.e. did not stabilize ferroportin) because of the irreversible loss of copper binding ability. Mutant R701W, which was found in a heterozygous very young patient with severe neurological problems, was unable to complement per se but did so in the presence of copper-glutathione or when the yeast copper ATPase Ccc2p was co-expressed, indicating that the protein was structurally able to bind copper but that metal loading involving the mammalian copper ATPase ATP7B was impaired. Notably, R701W exerted a dominant negative effect on wild type, and it induced the subcellular relocalization of ATP7B. Our results constitute the first evidence of "functional silencing" of ATP7B as a novel molecular defect in aceruloplasminemia. The possibility to reverse the deleterious effects of some aceruloplasminemia mutations may disclose new possible therapeutic strategies.

Processing of hemojuvelin requires retrograde trafficking to the Golgi in HepG2 cells

Hemojuvelin (HJV) was recently identified as a critical regulator of iron homeostasis. It is either associated with cell membranes through a glycosylphosphatidylinositol anchor or released as a soluble form. Membrane-anchored HJV acts as a coreceptor for bone morphogenetic proteins and activates the transcription of hepcidin, a hormone that regulates iron efflux from cells. Soluble HJV antagonizes bone morphogenetic protein signaling and suppresses hepcidin expression. In this study, we examined the trafficking and processing of HJV. Cellular HJV reached the plasma membrane without obtaining complex oligosaccharides, indicating that HJV avoided Golgi processing. Secreted HJV, in contrast, has complex oligosaccharides and can be derived from HJV with high-mannose oligosaccharides at the plasma membrane. Our results support a model in which retrograde trafficking of HJV before cleavage is the predominant processing pathway. Release of HJV requires it to bind to the transmembrane receptor neogenin. Neogenin does not, however, play a role in HJV trafficking to the cell surface, suggesting that it could be involved either in retrograde trafficking of HJV or in cleavage leading to HJV release.

Ceruloplasmin expression by human peripheral blood lymphocytes: a new link between immunity and iron metabolism

Ceruloplasmin (CP) is a multicopper oxidase involved in the acute phase reaction to stress. Although the physiological role of CP is uncertain, its role in iron (Fe) homeostasis and protection against free radical-initiated cell injury has been widely documented. Previous studies showed the existence of two molecular isoforms of CP: secreted CP (sCP) and a membrane glycosylphosphatidylinositol (GPI)-anchored form of CP (GPI-CP). sCP is produced mainly by the liver and is abundant in human serum whereas GPI-CP is expressed in mammalian astrocytes, rat leptomeningeal cells, and Sertolli cells. Herein, we show using RT-PCR that human peripheral blood lymphocytes (huPBL) constitutively express the transcripts for both CP molecular isoforms previously reported. Also, expression of CP in huPBL is demonstrated by immunofluorescence with confocal microscopy and flow cytometry analysis using cells isolated from healthy blood donors with normal Fe status. Importantly, the results obtained show that natural killer cells have a significantly higher CP expression compared to all other major lymphocyte subsets. In this context, the involvement of lymphocyte-derived CP on host defense processes via its anti/prooxidant properties is proposed, giving further support for a close functional interaction between the immune system and the Fe metabolism.

Cys-881 is essential for the trafficking and secretion of truncated mutant ceruloplasmin in aceruloplasminemia

BACKGROUND/AIMS

Aceruloplasminemia is an inherited iron overload disorder caused by a mutation in the ceruloplasmin gene and characterized by iron accumulation in both the liver and brain. The aim of this study was to elucidate the molecular pathogenesis of aceruloplasminemia by a functional analysis of mutant ceruloplasmin.

METHODS

The effects of nonsense mutations including Y694ter, W858ter and R882ter were studied by the expression in cultured cells.

RESULTS

A biogenesis study demonstrated that the Y694ter and W858ter mutants showed protein synthesis identical to that of wild type protein, however, the mutants were retained in the endoplasmic reticulum (ER), while R882ter mutant was secreted out. Site-directed mutagenesis analyses suggested that Cys-881 was necessary for the secretion of the truncated ceruloplasmin. The W858ter mutant decreased viability in the transfected cells. The expression and the promoter activity of glucose-regulated protein 78 that is an ER stress sensor protein, were up-regulated in the transfected cells.

