Stromal cell-derived receptor 2 and cytochrome b561 are functional ferric reductases

Ischemia-inhibited ferric chelate reductase 1 improves ferroptosis-mediated intestinal ischemia injury via Hippo signaling

The precise mechanism of ferroptosis as a regulatory cell death in intestinal ischemia injury induced by vascular intestinal obstruction (Vio) remains to be elucidated. Here, we evaluated iron levels, glutathione peroxidase 4 (GPX4) and Acyl-CoA synthetase long-chain family member 4 (ACSL4) changes after intestinal ischemia injury to validate ferroptosis. As an enzyme for Fe3+ reduction to Fe2+, Ferric Chelate Reductase 1 (FRRS1) is involved in the electron transport chain and the tricarboxylic acid (TCA) cycle in mitochondria. However, whether it is involved in ferroptosis and its role in intestinal ischemia injury need to be clarified. In the present study, FRRS1 was overexpressed in vivo and in vitro. The results showed that overexpression of FRRS1 prevented ischemia-induced iron levels, reactive oxygen species (ROS) production, lipid peroxidation, inflammatory responses, and cell death. Meanwhile, FRRS1 overexpression promoted GPX4 expression and suppressed ACSL4 levels. Further studies revealed that FRRS1 overexpression inhibited the activity of large tumor suppressor 1 (LATS1) / Yes-associated protein (YAP) / transcriptional co-activator with PDZ-binding motif (TAZ), a key component of Hippo signaling. In conclusion, this study demonstrates that FRRS1 is intimately involved in the inhibition of ferroptosis and thus protection of the intestine from intestinal ischemia injury, its downstream mechanism was related to Hippo signaling. These data provide new sight for the prevention and treatment of intestinal ischemia injury.

A CYBDOM protein impacts iron homeostasis and primary root growth under phosphate deficiency in Arabidopsis

Arabidopsis primary root growth response to phosphate (Pi) deficiency is mainly controlled by changes in apoplastic iron (Fe). Upon Pi deficiency, apoplastic Fe deposition in the root apical meristem activates pathways leading to the arrest of meristem maintenance and inhibition of cell elongation. Here, we report that a member of the uncharacterized cytochrome b561 and DOMON domain (CYBDOM) protein family, named CRR, promotes iron reduction in an ascorbate-dependent manner and controls apoplastic iron deposition. Under low Pi, the crr mutant shows an enhanced reduction of primary root growth associated with increased apoplastic Fe in the root meristem and a reduction in meristematic cell division. Conversely, CRR overexpression abolishes apoplastic Fe deposition rendering primary root growth insensitive to low Pi. The crr single mutant and crr hyp1 double mutant, harboring a null allele in another member of the CYDOM family, shows increased tolerance to high-Fe stress upon germination and seedling growth. Conversely, CRR overexpression is associated with increased uptake and translocation of Fe to the shoot and results in plants highly sensitive to Fe excess. Our results identify a ferric reductase implicated in Fe homeostasis and developmental responses to abiotic stress, and reveal a biological role for CYBDOM proteins in plants.

Comparison of insect and human cytochrome b561 proteins: Insights into candidate ferric reductases in insects

Cytochrome b561 (cytb561) proteins comprise a family of transmembrane oxidoreductases that transfer single electrons across a membrane. Most eukaryotic species, including insects, possess multiple cytb561 homologs. To learn more about this protein family in insects, we carried out a bioinformatics-based investigation of cytb561 family members from nine species representing eight insect orders. We performed a phylogenetic analysis to classify insect cytb561 ortholog groups. We then conducted sequence analyses and analyzed protein models to predict structural elements that may impact the biological functions and localization of these proteins, with a focus on possible ferric reductase activity. Our study revealed three ortholog groups, designated CG1275, Nemy, and CG8399, and a fourth group of less-conserved genes. We found that CG1275 and Nemy proteins are similar to a human ferric reductase, duodenal cytochrome b561 (Dcytb), and have many conserved amino acid residues that function in substrate binding in Dcytb. Notably, CG1275 and Nemy proteins contain a conserved histidine and other residues that play a role in ferric ion reduction by Dcytb. Nemy proteins were distinguished by a novel cysteine-rich cytoplasmic loop sequence. CG8399 orthologs are similar to a putative ferric reductase in humans, stromal cell-derived receptor 2. Like other members of the CYBDOM class of cytb561 proteins, these proteins contain reeler, DOMON, and cytb561 domains. Drosophila melanogaster CG8399 is the only insect cytb561 with known ferric reductase activity. Our investigation of the DOMON domain in CG8399 proteins revealed a probable heme-binding site and a possible site for ferric reduction. The fourth group includes a subgroup of proteins with a conserved "KXXXXKXH" non-cytoplasmic loop motif that may be a substrate binding site and is present in a potential ferric reductase, human tumor suppressor cytochrome b561. This study provides a foundation for future investigations of the biological functions of cytb561 genes in insects.

OsCYBDOMG1, a cytochrome b561 domain-containing protein, regulates salt tolerance and grain yield in rice

OsCYBDOMG1 positively regulates salt tolerance, plant growth, and grain yield by affecting ascorbate biosynthesis and redox state. Soil salinity is a major abiotic stress affecting rice growth and productivity. Many genes involved in the salt stress response have been identified, but the precise mechanisms underlying salt tolerance remain unclear. In this study, we isolated a salt-sensitive mutant of rice, rss5, which exhibited more severe wilting and chlorosis with a significant increase in lipid peroxidation, electrolyte leakage, and shoot Na⁺ concentration compared to wild-type plants. Map-based cloning, MutMap analysis, and genetic complementation revealed that a single-nucleotide mutation in a gene encoding a cytochrome b561 domain-containing protein (OsCYBDOMG1) was responsible for the mutant phenotype of rss5. The OsCYBDOMG1 gene was mainly expressed in young shoots and nodes, and the encoded protein was principally located in the plasma membrane and endoplasmic reticulum. Mutations of OsCYBDOMG1 resulted in decreased ascorbic acid (AsA) content and AsA/DHA (dehydroascorbate) ratio, which led to increased H₂O₂ accumulation and reduced salt tolerance. Moreover, plant growth and grain yield of rss5 and the OsCYBDOMG1 knockout mutant (cr-1) were significantly decreased compared to wild-type plants under normal conditions. The elite haplotype of OsCYBDOMG1 associated with higher salt tolerance and grain width and weight was mainly existed in japonica varieties. These results suggest that OsCYBDOMG1 plays an important role in the regulation of salt tolerance, plant growth, and grain yield in rice.

Iron Homeostasis in Insects

Iron is an essential micronutrient for all types of organisms; however, iron has chemical properties that can be harmful to cells. Because iron is both necessary and potentially damaging, insects have homeostatic processes that control the redox state, quantity, and location of iron in the body. These processes include uptake of iron from the diet, intracellular and extracellular iron transport, and iron storage. Early studies of iron-binding proteins in insects suggested that insects and mammals have surprisingly different mechanisms of iron homeostasis, including different primary mechanisms for exporting iron from cells and for transporting iron from one cell to another, and subsequent studies have continued to support this view. This review summarizes current knowledge about iron homeostasis in insects, compares insect and mammalian iron homeostasis mechanisms, and calls attention to key remaining knowledge gaps.

