Alternative splicing regulates the subcellular localization of divalent metal transporter 1 isoforms

Wheat ZIP3-2A encodes a metal transporter for Cd influx

Cadmium (Cd) accumulation in wheat grains occurs via three key physiological processes: root uptake, root-to-shoot translocation and/or shoot Cd distribution. Wheat ZIP transporters are hypothesized to involved in these pathways, their functional roles remain poorly characterized. Here, we identified and functionally characterized TpZIP3-2A from dwarf Polish wheat (DPW, Triticum polonicum L., AABB), which mediates Cd, Zn, Fe and Co transport. Expression pattern and subcellular localization analysis revealed that TpZIP3-2A encoding a plasma membrane protein, was mainly expressed in roots, flag leaves, peleas, lemmas and grains of DPW, which was growth stage-dependent. Its transcript in roots was up-regulated by Zn or Fe deficiency and Co or Cd supplementation. Expression of TpZIP3-2A in yeast increased the sensitivities and concentrations of Cd (by 104.84 %), Zn (by 36.12 %) and Co (by 81.87 %). The TpZIP3-2A-overexpressing lines of Arabidopsis showed sensitivity to Cd or Co stress and tolerance to Zn or Fe deficiency, and had higher root and shoot concentrations of Cd (61.17 % and 125.82 %), Co (27.65 % and 88.81 %), Zn (36.87 % and 70.04 %) and Fe (46.02 % in shoots), as well as root-to-shoot translocation factors (24.27 %-54.50 %). However, Tpzip3-2a (a mutation of TpZIP3-2A lacking the first transmembrane domain) had no ability to transport Cd, Zn and Fe. The study provides new insights into the molecular mechanisms regulating Cd uptake and translocation in wheat and offers potential applications for breeding low-Cd accumulating wheat cultivars.

The Roles of DMT1 in Inflammatory and Degenerative Diseases

Iron homeostasis is critical for multiple physiological and pathological processes. DMT1, a core iron transporter, is expressed in almost all cells and organs and altered in response to various conditions, whereas, there is few reviews focusing on DMT1 in diseases associated with aberrant iron metabolism. Based on available knowledge, this review described a full view of DMT1 and summarized the roles of DMT1 and DMT1-mediated iron metabolism in the onset and development of inflammatory and degenerative diseases. This review also provided an overview of DMT1-related treatment in these disorders, highlighting its therapeutic potential in chronic inflammatory and degenerative diseases.

Uptake of iron from ferrous fumarate can be mediated by clathrin-dependent endocytosis in Hutu-80 cells

Iron uptake in the intestinal epithelium is associated with transport of ferrous iron via the DMT1 transporter (SLC11a2; NRAMP2). In later years, uptake of iron from complex sources, such as nanoparticles, has been found to be mediated through endocytosis. Here we propose that iron from the simple salt ferrous fumarate, a common iron supplement, can be absorbed by clathrin-mediated endocytosis. We used siRNA to silence DMT1 transporter expression, pharmacological inhibition of endocytosis, and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) to show that iron uptake from ferrous fumarate can be mediated by both transport via DMT1 and by clathrin-dependent endocytosis in Hutu-80 cells. Iron uptake (ferritin L) from ferrous fumarate (0.5 mM, 24 h) in DMT1 silenced cells was significantly decreased (60% ± 11%) in comparison to iron controls while a 1-h dose of ferrous fumarate (0.5 mM) significantly decreased ferritin L formation in the presence of the clathrin inhibitor chlorpromazine (61% ± 10%, in post-confluent cells and 37% ± 9% in non-confluent cells). A pilot showed a similar trend for Ferritin (H) levels (confluent cells) and for total cellular iron load (non-confluent cells). ToF-SIMS analysis revealed diminished membrane-associated iron load in endocytosis-inhibited ferrous fumarate treated cells. The reported results support a clathrin-mediated endocytosis mechanism for uptake of iron from ferrous fumarate in addition to iron uptake by DMT1. More studies are needed to understand what determines which uptake mechanism are employed and to which extent.