CONCLUSIONS

The truncated mutant containing Cys-881 was able to pass through the ER and was secreted, while the truncated mutant protein without Cys-881 appeared to accumulate in the ER thus leading to ER stress and eventually resulting in cell death.

Identification and in silico characterization of a novel compound heterozygosity associated with hereditary aceruloplasminemia

BACKGROUND

Hereditary aceruloplasminemia is an adult-onset autosomal recessive disease characterized by increased iron overload in the liver, pancreas, retina, and central nervous system. So far, 45 families with cases of aceruloplasminemia have been reported world-wide and mainly missense and nonsense mutations in the ceruloplasmin gene were detected.

MATERIAL AND METHODS

Here, we report the identification, clinical characterization, and in silico analysis of a novel compound heterozygosity in the ceruloplasmin gene of a 31-year-old man with iron overload.

RESULTS

Increased serum ferritin levels, elevated iron saturation, as well as results of iron quantification in the liver and magnetic resonance imaging-based measurement of T2 relaxation times of the substantia nigra consistently suggested iron overload. By sequencing the ceruloplasmin gene, so far unknown nucleotide replacements G229C, and C2131A were detected in exons 2 and 12, respectively. In silico analyses showed that the resulting amino acid changes Asp58His and Gln692Lys are located at highly conserved positions. The Asp58His mutation is located on the surface of the protein, alters polarity, and may interfere with copper incorporation or ceruloplasmin trafficking. The Gln692Lys mutation is mapped to a beta-strand of domain 4 and may lead to conformational change of the cupredoxin fold.

CONCLUSIONS

As causative for aceruloplasminemia, a formerly unknown compound heterozygosity in the ceruloplasmin gene was identified. In silico characterization suggests an impact on ceruloplasmin conformation and function.

Brain iron metabolism: neurobiology and neurochemistry

New findings obtained during the past years, especially the discovery of mutations in the genes associated with brain iron metabolism, have provided key insights into the homeostatic mechanisms of brain iron metabolism and the pathological mechanisms responsible for neurodegenerative diseases. The accumulated evidence demonstrates that misregulation in brain iron metabolism is one of the initial causes for neuronal death in some neurodegenerative disorders. The errors in brain iron metabolism found in these disorders have a multifactorial pathogenesis, including genetic and nongenetic factors. The disturbances of iron metabolism might occur at multiple levels, including iron uptake and release, storage, intracellular metabolism and regulation. It is the increased brain iron that triggers a cascade of deleterious events, leading to neuronal death in these diseases. In the article, the recent advances in studies on neurochemistry and neuropathophysiology of brain iron metabolism were reviewed.

Biochemical features of ceruloplasmin gene mutations linked to aceruloplasminemia

Aceruloplasminemia is a neurodegenerative disease characterized by parenchymal iron accumulation owing to mutations in the ceruloplasmin gene. Ceruloplasmin is expressed in the central nervous system in which most of the ceruloplasmin is located on the surface of astrocytes in a glycosylphosphatidylinositol (GPI)-anchored form. We herein describe the biochemical features of wild-type and mutant GPI-anchored ceruloplasmin. An overexpression of wild-type GPI-anchored ceruloplasmin in Chinese hamster ovary cells led to the formation of aggresome-like inclusions, especially in the presence of proteasome inhibitors. As expected from the properties of aggresomes, the inclusions were colocalized with gamma-tubulin and a disruption of microtubules using nocodazole blocked the formation of such inclusions. Aceruloplasminemia-linked mutant proteins failed to form such inclusions even after treatment with proteasomal inhibitors. An immunofluorescent analysis indicated that the mutant proteins were thus retained in the endoplasmic reticulum (ER), whereas the transfected cells showed a decreased viability. The expression of glucose-regulated protein 78 that is one of the ER stress sensor proteins, and the activity of glucose-regulated protein 78 promoter was upregulated in the cells transfected with the mutants. These findings indicated that when the overexpressed cytoplasmic wild-type ceruloplasmin was not subjected to degradation by the proteasome-ubiquitin system, then the wild-type protein was transported along the microtubules, thus forming inclusions at the microtubule organizing center, whereas the mutant ceruloplasmin failed to form any such inclusions, because the mutant protein might not have been translocated across the ER into the cytoplasm. Therefore, the mutant protein was considered to have accumulated in the ER thus leading to the ER stress, which resulted in cell death.