Analysis of genome and methylation changes in Chinese indigenous chickens over time provides insight into species conservation

Conservation of natural resources is a vital and challenging task. Numerous animal genetic resources have been effectively conserved worldwide. However, the effectiveness of conservation programmes and the variation information of species have rarely been evaluated. Here, we performed whole-genome and whole-genome bisulfite sequencing of 90 Chinese indigenous chickens, which belonged to the Tibetan, Wenchang and Bian chicken breeds, and have been conserved under different conservation programmes. We observed that low genetic diversity and high DNA methylation variation occurs during ex situ in vivo conservation, while higher genetic diversity and differentiation occurs during in situ conservation. Further analyses revealed that most DNA methylation signatures are unique within ex situ in vivo conservation. Moreover, a high proportion of differentially methylated regions is found in genomic selection regions, suggesting a link between the effects of genomic variation and DNA methylation. Altogether our findings provide valuable information about genetic and DNA methylation variations during different conservation programmes, and hold practical relevance for species conservation.

Comparisons of genomic and methylomic changes during the conservation of indigenous chicken breeds in China provide insight into conservation programmes for these breeds and their adaptations to unique environments.

Metabolic-scale gene activation screens identify SLCO2B1 as a heme transporter that enhances cellular iron availability

Iron is the most abundant transition metal essential for numerous cellular processes. Although most mammalian cells acquire iron through transferrin receptors, molecular players of iron utilization under iron restriction are incompletely understood. To address this, we performed metabolism-focused CRISPRa gain-of-function screens, which revealed metabolic limitations under stress conditions. Iron restriction screens identified not only expected members of iron utilization pathways but also SLCO2B1, a poorly characterized membrane carrier. SLCO2B1 expression is sufficient to increase intracellular iron, bypass the essentiality of the transferrin receptor, and enable proliferation under iron restriction. Mechanistically, SLCO2B1 mediates heme analog import in cellular assays. Heme uptake by SLCO2B1 provides sufficient iron for proliferation through heme oxygenases. Notably, SLCO2B1 is predominantly expressed in microglia in the brain, and primary Slco2b1^-/- mouse microglia exhibit strong defects in heme analog import. Altogether, our work identifies SLCO2B1 as a microglia-enriched plasma membrane heme importer and provides a genetic platform to identify metabolic limitations under stress conditions.

Machine learning prediction and tau-based screening identifies potential Alzheimer's disease genes relevant to immunity

With increased research funding for Alzheimer's disease (AD) and related disorders across the globe, large amounts of data are being generated. Several studies employed machine learning methods to understand the ever-growing omics data to enhance early diagnosis, map complex disease networks, or uncover potential drug targets. We describe results based on a Target Central Resource Database protein knowledge graph and evidence paths transformed into vectors by metapath matching. We extracted features between specific genes and diseases, then trained and optimized our model using XGBoost, termed MPxgb(AD). To determine our MPxgb(AD) prediction performance, we examined the top twenty predicted genes through an experimental screening pipeline. Our analysis identified potential AD risk genes: FRRS1, CTRAM, SCGB3A1, FAM92B/CIBAR2, and TMEFF2. FRRS1 and FAM92B are considered dark genes, while CTRAM, SCGB3A1, and TMEFF2 are connected to TREM2-TYROBP, IL-1β-TNFα, and MTOR-APP AD-risk nodes, suggesting relevance to the pathogenesis of AD.

LncRNA HOXD-AS1 affects proliferation and apoptosis of cervical cancer cells by promoting FRRS1 expression via transcription factor ELF1

To investigate the function of lncRNA HOXD-AS1 in cervical squamous cell carcinoma (CESC) and the underlying mechanism. The expressions of HOXD-AS1 and FRRS1 were analyzed on the online software GEPIA based on CESC-related information in The Cancer Genome Atlas (TCGA). Cervical cancer cells (SiHa and Hela) were accordingly transfected with pCDNA3.1-HOXD-AS1, sh-HOXD-AS1, sh-FRRS1 or pCDNA3.1-ELF1. After cell transfection, CCK-8, EDU and flow cytometry were applied for measurement of cell vitality, quantity and apoptosis, respectively. The relationship between HOXD-AS1 and FRRS1 was predicted on the online software LncMap and further verified by RNA binding protein immunoprecipitation. Nude mice were injected with stabilized SiHa cells transfected with sh-HOXD-AS1 to assess the tumorigenic ability of HOXD-AS1 in vivo. Immunohistochemistry detected the expression of the proliferation marker Ki-67. The levels of HOXD-AS1, ELF1 and FRRS1 were measured in vivo and in vitro. HOXD-AS1 and FRRS1 were overexpressed in CESC. After transfection of sh-HOXD-AS1, sh-ELF1 or sh-FRRS1, the proliferation of SiHa and Hela cells was inhibited and their apoptosis was promoted; while HOXD-AS1 overexpression had opposite effects on CESC development. Co-transfection of sh-FRRS1 and pCDNA3.1-HOXD-AS1 could abolish the tumor suppressive effect of FRRS1 knockdown. HOXD-AS1 elevated the level of FRRS1 by binding ELF1. Furthermore, HOXD-AS1 contributed to the CESC growth in mouse models. HOXD-AS1 promotes CESC by up-regulating FRRS1 via ELF1.

Transcriptome Profiling of Atlantic Salmon (Salmo salar) Parr With Higher and Lower Pathogen Loads Following Piscirickettsia salmonis Infection

Salmonid rickettsial septicemia (SRS), caused by Piscirickettsia salmonis, is one of the most devastating diseases of salmonids. However, the transcriptomic responses of Atlantic salmon (Salmon salar) in freshwater to an EM-90-like isolate have not been explored. Here, we infected Atlantic salmon parr with an EM-90-like isolate and conducted time-course qPCR analyses of pathogen load and four biomarkers (campb, hampa, il8a, tlr5a) of innate immunity on the head kidney samples. Transcript expression of three of these genes (except hampa), as well as pathogen level, peaked at 21 days post-injection (DPI). Multivariate analyses of infected individuals at 21 DPI revealed two infection phenotypes [lower (L-SRS) and higher (H-SRS) infection level]. Five fish from each group (Control, L-SRS, and H-SRS) were selected for transcriptome profiling using a 44K salmonid microarray platform. We identified 1,636 and 3,076 differentially expressed probes (DEPs) in the L-SRS and H-SRS groups compared with the control group, respectively (FDR = 1%). Gene ontology term enrichment analyses of SRS-responsive genes revealed the activation of a large number of innate (e.g. “phagocytosis”, “defense response to bacterium”, “inflammatory response”) and adaptive (e.g. “regulation of T cell activation”, “antigen processing and presentation of exogenous antigen”) immune processes, while a small number of general physiological processes (e.g. “apoptotic process”, development and metabolism relevant) was enriched. Transcriptome results were confirmed by qPCR analyses of 42 microarray-identified transcripts. Furthermore, the comparison of individuals with differing levels of infection (H-SRS vs. L-SRS) generated insights into the biological processes possibly involved in disease resistance or susceptibility. This study demonstrated a low mortality (~30%) EM-90-like infection model and broadened the current understanding of molecular pathways underlying P. salmonis-triggered responses of Atlantic salmon, identifying biomarkers that may assist to diagnose and combat this pathogen.

Of mitochondrion and COVID-19

COVID-19, a pandemic disease caused by a viral infection, is associated with a high mortality rate. Most of the signs and symptoms, e.g. cytokine storm, electrolytes imbalances, thromboembolism, etc., are related to mitochondrial dysfunction. Therefore, targeting mitochondrion will represent a more rational treatment of COVID-19. The current work outlines how COVID-19's signs and symptoms are related to the mitochondrion. Proper understanding of the underlying causes might enhance the opportunity to treat COVID-19.