Iron Metabolism and Inflammatory Mediators in Patients with Renal Dysfunction

Chronic kidney disease (CKD) affects around 850 million people worldwide, posing significant challenges in healthcare due to complications like renal anemia, end-stage kidney disease, and cardiovascular diseases. This review focuses on the intricate interplay between iron metabolism, inflammation, and renal dysfunction in CKD. Renal anemia, prevalent in CKD, arises primarily from diminished erythropoietin (EPO) production and iron dysregulation, which worsens with disease progression. Functional and absolute iron deficiencies due to impaired absorption and chronic inflammation are key factors exacerbating erythropoiesis. A notable aspect of CKD is the accumulation of uremic toxins, such as indoxyl sulfate (IS), which hinder iron metabolism and worsen anemia. These toxins directly affect renal EPO synthesis and contribute to renal hypoxia, thus playing a critical role in the pathophysiology of renal anemia. Inflammatory cytokines, especially TNF-α and IL-6, further exacerbate CKD progression and disrupt iron homeostasis, thereby influencing anemia severity. Treatment approaches have evolved to address both iron and EPO deficiencies, with emerging therapies targeting hepcidin and employing hypoxia-inducible factor (HIF) stabilizers showing potential. This review underscores the importance of integrated treatment strategies in CKD, focusing on the complex relationship between iron metabolism, inflammation, and renal dysfunction to improve patient outcomes.

Iron-responsive element of Divalent metal transporter 1 (Dmt1) controls Notch-mediated cell fates

Notch receptor activation is regulated by the intramembrane protease γ-secretase, which cleaves and liberates the Notch intracellular domain (Nicd) that regulates gene transcription. While γ-secretase cleavage is necessary, we demonstrate it is insufficient for Notch activation and requires vesicular trafficking. Here, we report Divalent metal transporter 1 (Dmt1, Slc11A2) as a novel and essential regulator of Notch signalling. Dmt1-deficient cells are defective in Notch signalling and have perturbed endolysosomal trafficking and function. Dmt1 encodes for two isoforms, with and without an iron response element (ire). We show that isoform-specific silencing of Dmt1-ire and Dmt1+ire has opposite consequences on Notch-dependent cell fates in cell lines and intestinal organoids. Loss of Dmt1-ire suppresses Notch activation and promotes differentiation, whereas loss of Dmt1+ire causes Notch activation and maintains stem-progenitor cell fates. Dmt1 isoform expression correlates with Notch and Wnt signalling in Apc-deficient intestinal organoids and human colorectal cancers. Consistently, Dmt1-ire silencing induces Notch-dependent differentiation in colorectal cancer cells. These data identify Dmt1 isoforms as binary switches controlling Notch cell fate decisions in normal and tumour cells.

Cardiac iron metabolism during aging - Role of inflammation and proteolysis

Iron is the most abundant trace element in the human body. Since iron can switch between its 2-valent and 3-valent form it is essential in various physiological processes such as energy production, proliferation or DNA synthesis. Especially high metabolic organs such as the heart rely on iron-associated iron-sulfur and heme proteins. However, due to switches in iron oxidation state, iron overload exhibits high toxicity through formation of reactive oxygen species, underlining the importance of balanced iron levels. Growing evidence demonstrates disturbance of this balance during aging. While age-associated cardiovascular diseases are often related to iron deficiency, in physiological aging cardiac iron accumulates. To understand these changes, we focused on inflammation and proteolysis, two hallmarks of aging, and their role in iron metabolism. Via the IL-6-hepcidin axis, inflammation and iron status are strongly connected often resulting in anemia accompanied by infiltration of macrophages. This tight connection between anemia and inflammation highlights the importance of the macrophage iron metabolism during inflammation. Age-related decrease in proteolytic activity additionally affects iron balance due to impaired degradation of iron metabolism proteins. Therefore, this review accentuates alterations in iron metabolism during aging with regards to inflammation and proteolysis to draw attention to their implications and associations.

The Endolysosomal Transporter DMT1 is Required for Morphine Regulation of Neuronal Ferritin Heavy Chain

NeuroHIV and other neurologic disorders present with altered iron metabolism in central nervous system neurons. Many people with HIV also use opioids, which can worsen neuroHIV symptoms by further dysregulating neuronal iron metabolism. Our previous work demonstrated that the μ-opioid agonist morphine causes neuronal endolysosomes to release their iron stores, and neurons respond by upregulating ferritin heavy chain (FHC), an iron storage protein associated with cognitive impairment in neuroHIV. Here, we investigated if this process required divalent metal transporter 1 (DMT1), a well-known iron transporter expressed on endolysosomes. We first optimized conditions to detect DMT1 isoforms (DMT1 1B ± iron responsive element) using fluorescently labeled rat DMT1 constructs expressed in HEK-293 cells. We also expressed these constructs in primary rat cortical neurons to compare their expression and subcellular distribution with endogenous DMT1 isoforms. We found endogenous DMT1 isoforms in the cytoplasm that colocalized with lysosomal-associated protein 1 (LAMP1), a marker of endolysosomes. Next, we blocked endogenous DMT1 isoforms using ebselen, a potent pharmacological inhibitor of DMT1 iron transport. Ebselen pre-treatment blocked morphine's ability to upregulate FHC protein, suggesting this pathway requires DMT1 iron transport from endolysosomes. This was further validated using viral-mediated genetic silencing of DMT1±IRE in cortical neurons, which also blocked FHC upregulation in the presence of morphine. Overall, our work demonstrates that the μ-opioid agonist morphine utilizes the endolysosomal iron transporter DMT1 to modulate neuronal cellular iron metabolism, upregulate FHC protein, and contribute to cognitive decline in neuroHIV. Morphine requires DMT1 to upregulate neuronal FHC. Cortical neurons treated with morphine release their endolysosomal iron stores to the cytoplasm and upregulate FHC, an iron storage protein associated with dendritic spine deficits and cognitive impairment in neuroHIV. This pathway requires the endolysosomal iron transporter DMT1, as pharmacological and genetic inhibitors of the transporter completely block morphine's ability to upregulate FHC. Created with BioRender.com .