Iron metabolism in Parkinsonian syndromes

Growing evidence suggests an involvement of iron in the pathophysiology of neurodegenerative diseases. Several of the diseases are associated with parkinsonian syndromes, induced by degeneration of basal ganglia regions that contain the highest amount of iron within the brain. The group of neurodegenerative disorders associated with parkinsonian syndromes with increased brain iron content can be devided into two groups: (1) parkinsonian syndromes associated with brain iron accumulation, including Parkinson's disease, diffuse Lewy body disease, parkinsonian type of multiple system atrophy, progressive supranuclear palsy, corticobasal ganglionic degeneration, and Westphal variant of Huntington's disease; and (2) monogenetically caused disturbances of brain iron metabolism associated with parkinsonian syndromes, including aceruloplasminemia, hereditary ferritinopathies affecting the basal ganglia, and panthotenate kinase associated neurodegeneration type 2. Although it is still a matter of debate whether iron accumulation is a primary cause or secondary event in the first group, there is no doubt that iron-induced oxidative stress contributes to neurodegeneration. Parallels concerning pathophysiological as well as clinical aspects can be drawn between disorders of both groups. Results from animal models and reduction of iron overload combined with at least partial relief of symptoms by application of iron chelators in patients of the second group give hope that targeting the iron overload might be one possibility to slow down the neurodegenerative cascade also in the first group of inevitably progressive neurodegenerative disorders.

Role of iron in neurodegenerative disorders

Although the pathophysiology underlying a number of neurodegenerative diseases is complex and, in many aspects, only partly understood, increased iron levels in pathologically relevant brain areas and iron-mediated oxidative stress seem to play a central role in many of them. Much has been learned from monogenetically caused disturbances of brain iron metabolism including pantothenate kinase-associated neurodegeneration type 2, hereditary ferritinopathies affecting the basal ganglia, and aceruloplasminemia that may well be applied to the most common neurodegenerative disorders associated with brain iron accumulation including Parkinson disease and Alzheimer disease. Iron-mediated oxidative stress in neurodegenerative diseases caused by other genetic pathways like Huntington disease and Friedreich ataxia underscore the complex interaction of this trace metal and genetic variations. Therapeutical strategies derived from application of iron chelators in monogenetically caused disturbances of brain iron metabolism and new iron and oxidative stress diminishing substances in animal models of Parkinson disease are promising and warrant further investigational effort.

Hepatic iron overload associated with a decreased serum ceruloplasmin level in a novel clinical type of aceruloplasminemia

BACKGROUND & AIMS

Aceruloplasminemia is a novel hereditary iron overload disease caused by a mutation in the ceruloplasmin gene and characterized by a complete deficiency of serum ceruloplasmin and iron accumulation in the liver and brain.

METHODS

We herein studied a novel clinical type of aceruloplasminemia in which a low amount of ceruloplasmin was detected in the serum of a patient. The patient presented with an asymptomatic hepatic iron overload, retinal degeneration, and diabetes mellitus. Magnetic resonance imaging of the liver and basal ganglia showed T2-hypointensity signals associated with parenchymal iron accumulation because of an absence of the ferroxidase activity in ceruloplasmin.

RESULTS

A gene analysis showed a novel G969S mutation in the ceruloplasmin gene. A biochemical analysis of the patients' serum and a biogenesis study of G969S mutant ceruloplasmin using mammalian cell culture system resulted in the synthesis and secretion of only apoceruloplasmin without any ferroxidase activity.

CONCLUSIONS

This novel clinical type of aceruloplasminemia should therefore be considered in the differential diagnosis of unexplained hemochromatosis, which is associated with a decrease in the serum ceruloplasmin level.