Cell Death-Osis of Dopaminergic Neurons and the Role of Iron in Parkinson's Disease

Significance: There is still no cure for neurodegenerative diseases, such as Parkinson's disease (PD). Current treatments are based on the attempt to reduce dopaminergic neuronal loss, and multidisciplinary approaches have been used to provide only a temporary symptoms' relief. In addition to the difficulties of drugs developed against PD to access the brain, the specificity of those inhibitory compounds could be a concern. This because neurons might degenerate by activating distinct signaling pathways, which are often initiated by the same stimulus. Recent Advances: Apoptosis, necroptosis, and ferroptosis were shown to significantly contribute to PD progression and, so far, are the main death programs described as capable to alter brain homeostasis. Their activation is characterized by different biochemical and morphological features, some of which might even share the same molecular players. Critical Issues: If there is a pathological need to engage, in PD, multiple death programs, sequentially or simultaneously, is not clear yet. Possibly the activation of apoptosis, necroptosis, and/or ferroptosis correlates to different PD stages and symptom severities. This would imply that the efficacy of therapeutic approaches against neuronal death might depend on the death program they target and the relevance of this death pathway on a specific PD phase. Future Directions: In this review, we describe the molecular mechanisms underlying the activation of apoptosis, necroptosis, and ferroptosis in PD. Understanding the interrelationship between different death pathways' activation in PD is of utmost importance for the development of therapeutic approaches against disease progression. Antioxid. Redox Signal. 35, 453-473.

Functional Assembly of Caenorhabditis elegans Cytochrome b-2 (Cecytb-2) into Phospholipid Bilayer Nanodisc with Enhanced Iron Reductase Activity

Among seven homologs of cytochrome b₅₆₁ in a model organism C. elegans, Cecytb-2 was confirmed to be expressed in digestive organs and was considered as a homolog of human Dcytb functioning as a ferric reductase. Cecytb-2 protein was expressed in Pichia pastoris cells, purified, and reconstituted into a phospholipid bilayer nanodisc. The reconstituted Cecytb-2 in nanodisc environments was extremely stable and more reducible with ascorbate than in a detergent-micelle state. We confirmed the ferric reductase activity of Cecytb-2 by analyzing the oxidation of ferrous heme upon addition of ferric substrate under anaerobic conditions, where clear and saturable dependencies on the substrate concentrations following the Michaelis–Menten equation were observed. Further, we confirmed that the ferric substrate was converted to a ferrous state by using a nitroso-PSAP assay. Importantly, we observed that the ferric reductase activity of Cecytb-2 became enhanced in the phospholipid bilayer nanodisc.

Direct measurements of ferric reductase activity of human 101F6 and its enhancement upon reconstitution into phospholipid bilayer nanodisc

We studied human 101F6 protein to clarify its physiological function as a ferric reductase and its relationship to tumor suppression activity. We found for the first time that purified 101F6 both in detergent micelle state and in phospholipid bilayer nanodisc state has an authentic ferric reductase activity by single turnover kinetic analyses. The kinetic analysis on the ferrous heme oxidation of reduced 101F6 upon the addition of a ferric substrate, ferric ammonium citrate (FAC), showed concentration-dependent accelerations of its reaction with reasonable values of K_M and V_max. We further verified the authenticity of the ferric reductase activity of 101F6 using nitroso-PSAP as a Fe²⁺-specific colorimetric chelator. 101F6 in nanodisc state showed higher efficiency for FAC than in detergent micelle state.

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Human tumor suppressor 101F6 protein was reconstituted into nanodisc.

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101F6 functions as a ferric reductase both in detergent micelle and in nanodisc.

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101F6 in nanodisc showed higher efficiency in the reductase activity.

Transcriptomic response and hydrocarbon accumulation in the eastern oyster (Crassostrea virginica) exposed to crude oil

The adverse effect of crude oil on marine invertebrates is well known. To have a better understanding of its effects on marine invertebrates, Crassostrea virginica was exposed to different concentrations (50, 100 and 200 μg/L) of a mixture of super-light and light crude oil for two weeks, evaluating the transcriptomic response of the digestive gland using RNA-Seq and their accumulation in soft tissues. A total of 33,469,374 reads were assembled, which resulted in 61,356 genome assemblies ('Genes'). Trinotate was used for transcript annotation. At the end of this process, 86,409 transcripts were maintained, comprising a broad set of enzymes from xenobiotics metabolism, oxidative stress, stress and immune responses, and energetic metabolism. The enrichment analysis revealed a change in biological processes and molecular functions, finding from 100 to 200 μg/L. Moreover, the differential gene expression analysis showed a dose-dependent transcriptional response, generally up to 100 μg/L and in some cases up to 200 μg/L, which suggested that oysters' response decreased after 100 μg/L; the analysis of crude oil presence in soft tissues indicated that C. virginica is a suitable candidate for ecotoxicology. Finally, these results should contribute to expanding current genomic resources for C. virginica. Furthermore, they will help to develop new studies in aquatic toxicology focused on knowledge in depth of metabolic pathways, jointly with other approaches (such as proteomics) to allow obtaining a complete idea about the eastern oyster response to crude oil.

Multilevel Impacts of Iron in the Brain: The Cross Talk between Neurophysiological Mechanisms, Cognition, and Social Behavior

Iron is a critical element for most organisms, which plays a fundamental role in the great majority of physiological processes. So much so, that disruption of iron homeostasis has severe multi-organ impacts with the brain being particularly sensitive to such modifications. More specifically, disruption of iron homeostasis in the brain can affect neurophysiological mechanisms, cognition, and social behavior, which eventually contributes to the development of a diverse set of neuro-pathologies. This article starts by exploring the mechanisms of iron action in the brain and follows with a discussion on cognitive and behavioral implications of iron deficiency and overload and how these are framed by the social context. Subsequently, we scrutinize the implications of the disruption of iron homeostasis for the onset and progression of psychosocial disorders. Lastly, we discuss the links between biological, psychological, and social dimensions and outline potential avenues of research. The study of these interactions could ultimately contribute to a broader understanding of how individuals think and act under physiological and pathophysiological conditions.

Brain iron transport

Brain iron is a crucial participant and regulator of normal physiological activity. However, excess iron is involved in the formation of free radicals, and has been associated with oxidative damage to neuronal and other brain cells. Abnormally high brain iron levels have been observed in various neurodegenerative diseases, including neurodegeneration with brain iron accumulation, Alzheimer's disease, Parkinson's disease and Huntington's disease. However, the key question of why iron levels increase in the relevant regions of the brain remains to be answered. A full understanding of the homeostatic mechanisms involved in brain iron transport and metabolism is therefore critical not only for elucidating the pathophysiological mechanisms responsible for excess iron accumulation in the brain but also for developing pharmacological interventions to disrupt the chain of pathological events occurring in these neurodegenerative diseases. Numerous studies have been conducted, but to date no effort to synthesize these studies and ideas into a systematic and coherent summary has been made, especially concerning iron transport across the luminal (apical) membrane of the capillary endothelium and the membranes of different brain cell types. Herein, we review key findings on brain iron transport, highlighting the mechanisms involved in iron transport across the luminal (apical) as well as the abluminal (basal) membrane of the blood-brain barrier, the blood-cerebrospinal fluid barrier, and iron uptake and release in neurons, oligodendrocytes, astrocytes and microglia within the brain. We offer suggestions for addressing the many important gaps in our understanding of this important topic, and provide new insights into the potential causes of abnormally increased iron levels in regions of the brain in neurodegenerative disorders.