Impact of maternal iron deficiency anemia on fetal iron status and placental iron transporters in human pregnancy

Iron deficiency anemia is associated with maternal morbidity and poor pregnancy outcomes. Heme and non-heme iron transport proteins expressed in the placenta help in adequate iron supply from anemic mother to fetus. Here we examined the expression of placental iron trafficking molecules and their association with maternal and neonatal iron status in pregnant women with iron deficiency anemia (IDA). Pregnant women who received prenatal care at Christian Medical College, Vellore, India for childbirth were recruited. Pregnant women who were 18-35 years old with gestational age (GA) of ≥36 weeks were eligible to participate in the study. In a prospective cohort of pregnant women, 22 % were iron deficiency anemia and 42 % were iron replete. Samples were collected (Maternal blood, placental tissue, and cord blood) from pregnant women with a gestational age of ≥38 weeks at the time of delivery. The mean gestational age at the first visit and delivery was 12.8 ± 2.72 weeks and 39 ± 1.65 weeks, respectively. Hemoglobin (9.3 ± 0.9 g/dl) and ferritin (15.4(0.8-28.3) ng/ml) levels at delivery were significantly decreased in IDA as compared to controls. The fetal hemoglobin and ferritin levels were in the normal range in both groups. There was no correlation between maternal and cord blood hepcidin with fetal iron status in IDA. We further analyzed the expression of iron transport genes in the placenta of controls and the IDA group. Under maternal iron insufficiency, the expression of placental iron transporters DMT1, FPN1, and GDF15 was upregulated at the protein level. In IDA, placental GDF15 and ferroportin protein had an association with fetal iron status. These findings confirm that placental iron traffickers respond to maternal iron deficiency by increasing their expression and allowing sufficient iron to pass to the fetus.

Internalization and Decrease of Duodenal DMT1 Involved in Transient Suppression of Iron Uptake in Short-Acting Mucosal Block

Mucosal block (MB) is induced by the oral administration of excess iron (10 mg) and suppresses intestinal iron absorption for 3-72 h. The inhibition of iron absorption is accompanied by the downregulation of molecules associated with intestinal iron absorption. Recently, we found that a smaller amount of iron (1 mg) also induced a transient suppression of iron uptake without affecting gene expression levels (short-acting mucosal block, SAMB), which is specific to iron-deficient rats. In this study, we investigated how the nonheme iron transporters divalent metal transporter 1 (DMT1) and ferroportin (FPN) are involved in the transient suppression of iron uptake in SAMB. To induce SAMB, a test solution containing 1 mg iron was infused into the duodenum loop in iron-sufficient and iron-deficient rats. Total duodenal DMT1 and DMT1-IRE expression were increased during iron deficiency. After 15 min of 1 mg iron loading, the fluorescence intensity of duodenal DMT1 in iron-deficient rats was decreased and was comparable to that in iron-sufficient rats. Internalized DMT1-IRE as puncta was observed at 15 and 60 min after 1 mg iron loading, and the number of punctas was significantly increased after 60 min compared with control. There was no effect of 1 mg iron loading on the intracellular distribution of duodenal FPN. Our results suggest that the decrease and internalization of DMT1-IRE protein may be related, at least in part, to iron uptake suppression in SAMB.

Adaptive Changes Allow Targeting of Ferroptosis for Glioma Treatment

Ferroptosis is a type of regulated cell death that plays an essential role in various brain diseases, including cranial trauma, neuronal diseases, and brain tumors. It has been reported that cancer cells rely on their robust antioxidant capacity to escape ferroptosis. Therefore, ferroptosis exploitation could be an effective strategy to prevent tumor proliferation and invasion. Glioma is a common malignant craniocerebral tumor exhibiting complicated drug resistance and survival mechanisms, resulting in a high mortality rate and short survival time. Recent studies have determined that metabolic alterations in glioma offer exploitable therapeutic targets. These metabolic alterations allow targeted therapy to achieve some initial efficacy but have failed to inhibit glioma growth, invasion, and drug resistance effectively. It has been proposed that the reason for the high malignancy and drug resistance observed with glioma is that these tumors can effectively evade ferroptosis. Ferroptosis-inducing drugs were found to exert a positive effect by targeting this particular characteristic of glioma cells. Moreover, gliomas develop enhanced drug resistance through anti-ferroptosis mechanisms. In this study, we provided an overview of the mechanisms by which glioma aggressiveness and drug resistance are mediated by the evasion of ferroptosis. This information might provide new targets for glioma therapy as well as new insights and ideas for future research.