Decreased hephaestin activity in the intestine of copper-deficient mice causes systemic iron deficiency

Copper and iron metabolism intersect in mammals. Copper deficiency simultaneously leads to decreased iron levels in some tissues and iron deficiency anemia, whereas it results in iron overload in other tissues such as the intestine and liver. The copper requirement of the multicopper ferroxidases hephaestin and ceruloplasmin likely explains this link between copper and iron homeostasis in mammals. We investigated the effect of in vivo and in vitro copper deficiency on hephaestin (Heph) expression and activity. C57BL/6J mice were separated into 2 groups on the day of parturition. One group was fed a copper-deficient diet and another was fed a control diet for 6 wk. Copper-deficient mice had significantly lower hephaestin and ceruloplasmin (approximately 50% of controls) ferroxidase activity. Liver hepcidin expression was significantly downregulated by copper deficiency (approximately 60% of controls), and enterocyte mRNA and protein levels of ferroportin1 were increased to 2.5 and 10 times, respectively, relative to controls, by copper deficiency, indicating a systemic iron deficiency in the copper-deficient mice. Interestingly, hephaestin protein levels were significantly decreased to approximately 40% of control, suggesting that decreased enterocyte copper content leads to decreased hephaestin synthesis and/or stability. We also examined the effect of copper deficiency on hephaestin in vitro in the HT29 cell line and found dramatically decreased hephaestin synthesis and activity. Both in vivo and in vitro studies indicate that copper is required for the proper processing and/or stability of hephaestin.

Novel mutation in the ceruloplasmin gene causing a cognitive and movement disorder with diabetes mellitus

In a Chinese woman who had diabetes mellitus, undetectable ceruloplasmin, hand tremor, neck dystonia, and cognitive disturbances, genetic analyses revealed a novel homozygous mutation (848G > C or W283S) in exon 5 in the ceruloplasmin gene. Another member with a milder phenotype was also affected by this mutation. The healthy sister was heterozygous at the same position. Aceruloplasminemia has not yet been reported in China. This case suggests that increased awareness should be paid to this disorder in the presence of the typical symptoms.

Functional relevance of ceruloplasmin mutations in Parkinson's disease

Increased iron levels of the substantia nigra and the discovery of ceruloplasmin mutations in patients with Parkinson's disease (PD) imply impaired iron metabolism in this neurodegenerative disorder. Ceruloplasmin has ferroxidase activity oxidizing iron(II) to iron(III). In the present study, we analyzed the amount of ceruloplasmin, iron, ferritin, and transferrin and the ceruloplasmin ferroxidase activity in serum of patients with the diagnosis of PD carrying the ceruloplasmin mutations I63T, D544E, and R793H. The impact of these missense mutations on the biosynthesis of holo-ceruloplasmin was investigated in cell culture experiments. Functional relevance was found for the ceruloplasmin mutations I63T and D544E. In vivo, the I63T mutation resulted in half the normal ceruloplasmin concentration and markedly reduced ferroxidase activity in serum from a heteroallelic PD patient. In cell culture, the I63T glycosylphosphatidylinositol (GPI)-linked ceruloplasmin isoform was retained in the endoplasmatic reticulum of human embryonic kidney cells. Furthermore, the D544E polymorphism resulted in significantly reduced serum ceruloplasmin levels and ferroxidase activity in heteroallelic patients and in expression of mainly apo-ceruloplasmin in cell culture. Our studies indicate that altered activity of ceruloplasmin may present a vulnerability factor for iron induced oxidative stress in PD.

Aceruloplasminemia in an asymptomatic patient with a new mutation. Diagnosis and family genetic analysis

A 39-year-old asymptomatic man showed elevated serum ferritin levels, mild hypertransaminasemia and serum ceruloplasmin almost undetectable. There was histological iron accumulation within the hepatocytes and also in the central nervous system (MRI). A genetic analysis revealed a new missense mutation in the ceruloplasmin gene. Two of the other four siblings were also affected by this mutation.