Targeting iron metabolism in drug discovery and delivery

Iron fulfils a central role in many essential biochemical processes in human physiology; thus, proper processing of iron is crucial. Although iron metabolism is subject to relatively strict physiological control, numerous disorders, such as cancer and neurodegenerative diseases, have recently been linked to deregulated iron homeostasis. Consequently, iron metabolism constitutes a promising and largely unexploited therapeutic target for the development of new pharmacological treatments for these diseases. Several iron metabolism-targeted therapies are already under clinical evaluation for haematological disorders, and these and newly developed therapeutic agents are likely to have substantial benefit in the clinical management of iron metabolism-associated diseases, for which few efficacious treatments are currently available.

Development of a RNA-Seq Based Prognostic Signature in Lung Adenocarcinoma

Background

Precision therapy for lung cancer will require comprehensive genomic testing to identify actionable targets as well as ascertain disease prognosis. RNA-seq is a robust platform that meets these requirements, but microarray-derived prognostic signatures are not optimal for RNA-seq data. Thus, we undertook the first prognostic analysis of lung adenocarcinoma RNA-seq data and generated a prognostic signature.

Methods

Lung adenocarcinoma RNA-seq and clinical data from The Cancer Genome Atlas (TCGA) were divided chronologically into training (n = 255) and validation (n = 157) cohorts. In the training cohort, prognostic association was assessed by univariate Cox analysis. A prognostic signature was built with stepwise multivariable Cox analysis. Outcomes by risk group, stage, and mutation status were analyzed with Kaplan-Meier and multivariable Cox analyses. All the statistical tests were two-sided.

Results

In the training cohort, 96 genes had prognostic association with P values of less than or equal to 1.00x10^-4, including five long noncoding RNAs (lncRNAs). Stepwise regression generated a four-gene signature, including one lncRNA. Signature high-risk cases had worse overall survival (OS) in the TCGA validation cohort (hazard ratio [HR] = 3.07, 95% confidence interval [CI] = 2.00 to 14.62) and a University of Michigan institutional cohort (n = 67; HR = 2.05, 95% CI = 1.18 to 4.55), and worse metastasis-free survival in the TCGA validation cohort (HR = 3.05, 95% CI = 2.31 to 13.37). The four-gene prognostic signature also statistically significantly stratified overall survival in important clinical subsets, including stage I (HR = 2.78, 95% CI = 1.91 to 11.13), EGFR wild-type (HR = 3.01, 95% CI = 1.73 to 14.98), and EGFR mutant (HR = 8.99, 95% CI = 62.23 to 141.44). The four-gene prognostic signature also stood out on top when compared with other prognostic signatures.

Conclusions

Here, we present the first RNA-seq prognostic signature for lung adenocarcinoma that can provide a powerful prognostic tool for precision oncology as part of an integrated RNA-seq clinical sequencing program.

Genome-wide analysis of the zebrafish Klf family identifies two genes important for erythroid maturation

Krüppel-like transcription factors (Klfs), each of which contains a CACCC-box binding domain, have been investigated in a variety of developmental processes, such as angiogenesis, neurogenesis and somatic-cell reprogramming. However, the function and molecular mechanism by which the Klf family acts during developmental hematopoiesis remain elusive. Here, we report identification of 24 Klf family genes in zebrafish using bioinformatics. Gene expression profiling shows that 6 of these genes are expressed in blood and/or vascular endothelial cells during embryogenesis. Loss of function of 2 factors (klf3 or klf6a) leads to a decreased number of mature erythrocytes. Molecular studies indicate that both Klf3 and Klf6a are essential for erythroid cell differentiation and maturation but that these two proteins function in distinct manners. We find that Klf3 inhibits the expression of ferric-chelate reductase 1b (frrs1b), thereby promoting the maturation of erythroid cells, whereas Klf6a controls the erythroid cell cycle by negatively regulating cdkn1a expression to determine the rate of red blood cell proliferation. Taken together, our study provides a global view of the Klf family members that contribute to hematopoiesis in zebrafish and sheds new light on the function and molecular mechanism by which Klf3 and Klf6a act during erythropoiesis in vertebrates.

Duodenal cytochrome b (DCYTB) in iron metabolism: an update on function and regulation

Iron and ascorbate are vital cellular constituents in mammalian systems. The bulk-requirement for iron is during erythropoiesis leading to the generation of hemoglobin-containing erythrocytes. Additionally, both iron and ascorbate are required as co-factors in numerous metabolic reactions. Iron homeostasis is controlled at the level of uptake, rather than excretion. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance non-heme iron absorption in the gut, ascorbate regulates iron homeostasis. The involvement of ascorbate in dietary iron absorption extends beyond the direct chemical reduction of non-heme iron by dietary ascorbate. Among other activities, intra-enterocyte ascorbate appears to be involved in the provision of electrons to a family of trans-membrane redox enzymes, namely those of the cytochrome b₅₆₁ class. These hemoproteins oxidize a pool of ascorbate on one side of the membrane in order to reduce an electron acceptor (e.g., non-heme iron) on the opposite side of the membrane. One member of this family, duodenal cytochrome b (DCYTB), may play an important role in ascorbate-dependent reduction of non-heme iron in the gut prior to uptake by ferrous-iron transporters. This review discusses the emerging relationship between cellular iron homeostasis, the emergent “IRP1-HIF2α axis”, DCYTB and ascorbate in relation to iron metabolism.

Molecular mechanisms of non-transferrin-bound and transferring-bound iron uptake in primary hippocampal neurons

The molecular mechanisms of iron trafficking in neurons have not been elucidated. In this study, we characterized the expression and localization of ferrous iron transporters Zip8, Zip14 and divalent metal transporter 1 (DMT1), and ferrireductases Steap2 and stromal cell-derived receptor 2 in primary rat hippocampal neurons. Steap2 and Zip8 partially co-localize, indicating these two proteins may function in Fe(3+) reduction prior to Fe(2+) permeation. Zip8, DMT1, and Steap2 co-localize with the transferrin receptor/transferrin complex, suggesting they may be involved in transferrin receptor/transferrin-mediated iron assimilation. In brain interstitial fluid, transferring-bound iron (TBI) and non-transferrin-bound iron (NTBI) exist as potential iron sources. Primary hippocampal neurons exhibit significant iron uptake from TBI (Transferrin-(59) Fe(3+)) and NTBI, whether presented as (59) Fe(2+) -citrate or (59) Fe(3+) -citrate; reductase-independent (59) Fe(2+) uptake was the most efficient uptake pathway of the three. Kinetic analysis of Zn(2+) inhibition of Fe(2+) uptake indicated that DMT1 plays only a minor role in the uptake of NTBI. In contrast, localization and knockdown data indicate that Zip8 makes a major contribution. Data suggest also that cell accumulation of (59) Fe from TBI relies at least in part on an endocytosis-independent pathway. These data suggest that Zip8 and Steap2 play a major role in iron accumulation from NTBI and TBI by hippocampal neurons. Analysis of the expression and localization of known iron uptake transporters demonstrated that Zip8 makes a major contribution to iron accumulation in primary cultures of rat embryonic hippocampal neurons. These cells exhibit uptake pathways for ferrous and ferric iron (non-transferrin-bound iron, NTBI in figure) and for transferrin-bound iron; the ferrireductases Steap2 and SDR2 support the uptake of ferric iron substrates. Zip8 and Steap2 are strongly expressed in the plasma membrane of both soma and processes, implying a crucial role in iron accumulation from NTBI and transferrin-bound iron (TBI) by hippocampal neurons.