Aceruloplasminemia: an inherited neurodegenerative disease with impairment of iron homeostasis

In 1987, Miyajima et al. first characterized an autosomal recessive, adult-onset neurodegenerative disorder resembling Parkinson's disease associated with near-absent circulating serum ceruloplasmin levels. Coined "familial apoceruloplasmin deficiency", they described a patient with a presenting triad of diabetes mellitus, retinal degeneration, and neurodegeneration with blepharospasm. Neuropathological evaluation revealed abundant iron deposition in selected neurons of the basal ganglia and substantia nigra with associated neuronal dropout and spongioform degeneration without evidence of reactive gliosis. Subsequently, mutations in the ceruloplasmin gene have been determined to result in the excessive iron accumulation seen in the pancreas, retina, and brain. Elevated serum ferritin suggests a systemic iron overload syndrome, yet affected patients had low transferrin saturation and a mild anemia. This new disease, "aceruloplasminemia", reveals a role for ceruloplasmin as an essential ferroxidase critical for iron homeostasis. This multicopper oxidase promotes efficient iron efflux such that individuals lacking ceruloplasmin develop a presumed oxidative injury secondary to iron accumulation and significant neuronal damage. Aceruloplasminemic mice provide a valuable model to further study the mechanisms by which ceruloplasmin regulates iron trafficking and the role of iron in oxidative injury. Despite the dependence of ceruloplasmin on copper for its function, aceruloplasminemia represents an iron storage disease and not a defect in copper metabolism. However, recent evidence in Saccharomyces cerevisiae indicates that Fet3, the yeast homologue of ceruloplasmin, functions as an essential cuprous oxidase. Further investigation into the mechanisms by which ceruloplasmin regulates iron and copper homeostasis will provide valuable insight into the pathogenesis of metallo-mediated diseases and elucidate mechanisms for transition metal (copper, iron) neuropathology.

Role of copper in the proteosome-mediated degradation of the multicopper oxidase hephaestin

To elucidate the mechanisms of cuproprotein biosynthesis in the secretory pathway, a polyclonal antiserum was generated against hephaestin, a multicopper oxidase essential for enteric iron absorption. Immunoblot analysis and pulse-chase metabolic labeling revealed that hephaestin is synthesized as a single-chain polypeptide modified by N-linked glycosylation to a mature 161-kDa species. Cell surface biotinylation and immunofluorescent studies of polarized, differentiated colon carcinoma cells detected hephaestin on the basolateral surface under steady-state conditions. However, a decrease in the intracellular copper concentration resulted in a marked diminution in the abundance of this protein. Metabolic studies revealed no effect of decreased intracellular copper on the rate of hephaestin synthesis but a dramatic, specific, and reproducible increase in the turnover of the mature 161-kDa protein. Surprisingly, inhibitor studies revealed that this turnover occurs exclusively in the proteasome, and consistent with this finding, in vitro studies identified polyubiquitinated hephaestin under conditions abrogating copper incorporation into this protein. Taken together, these studies demonstrate the presence of a quality control system for posttranslational protein modification occurring beyond the endoplasmic reticulum that, in the case of hephaestin, directly links the rate of enteric iron uptake to nutritional copper status.