The active role of vitamin C in mammalian iron metabolism: much more than just enhanced iron absorption!

Ascorbate is a cofactor in numerous metabolic reactions. Humans cannot synthesize ascorbate owing to inactivation of the gene encoding the enzyme l-gulono-γ-lactone oxidase, which is essential for ascorbate synthesis. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance nonheme iron absorption in the gut, ascorbate within mammalian systems can regulate cellular iron uptake and metabolism. Ascorbate modulates iron metabolism by stimulating ferritin synthesis, inhibiting lysosomal ferritin degradation, and decreasing cellular iron efflux. Furthermore, ascorbate cycling across the plasma membrane is responsible for ascorbate-stimulated iron uptake from low-molecular-weight iron-citrate complexes, which are prominent in the plasma of individuals with iron-overload disorders. Importantly, this iron-uptake pathway is of particular relevance to astrocyte brain iron metabolism and tissue iron loading in disorders such as hereditary hemochromatosis and β-thalassemia. Recent evidence also indicates that ascorbate is a novel modulator of the classical transferrin-iron uptake pathway, which provides almost all iron for cellular demands and erythropoiesis under physiological conditions. Ascorbate acts to stimulate transferrin-dependent iron uptake by an intracellular reductive mechanism, strongly suggesting that it may act to stimulate iron mobilization from the endosome. The ability of ascorbate to regulate transferrin iron uptake could help explain the metabolic defect that contributes to ascorbate-deficiency-induced anemia.

How are cytochrome b561 electron currents controlled by membrane voltage and substrate availability?

Direct recordings of electron currents mediated by cytochromes b561 (CYB561) are not available yet, despite the importance of these proteins in a variety of physiological functions, including neurotransmitter synthesis and dietary iron uptake. Here, we used the two-electrode voltage-clamp technique applied to Xenopus oocytes to demonstrate, for the first time, the generation of electron currents by a Drosophila member of the CYB561 superfamily named stromal cell-derived receptor 2 (SDR2). This experimental method, along with the theoretical development of a three-state kinetic model, supports the hypothesis that electron donor/acceptor concentrations and transmembrane voltage mutually control SDR2-mediated electron transport activity in a complex but predictable manner.

Cytochromes b561: ascorbate-mediated trans-membrane electron transport

SIGNIFICANCE

Cytochromes b561 (CYB561s) constitute a family of trans-membrane (TM), di-heme proteins, occurring in a variety of organs and cell types, in plants and animals, and using ascorbate (ASC) as an electron donor. CYB561s function as monodehydroascorbate reductase, regenerating ASC, and as Fe³⁺-reductases, providing reduced iron for TM transport. A CYB561-core domain is also associated with dopamine β-monooxygenase redox domains (DOMON) in ubiquitous CYBDOM proteins. In plants, CYBDOMs form large protein families. Physiological functions supported by CYB561s and CYBDOMs include stress defense, cell wall modifications, iron metabolism, tumor suppression, and various neurological processes, including memory retention. CYB561s, therefore, significantly broaden our view on the physiological roles of ASC.

RECENT ADVANCES

The ubiquitous nature of CYB561s is only recently being recognized. Significant advances have been made through the study of recombinant CYB561s, revealing structural and functional properties of a unique "two-heme four-helix" protein configuration. In addition, the DOMON domains of CYBDOMs are suggested to contain another heme b.

CRITICAL ISSUES

New CYB561 proteins are still being identified, and there is a need to provide an insight and overview on the various roles of these proteins and their structural properties.

FUTURE DIRECTIONS

Mutant studies will reveal in greater detail the mechanisms by which CYB561s and CYBDOMs participate in cell metabolism in plants and animals. Moreover, the availability of efficient heterologous expression systems should allow protein crystallization, more detailed (atomic-level) structural information, and insights into the intra-molecular mechanism of electron transport.

Mammalian iron homeostasis in health and disease: uptake, storage, transport, and molecular mechanisms of action

Iron is a crucial factor for life. However, it also has the potential to cause the formation of noxious free radicals. These double-edged sword characteristics demand a tight regulation of cellular iron metabolism. In this review, we discuss the various pathways of cellular iron uptake, cellular iron storage, and transport. Recent advances in understanding the reduction and uptake of non-transferrin-bound iron are discussed. We also discuss the recent progress in the understanding of transcriptional and translational regulation by iron. Furthermore, we discuss recent advances in the understanding of the regulation of cellular and systemic iron homeostasis and several key diseases resulting from iron deficiency and overload. We also discuss the knockout mice available for studying iron metabolism and the related human conditions.

Expression and localization of duodenal cytochrome b in the rat hippocampus after kainate-induced excitotoxicity

Brain iron accumulation and oxidative stress are common features of many neurodegenerative diseases, and could be due in part to increased iron influx across the blood-brain interface. The iron transport protein, divalent metal transporter 1 (DMT1) is found in reactive astrocytes of the lesioned hippocampal CA fields after excitotoxicity induced by the glutamate analog kainate (KA), but in order for iron to be transported by DMT1, it must be converted from the ferric to the ferrous form. The present study was carried out to investigate the expression of a ferric reductase, duodenal cytochrome b (DCYTB), in the rat hippocampus after KA injury. Quantitative reverse transcriptase-polymerase chain reaction showed significant increases in DCYTB mRNA expression of 2.5, 2.7, and 5.2-fold in the hippocampus at 1week, 2weeks and 1month post-KA lesions respectively compared to untreated controls, and 3.0-fold compared to 1month post-saline injection. DCYTB-positive cells were double labeled with glial fibrillary acidic protein, and electron microscopy showed that the DCYTB-positive cells had dense bundles of glial filaments, characteristic of astrocytes, and were present as end-feet around unlabeled brain capillary endothelial cells. DMT1 labeling in astrocytes and increased iron staining were also observed in the lesioned hippocampus. Together, the present findings of DCYTB and DMT1 localization in astrocytes suggest that DCYTB is a ferric reductase for reduction of ferric iron, for transport by DMT1 into the brain. We postulate that the coordinated action of these two proteins could be important in iron influx across the blood-brain interface, in areas undergoing neurodegeneration.

A gene trap mutagenesis screen for genes underlying cellular response to the mood stabilizer lithium

Identifying the biological pathways mediating the action of a therapeutic compound may help the development of more specific treatments while also increasing our understanding of the underlying disease pathology. Salts of the metal lithium are commonly used as a front-line mood stabilizing treatment for bipolar disorder. Lithium's action has been variously linked to inositol phosphate metabolism and the WNT/Glycogen Synthase Kinase 3β (GSK3β)/β-Catenin signalling cascade, but, to date, little is known about which of these provides the principal therapeutic benefit for patients and, more specifically, which constituent genes, through presumed sequence variation, determine differences in patient response to treatment. Here, we describe a functional screen in which SH-SY5Y neuroblastoma cells were randomly mutated through genomic integration of the pMS1 poly A ‘gene trap’ plasmid vector. Lithium normally induces differentiation of neuroblastoma cells, but a small proportion of mutated cells continued to proliferate and formed colonies. Rapid amplification of cDNA ends (RACE)-PCR was used to identify the ‘trapped’ gene in each of these lithium-resistant colonies. Heterozygous, gene trap integrations were identified within ten genes, eight of which are likely to produce loss-of-function mutations including MED10, MSI2 and three long intergenic non-coding (LINC) RNAs. Both MED10 and MSI2 have been previously linked with WNT/GSK3β/β-Catenin pathway function suggesting that this is an important mediator of lithium action in this screen. The methodology applied here provides a rapid, objective and economic approach to define the genetic contribution to drug action, but could also be readily adapted to any desired in vitro functional selection/screening paradigm.

Roles of conserved Arg(72) and Tyr(71) in the ascorbate-specific transmembrane electron transfer catalyzed by Zea mays cytochrome b561

Cytochromes b561, novel transmembrane electron transport proteins residing in eukaryotic cells, have a number of common features including six transmembrane α-helices and two heme ligation sites. Our recent studies on recombinant Zea mays cytochrome b561 suggested that concerted proton/electron transfer mechanism was functioning in plant cytochromes b561 as well and that conserved Lys(83) on a cytosolic loop had important roles for ascorbate-binding and a succeeding electron transfer. In the present study, we conducted site-directed mutagenesis analyses on conserved Arg(72) and Tyr(71). Removal of a positive charge at Arg(72) did not affect significantly on the final heme reduction level with ascorbate as reductant. However, characteristic pH-dependent initial time-lag upon electron acceptance from ascorbate was completely lost for R72A and R72E mutants. Substitution of Tyr(71) with Ala or Phe affected both on the final heme reduction level and on the pH-dependent initial time-lag, causing acceleration of the electron transfer. These observations were interpreted as existence of specific interactions of Tyr(71) and Arg(72) with ascorbate. However, their mechanistic roles were distinctly different from that of Lys(83), as exemplified by K83A/Y71A double mutant, and might be related for expelling of monodehydroascorbate radical from the substrate-binding site to prevent a back-flow of electrons.

Transcriptome analysis of the ependymal barrier during murine neurocysticercosis

BACKGROUND

Central nervous system (CNS) barriers play a pivotal role in the protection and homeostasis of the CNS by enabling the exchange of metabolites while restricting the entry of xenobiotics, blood cells and blood-borne macromolecules. While the blood-brain barrier and blood-cerebrospinal fluid barrier (CSF) control the interface between the blood and CNS, the ependyma acts as a barrier between the CSF and parenchyma, and regulates hydrocephalic pressure and metabolic toxicity. Neurocysticercosis (NCC) is an infection of the CNS caused by the metacestode (larva) of Taenia solium and a major cause of acquired epilepsy worldwide. The common clinical manifestations of NCC are seizures, hydrocephalus and symptoms due to increased intracranial pressure. The majority of the associated pathogenesis is attributed to the immune response against the parasite. The properties of the CNS barriers, including the ependyma, are affected during infection, resulting in disrupted homeostasis and infiltration of leukocytes, which correlates with the pathology and disease symptoms of NCC patients.

RESULTS

In order to characterize the role of the ependymal barrier in the immunopathogenesis of NCC, we isolated ependymal cells using laser capture microdissection from mice infected or mock-infected with the closely related parasite Mesocestoides corti, and analyzed the genes that were differentially expressed using microarray analysis. The expression of 382 genes was altered. Immune response-related genes were verified by real-time RT-PCR. Ingenuity Pathway Analysis (IPA) software was used to analyze the biological significance of the differentially expressed genes, and revealed that genes known to participate in innate immune responses, antigen presentation and leukocyte infiltration were affected along with the genes involved in carbohydrate, lipid and small molecule biochemistry. Further, MHC class II molecules and chemokines, including CCL12, were found to be upregulated at the protein level using immunofluorescence microscopy. This is important, because these molecules are members of the most significant pathways by IPA analyses.

CONCLUSION

Thus, our study indicates that ependymal cells actively express immune mediators and likely contribute to the observed immunopathogenesis during infection. Of particular interest is the major upregulation of antigen presentation pathway-related genes and chemokines/cytokines. This could explain how the ependyma is a prominent source of leukocyte infiltration into ventricles through the disrupted ependymal lining by way of pial vessels present in the internal leptomeninges in murine NCC.

Tissue-restricted expression of Nrf2 and its target genes in zebrafish with gene-specific variations in the induction profiles

The Keap1-Nrf2 system serves as a defense mechanism against oxidative stress and electrophilic toxicants by inducing more than one hundred cytoprotective proteins, including antioxidants and phase 2 detoxifying enzymes. Since induction profiles of Nrf2 target genes have been studied exclusively in cultured cells, and not in animal models, their tissue-specificity has not been well characterized. In this paper, we examined and compared the tissue-specific expression of several Nrf2 target genes in zebrafish larvae by whole-mount in situ hybridization (WISH). Seven zebrafish genes (gstp1, mgst3b, prdx1, frrs1c, fthl, gclc and hmox1a) suitable for WISH analysis were selected from candidates for Nrf2 targets identified by microarray analysis. Tissue-restricted induction was observed in the nose, gill, and/or liver for all seven genes in response to Nrf2-activating compounds, diethylmaleate (DEM) and sulforaphane. The Nrf2 gene itself was dominantly expressed in these three tissues, implying that tissue-restricted induction of Nrf2 target genes is defined by tissue-specific expression of Nrf2. Interestingly, the induction of frrs1c and gclc in liver and nose, respectively, was quite low and that of hmox1a was restricted in the liver. These results indicate the existence of gene-specific variations in the tissue specificity, which can be controlled by factors other than Nrf2.

Mechanisms of brain iron transport: insight into neurodegeneration and CNS disorders

Trace metals such as iron, copper, zinc, manganese, and cobalt are essential cofactors for many cellular enzymes. Extensive research on iron, the most abundant transition metal in biology, has contributed to an increased understanding of the molecular machinery involved in maintaining its homeostasis in mammalian peripheral tissues. However, the cellular and intercellular iron transport mechanisms in the central nervous system (CNS) are still poorly understood. Accumulating evidence suggests that impaired iron metabolism is an initial cause of neurodegeneration, and several common genetic and sporadic neurodegenerative disorders have been proposed to be associated with dysregulated CNS iron homeostasis. This review aims to provide a summary of the molecular mechanisms of brain iron transport. Our discussion is focused on iron transport across endothelial cells of the blood-brain barrier and within the neuro- and glial-vascular units of the brain, with the aim of revealing novel therapeutic targets for neurodegenerative and CNS disorders.

High-yield production, purification and characterization of functional human duodenal cytochrome b in an Escherichia coli system

Human duodenal cytochrome b (Dcytb) is a transmembrane hemoprotein found in the duodenal brush border membrane and in erythrocytes. Dcytb has been linked to uptake of dietary iron and to ascorbate recycling in erythrocytes. Detailed biophysical and biochemical characterization of Dcytb has been limited by difficulties in expressing sufficient amounts of functional recombinant protein in yeast and insect cell systems. We have developed an Escherichia coli Rosetta-gami B(DE3) cell system for production of recombinant His-tagged human Dcytb with a yield of ∼26 mg of purified, ascorbate-reducible cytochrome per liter of culture. The recombinant protein is readily solubilized with n-dodecyl-β-D-maltoside and purified to electrophoretic homogeneity by one-step chromatography on cobalt affinity resin. The purified recombinant Dcytb has a heme to protein ratio very close to the theoretical value of 2 and retains functional reactivity with ascorbate, as assessed by spectroscopic and kinetic measurements. Ascorbate showed a marked kinetic selectivity for the high-potential heme center over the low-potential heme center in purified Dcytb. This new E. coli expression system for Dcytb offers ∼7-fold improvement in yield and other substantial advantages over existing expression systems for reliable production of functional Dcytb at levels suitable for biochemical, biophysical and structural characterization.