Hereditary Causes of Disturbed Iron Homeostasis in the Central Nervous System

Iron is essential for oxidation-reduction catalysis and bioenergetics; however, unless appropriately shielded, this metal plays a crucial role in the formation of toxic oxygen radicals that can attack all biological molecules. Organisms are equipped with specific proteins designed for iron acquisition, export and transport, and storage, as well as with sophisticated mechanisms that maintain the intracellular labile iron pool at an appropriate level. Despite these homeostatic mechanisms, organisms often face the threat of either iron deficiency or iron overload. This review describes several hereditary iron-overloading conditions that are confined to the brain. Recently, a mutation in the L-subunit of ferritin has been described that causes the formation of aberrant L-ferritin with an altered C-terminus. Individuals with this mutation in one allele of L-ferritin have abnormal aggregates of ferritin and iron in the brain, primarily in the globus pallidus. Patients with this dominantly inherited late-onset disease present with symptoms of extrapyramidal dysfunction. Mice with a targeted disruption of a gene for iron regulatory protein 2 (IRP2), a translational repressor of ferritin, misregulate iron metabolism in the intestinal mucosa and the central nervous system. Significant amounts of ferritin and iron accumulate in white matter tracts and nuclei, and adult IRP2-deficient mice develop a movement disorder consisting of ataxia, bradykinesia, and tremor. Mutations in the frataxin gene are responsible for Friedreich's ataxia, the most common of the inherited ataxias. Frataxin appears to regulate mitochondrial iron-sulfur cluster formation, and the neurologic and cardiac manifestations of Friedreich's ataxia are due to iron-mediated mitochondrial toxicity. Patients with Hallervorden-Spatz syndrome, an autosomal recessive, progressive neurodegenerative disorder, have mutations in a novel pantothenate kinase gene (PANK2). The cardinal feature of this extrapyramidal disease is pathologic iron accumulation in the globus pallidus. The defect in PANK2 is predicted to cause the accumulation of cysteine, which binds iron and causes oxidative stress in the iron-rich globus pallidus. Finally, aceruloplasminemia is an autosomal recessive disorder of iron metabolism caused by loss-of-function mutations in ceruloplasmin gene that leads to misregulation of both systemic and central nervous system iron trafficking. Affected individuals suffer from extrapyramidal signs, cerebellar ataxia, progressive neurodegeneration of retina, and diabetes mellitus. Excessive iron depositions are found in the brain, liver, pancreas, and other parenchymal cells, but plasma iron concentrations are decreased. These conditions are not common, but awareness about them is important for differential diagnosis of various neurodegenerative disorders.

Iron metabolism in the reticuloendothelial system

Comprised mainly of monocytes and tissue macrophages, the reticuloendothelial system (RES) plays two major roles in iron metabolism: it recycles iron from senescent red blood cells and it serves as a large storage depot for excess iron. Although iron recycling by the RES represents the largest pathway of iron efflux in the body, the precise mechanisms involved have remained elusive. However, studies characterizing the function and regulation of Nramp1, DMT1, HFE, FPN1, CD163, and hepcidin are rapidly expanding our knowledge of the molecular aspects of RE iron handling. This review summarizes fundamental physiological and biochemical aspects of iron metabolism in the RES and focuses on how recent studies have advanced our understanding of these areas. Also discussed are novel insights into the molecular mechanisms contributing to the abnormal RE iron metabolism characteristic of hereditary hemochromatosis and the anemia of chronic disease.

Mechanisms of biosynthesis of mammalian copper/zinc superoxide dismutase

Copper/zinc superoxide dismutase (SOD1) is an abundant intracellular enzyme with an essential role in antioxidant defense. The activity of SOD1 is dependent upon the presence of a bound copper ion incorporated by the copper chaperone for superoxide dismutase, CCS. To elucidate the cell biological mechanisms of this process, SOD1 synthesis and turnover were examined following 64Cu metabolic labeling of fibroblasts derived from CCS+/+ and CCS-/- embryos. The data indicate that copper is rapidly incorporated into both newly synthesized SOD1 and preformed SOD1 apoprotein, that each process is dependent upon CCS and that once incorporated, copper is unavailable for cellular exchange. The abundance of apoSOD1 is inversely proportional to the intracellular copper content and immunoblot and gel filtration analysis indicate that this apoprotein exists as a homodimer that is distinguishable from SOD1. Despite these distinct differences, the abundance and half-life of SOD1 is equivalent in CCS+/+ and CCS-/- fibroblasts, indicating that neither CCS nor copper incorporation has any essential role in the stability or turnover of SOD1 in vivo. Taken together, these data provide a cell biological model of SOD1 biosynthesis that is consistent with the concept of limited intracellular copper availability and indicate that the metallochaperone CCS is a critical determinant of SOD1 activity in mammalian cells. These kinetic and biochemical findings also provide an important framework for understanding the role of mutant SOD1 in the pathogenesis of familial amyotrophic lateral sclerosis.

Glycosylphosphatidylinositol-anchored ceruloplasmin is required for iron efflux from cells in the central nervous system

Ceruloplasmin (Cp) is a ferroxidase that converts highly toxic ferrous iron to its non-toxic ferric form. A glycosylphosphatidylinositol (GPI)-anchored form of this enzyme is expressed by astrocytes in the mammalian central nervous system, whereas the secreted form is expressed by the liver and found in serum. Lack of this enzyme results in iron accumulation in the brain and neurodegeneration. Herein, we show using astrocytes purified from the central nervous system of Cp-null mice that GPI-Cp is essential for iron efflux and not involved in regulating iron influx. We also show that GPI-Cp colocalizes on the astrocyte cell surface with the divalent metal transporter IREG1 and is physically associated with IREG1. In addition, IREG1 alone is unable to efflux iron from astrocytes in the absence of GPI-Cp or secreted Cp. We also provide evidence that the divalent metal influx transporter DMT1 is expressed by astrocytes and is likely to mediate iron influx into these glial cells. The coordinated actions of GPI-Cp and IREG1 may be required for iron efflux from neural cells, and disruption of this balance could lead to iron accumulation in the central nervous system and neurodegeneration.

Iron misregulation in the brain: a primary cause of neurodegenerative disorders

High iron concentrations in the brains of patients and the discovery of mutations in the genes associated with iron metabolism in the brain suggest that iron misregulation in the brain plays a part in neuronal death in some neurodegenerative disorders, such as Alzheimer's, Parkinson's, and Huntington's diseases and Hallervorden-Spatz syndrome. Iron misregulation in the brain may have genetic and non-genetic causes. The disrupted expression or function of proteins involved in iron metabolism increases the concentration of iron in the brain. Disturbances can happen at any of several stages in iron metabolism (including uptake and release, storage, intracellular metabolism, and regulation). Increased brain iron triggers a cascade of deleterious events that lead to neurodegeneration. An understanding of the process of iron regulation in the brain, the proteins important in this process, and the effects of iron misregulation could help to treat or prevent neurodegenerative disorders.

Metabolic effects of infection on protein and energy status

A review of some of the seminal studies of metabolism during various infections indicates that similar patterns of metabolic alterations occur during these illnesses. This patterned metabolic array occurs whether the infection is caused by a gram-positive or a gram-negative bacterium, a rickettsia or a virus, or is respiratory or systemic. In all instances, the previously healthy host responds to infection with cytokine-mediated alterations that appear to occur in proportion to the infectious challenge and to the likelihood of death. These alterations also can occur in the vaccinated host, if the infectious challenge is sufficiently great. Because of their widespread occurrence and seemingly ingrained status, these metabolic alterations may be presumed to be of survival benefit to the host. Whether this patterned array of metabolic sequelae is of benefit to the host, or even to the species, its widespread and systematic occurrence allows it to be of value in assessing the safety and efficacy of vaccines and drugs to prevent or treat a wide variety of infections. In this era of bioterrorism, wherein drugs and vaccines may have to be approved for human use without clinical trials and solely on the basis of animal data, these cytokine-mediated metabolic sequelae can aid in the rational selection of drug and vaccine candidates.

Mechanisms of copper incorporation into human ceruloplasmin

Ceruloplasmin is a multicopper oxidase essential for normal iron homeostasis. To elucidate the mechanisms of copper incorporation into this protein, holoceruloplasmin biosynthesis was examined by immunoblot analysis and (64)Cu metabolic labeling of Chinese hamster ovary cells transfected with cDNAs encoding wild-type or mutant ceruloplasmin. This analysis reveals that the incorporation of copper into newly synthesized apoceruloplasmin in vivo results in a detectable conformational change in the protein. Strikingly, despite the unique functional role of each copper site within ceruloplasmin, metabolic studies indicate that achieving this final conformation-driven state requires the occupation of all six copper-binding sites with no apparent hierarchy for copper incorporation at any given site. Consistent with these findings a missense mutation (G631R), resulting in aceruloplasminemia and predicted to alter the interactions at a single type I copper-binding site, results in the synthesis and secretion only of apoceruloplasmin. Analysis of copper incorporation into apoceruloplasmin in vitro reveals that this process is cooperative and that the failure of copper incorporation into copper-binding site mutants observed in vivo is intrinsic to the mutant proteins. These findings reveal a precise and sensitive mechanism for the formation of holoceruloplasmin under the limiting conditions of copper availability within the cell that may be generally applicable to the biosynthesis of cuproproteins within the secretory pathway.