Iron handling in hippocampal neurons: activity-dependent iron entry and mitochondria-mediated neurotoxicity

The characterization of iron handling in neurons is still lacking, with contradictory and incomplete results. In particular, the relevance of non-transferrin-bound iron (NTBI), under physiologic conditions, during aging and in neurodegenerative disorders, is undetermined. This study investigates the mechanisms underlying NTBI entry into primary hippocampal neurons and evaluates the consequence of iron elevation on neuronal viability. Fluorescence-based single cell analysis revealed that an increase in extracellular free Fe(2+) (the main component of NTBI pool) is sufficient to promote Fe(2+) entry and that activation of either N-methyl-d-aspartate receptors (NMDARs) or voltage operated calcium channels (VOCCs) significantly potentiates this pathway, independently of changes in intracellular Ca(2+) concentration ([Ca(2+) ](i) ). The enhancement of Fe(2+) influx was accompanied by a corresponding elevation of reactive oxygen species (ROS) production and higher susceptibility of neurons to death. Interestingly, iron vulnerability increased in aged cultures. Scavenging of mitochondrial ROS was the most powerful protective treatment against iron overload, being able to preserve the mitochondrial membrane potential and to safeguard the morphologic integrity of these organelles. Overall, we demonstrate for the first time that Fe(2+) and Ca(2+) compete for common routes (i.e. NMDARs and different types of VOCCs) to enter primary neurons. These iron entry pathways are not controlled by the intracellular iron level and can be harmful for neurons during aging and in conditions of elevated NTBI levels. Finally, our data draw the attention to mitochondria as a potential target for the treatment of the neurodegenerative processes induced by iron dysmetabolism.

Classification of subcellular location by comparative proteomic analysis of native and density-shifted lysosomes

One approach to the functional characterization of the lysosome lies in the use of proteomic methods to identify proteins in subcellular fractions enriched for this organelle. However, distinguishing between true lysosomal residents and proteins from other cofractionating organelles is challenging. To this end, we implemented a quantitative mass spectrometry approach based on the selective decrease in the buoyant density of liver lysosomes that occurs when animals are treated with Triton-WR1339. Liver lysosome-enriched preparations from control and treated rats were fractionated by isopycnic sucrose density gradient centrifugation. Tryptic peptides derived from gradient fractions were reacted with isobaric tag for relative and absolute quantitation eight-plex labeling reagents and analyzed by two-dimensional liquid chromatography matrix-assisted laser desorption ionization time-of-flight MS. Reporter ion intensities were used to generate relative protein distribution profiles across both types of gradients. A distribution index was calculated for each identified protein and used to determine a probability of lysosomal residence by quadratic discriminant analysis. This analysis suggests that several proteins assigned to the lysosome in other proteomics studies are not true lysosomal residents. Conversely, results support lysosomal residency for other proteins that are either not or only tentatively assigned to this location. The density shift for two proteins, Cu/Zn superoxide dismutase and ATP-binding cassette subfamily B (MDR/TAP) member 6, was corroborated by quantitative Western blotting. Additional balance sheet analyses on differential centrifugation fractions revealed that Cu/Zn superoxide dismutase is predominantly cytosolic with a secondary lysosomal localization whereas ATP-binding cassette subfamily B (MDR/TAP) member 6 is predominantly lysosomal. These results establish a quantitative mass spectrometric/subcellular fractionation approach for identification of lysosomal proteins and underscore the necessity of balance sheet analysis for localization studies.

A cytochrome b561 with ferric reductase activity from the parasitic blood fluke, Schistosoma japonicum

BACKGROUND

Iron has an integral role in numerous cellular reactions and is required by virtually all organisms. In physiological conditions, iron is abundant in a largely insoluble ferric state. Ferric reductases are an essential component of iron uptake by cells, reducing iron to the soluble ferrous form. Cytochromes b561 (cyts-b561) are a family of ascorbate reducing transmembrane proteins found in most eukaryotic cells. The identification of the ferric reductase duodenal cytochrome b (dcytb) and recent observations that other cyts-b561 may be involved in iron metabolism have opened novel perspectives for elucidating their physiological function.

METHODOLOGY/PRINCIPAL FINDINGS

Here we have identified a new member of the cytochrome b561 (Sjcytb561) family in the pathogenic blood fluke Schistosoma japonicum that localises to the outer surface of this parasitic trematode. Heterologous expression of recombinant Sjcyt-b561 in a Saccharomyces cerevisiae mutant strain that lacks plasma membrane ferrireductase activity demonstrated that the molecule could rescue ferric reductase activity in the yeast.

SIGNIFICANCE/CONCLUSIONS

This finding of a new member of the cytochrome b561 family further supports the notion that a ferric reductase function is likely for other members of this protein family. Additionally, the localisation of Sjcytb561 in the surface epithelium of these blood-dwelling schistosomes contributes further to our knowledge concerning nutrient acquisition in these parasites and may provide novel targets for therapeutic intervention.

Uptake of ferrous iron by cultured rat astrocytes

Astrocytes are considered to play an important role in iron homeostasis of the brain, yet the mechanisms involved in the uptake of iron into astrocytes remain elusive. To investigate the uptake of iron into astrocytes, we have applied ferric ammonium citrate (FAC) to rat astrocyte-rich primary cultures. These cultures express the mRNAs of two membrane-bound ferric reductases, Dcytb and SDR2, and reduce extracellular ferric iron (100 muM) with a rate of 3.2 +/- 0.4 nmol/(hr x mg). This reduction rate is substantially lower than the rate of cellular iron accumulation from 100 muM FAC [24.7 +/- 8.9 nmol/(hr x mg)], which suggests that iron accumulation from FAC does at best partially depend on extracellular ferric reduction. Nonetheless, when the iron in FAC was almost completely reduced by an excess of exogenous ascorbate, astrocytes accumulated iron in a time- and concentration-dependent manner with specific iron accumulation rates that increased linearly for concentrations of up to 100 muM ferrous iron. This accumulation was attenuated by lowering the incubation temperature, by the presence of ferrous iron chelators, or by lowering the pH from 7.4 to 6.8. These data indicate that, in addition to the DMT1-mediated uptake of ferrous iron, astrocytes can accumulate ferric and ferrous iron by mechanisms that are independent of DMT1 or transferrin.

Importance of the conserved lysine 83 residue of Zea mays cytochrome b(561) for ascorbate-specific transmembrane electron transfer as revealed by site-directed mutagenesis studies

Cytochromes b(561), a novel class of transmembrane electron transport proteins residing in a large variety of eukaryotic cells, have a number of common structural features including six hydrophobic transmembrane alpha-helices and two heme ligation sites. We found that recombinant Zea mays cytochrome b(561) obtained by a heterologous expression system using yeast Pichia pastoris cells could utilize the ascorbate/mondehydroascorbate radical as a physiological electron donor/acceptor. We found further that a concerted proton/electron transfer mechanism might be operative in Z. mays cytochrome b(561) as well upon the electron acceptance from ascorbate to the cytosolic heme center. The well-conserved Lys(83) residue in a cytosolic loop was found to have a very important role(s) for the binding of ascorbate and the succeeding electron transfer via electrostatic interactions based on the analyses of three site-specific mutants, K83A, K83E, and K83D. Further, unusual behavior of the K83A mutant in pulse radiolysis experiments indicated that Lys(83) might also be responsible for the intramolecular electron transfer to the intravesicular heme. On the other hand, pulse radiolysis experiments on two site-specific mutants, S118A and W122A, for the well-conserved residues in the putative monodehydroascorbate radical binding site showed that their electron transfer activities to the monodehydroascorbate radical were very similar to those of the wild-type protein, indicating that Ser(118) and Trp(122) do not have major roles for the redox events on the intravesicular side.

Auxin-responsive genes AIR12 code for a new family of plasma membrane b-type cytochromes specific to flowering plants

We report here on the identification of the major plasma membrane (PM) ascorbate-reducible b-type cytochrome of bean (Phaseolus vulgaris) and soybean (Glycine max) hypocotyls as orthologs of Arabidopsis (Arabidopsis thaliana) AIR12 (for auxin induced in root cultures). Soybean AIR12, which is glycosylated and glycosylphosphatidylinositol-anchored to the external side of the PM in vivo, was expressed in Pichia pastoris in a recombinant form, lacking the glycosylphosphatidylinositol modification signal and purified from the culture medium. Recombinant AIR12 is a soluble protein predicted to fold into a beta-sandwich domain and belonging to the DOMON (for dopamine beta-monooxygenase N terminus) domain superfamily. It is shown to be a b-type cytochrome with a symmetrical alpha-band at 561 nm, fully reduced by ascorbate, and fully oxidized by monodehydroascorbate radical. AIR12 is a high-potential cytochrome b showing a wide bimodal dependence from the redox potential between +80 mV and +300 mV. Optical absorption and electron paramagnetic resonance analysis indicate that AIR12 binds a single, highly axial low-spin heme, likely coordinated by methionine-91 and histidine-76, which are strongly conserved in AIR12 sequences. Phylogenetic analyses reveal that the auxin-responsive genes AIR12 represent a new family of PM b-type cytochromes specific to flowering plants. Circumstantial evidence suggests that AIR12 may interact with other redox partners within the PM to constitute a redox link between cytoplasm and apoplast.

Lipopolysaccharide (LPS) potentiates hydrogen peroxide toxicity in T98G astrocytoma cells by suppression of anti-oxidative and growth factor gene expression

Background

Lipopolysaccharide (LPS) is a cell wall component of Gram-negative bacteria with proved role in pathogenesis of sepsis. Brain injury was observed with both patients dead from sepsis and animal septic models. However, in vitro administration of LPS has not shown obvious cell damage to astrocytes and other relative cell lines while it does cause endothelial cell death in vitro. These observations make it difficult to understand the role of LPS in brain parenchymal injury.

Results

To test the hypothesis that LPS may cause biological changes in astrocytes and make the cells to become vulnerable to reactive oxygen species, a recently developed highly sensitive and highly specific system for large-scale gene expression profiling was used to examine the gene expression profile of a group of 1,135 selected genes in a cell line, T98G, a derivative of human glioblastoma of astrocytic origin. By pre-treating T98G cells with different dose of LPS, it was found that LPS treatment caused a broad alteration in gene expression profile, but did not cause obvious cell death. However, after short exposure to H₂O₂, cell death was dramatically increased in the LPS pretreated samples. Interestingly, cell death was highly correlated with down-regulated expression of antioxidant genes such as cytochrome b561, glutathione s-transferase a4 and protein kinase C-epsilon. On the other hand, expression of genes encoding growth factors was significantly suppressed. These changes indicate that LPS treatment may suppress the anti-oxidative machinery, decrease the viability of the T98G cells and make the cells more sensitive to H₂O₂ stress.

Conclusion

These results provide very meaningful clue for further exploring and understanding the mechanism underlying astrocyte injury in sepsis in vivo, and insight for why LPS could cause astrocyte injury in vivo, but not in vitro. It will also shed light on the therapeutic strategy of sepsis.

The role of Dcytb in iron metabolism: an update

Dcytb (duodenal cytochrome b) is an iron-regulated ferric reductase highly expressed in duodenal enterocytes. Its location and strong regulation by iron has indicated it plays an important role in iron absorption. Expression of Dcytb in cells (Caco-2 and MDCK) was found to increase both ferric reductase activity and stimulate uptake of 59Fe. An additional increase in cupric reductase activity was found in MDCK (Madin–Darby canine kidney) cells expressing Dcytb. Expression and purification of Dcytb in insect cells reveals that Dcytb is a di-haem protein and that the haems are reducible by ascorbate, indicating that ascorbate is the likely intracelluar electron donor. Studies underway in Dcytb-knockout mice reveal that Dcytb is the only iron-regulated ferric reductase in the duodenal mucosa and that loss of Dcytb affects iron absorption.

Genetics of iron regulation and the possible role of iron in Parkinson's disease

Parkinson's disease (PD) is acknowledged as the second most common neurodegenerative disorder after Alzheimer's Disease. Older age may be the only unequivocal risk factor for PD although the male to female ratio is consistently greater than 1 in populations of European ancestry. Characteristic features of PD include dopaminergic neuron death in the substantia nigra (SN) pars compacta, accumulation of alpha-synuclein inclusions known as Lewy bodies in the SN, and brain iron accumulation beyond that observed in non-PD brains of a similar age. In this review article, we will provide an overview of human and animal studies investigating the contributions of iron in PD, a summary of human studies of iron-related genes in PD, a review of the literature on the genetics of iron metabolism, and some hypotheses on possible roles for iron in the pathogenic processes of PD including potential interactions between iron and other factors associated with Parkinson's disease.

Iron trafficking inside the brain

Iron, an essential element for all cells of the body, including those of the brain, is transported bound to transferrin in the blood and the general extracellular fluid of the body. The demonstration of transferrin receptors on brain capillary endothelial cells (BCECs) more than 20 years ago provided the evidence for the now accepted view that the first step in blood to brain transport of iron is receptor-mediated endocytosis of transferrin. Subsequent steps are less clear. However, recent investigations which form the basis of this review have shed some light on them and also indicate possible fruitful avenues for future research. They provide new evidence on how iron is released from transferrin on the abluminal surface of BCECs, including the role of astrocytes in this process, how iron is transported in brain extracellular fluid, and how iron is taken up by neurons and glial cells. We propose that the divalent metal transporter 1 is not involved in iron transport through the BCECs. Instead, iron is probably released from transferrin on the abluminal surface of these cells by the action of citrate and ATP that are released by astrocytes, which form a very close relationship with BCECs. Complexes of iron with citrate and ATP can then circulate in brain extracellular fluid and may be taken up in these low-molecular weight forms by all types of brain cells or be bound by transferrin and taken up by cells which express transferrin receptors. Some iron most likely also circulates bound to transferrin, as neurons contain both transferrin receptors and divalent metal transporter 1 and can take up transferrin-bound iron. The most likely source for transferrin in the brain interstitium derives from diffusion from the ventricles. Neurons express the iron exporting carrier, ferroportin, which probably allows them to excrete unneeded iron. Astrocytes lack transferrin receptors. Their source of iron is probably that released from transferrin on the abluminal surface of BCECs. They probably to export iron by a mechanism involving a membrane-bound form of the ferroxidase, ceruloplasmin. Oligodendrocytes also lack transferrin receptors. They probably take up non-transferrin bound iron that gets incorporated in newly synthesized transferrin, which may play an important role for intracellular iron transport.

The DOMON domains are involved in heme and sugar recognition

We expand the functionally uncharacterized DOMON domain superfamily to identify several novel families, including the first prokaryotic representatives. Using several computational tools we show that it is involved in ligand binding--either as heme- or sugar-binding domains. We present evidence that the DOMON domain along with the DM13 domain comprises a novel electron-transfer system potentially involved in oxidative modification of animal cell-surface proteins. Other novel versions might function as sugar sensors of histidine kinases of bacterial two component systems.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online and also at ftp://ftp.ncbi.nih.gov/pub/aravind/domon/.

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.