Iron homeostasis and organismal aging

Iron is indispensable for normal body functions across species because of its critical roles in red blood cell function and many essential proteins and enzymes required for numerous physiological processes. Regulation of iron homeostasis is an intricate process involving multiple modulators at the systemic, cellular, and molecular levels. Interestingly, emerging evidence has demonstrated that many modulators of iron homeostasis contribute to organismal aging and longevity. On the other hand, the age-related dysregulation of iron homeostasis is often associated with multiple age-related pathologies including bone resorption and neurodegenerative diseases such as Alzheimer's disease. Thus, a thorough understanding on the interconnections between systemic and cellular iron balance and organismal aging may help decipher the etiologies of multiple age-related diseases, which could ultimately lead to developing therapeutic strategies to delay aging and treat various age-related diseases. Here we present the current understanding on the mechanisms of iron homeostasis. We also discuss the impacts of aging on iron homeostatic processes and how dysregulated iron metabolism may affect aging and organismal longevity.

HCV-Induced Immunometabolic Crosstalk in a Triple-Cell Co-Culture Model Capable of Simulating Systemic Iron Homeostasis

Iron is crucial to the regulation of the host innate immune system and the outcome of many infections. Hepatitis C virus (HCV), one of the major viral human pathogens that depends on iron to complete its life cycle, is highly skilled in evading the immune system. This study presents the construction and validation of a physiologically relevant triple-cell co-culture model that was used to investigate the input of iron in HCV infection and the interplay between HCV, iron, and determinants of host innate immunity. We recorded the expression patterns of key proteins of iron homeostasis involved in iron import, export and storage and examined their relation to the iron regulatory hormone hepcidin in hepatocytes, enterocytes and macrophages in the presence and absence of HCV. We then assessed the transcriptional profiles of pro-inflammatory cytokines Interleukin-6 (IL-6) and interleukin-15 (IL-15) and anti-inflammatory interleukin-10 (IL-10) under normal or iron-depleted conditions and determined how these were affected by infection. Our data suggest the presence of a link between iron homeostasis and innate immunity unfolding among liver, intestine, and macrophages, which could participate in the deregulation of innate immune responses observed in early HCV infection. Coupled with iron-assisted enhanced viral propagation, such a mechanism may be important for the establishment of viral persistence and the ensuing chronic liver disease.

Organic nitrogen nutrition: LHT1.2 protein from hybrid aspen (Populus tremula L. x tremuloides Michx) is a functional amino acid transporter and a homolog of Arabidopsis LHT1

The contribution of amino acids (AAs) to soil nitrogen (N) fluxes is higher than previously thought. The fact that AA uptake is pivotal for N nutrition in boreal ecosystems highlights plant AA transporters as key components of the N cycle. At the same time, very little is known about AA transport and respective transporters in trees. Tree genomes may contain 13 or more genes encoding the lysine histidine transporter (LHT) family proteins, and this complicates the study of their significance for tree N-use efficiency. With the strategy of obtaining a tool to study N-use efficiency, our aim was to identify and characterize a relevant AA transporter in hybrid aspen (Populus tremula L. x tremuloides Michx.). We identified PtrLHT1.2, the closest homolog of Arabidopsis thaliana (L.) Heynh AtLHT1, which is expressed in leaves, stems and roots. Complementation of a yeast AA uptake mutant verified the function of PtrLHT1.2 as an AA transporter. Furthermore, PtrLHT1.2 was able to fully complement the phenotypes of the Arabidopsis AA uptake mutant lht1 aap5, including early leaf senescence-like phenotype, reduced growth, decreased plant N levels and reduced root AA uptake. Amino acid uptake studies finally showed that PtrLHT1.2 is a high affinity transporter for neutral and acidic AAs. Thus, we identified a functional AtLHT1 homolog in hybrid aspen, which harbors the potential to enhance overall plant N levels and hence increase biomass production. This finding provides a valuable tool for N nutrition studies in trees and opens new avenues to optimizing tree N-use efficiency.

A guide to plasma membrane solute carrier proteins

This review aims to serve as an introduction to the solute carrier proteins (SLC) superfamily of transporter proteins and their roles in human cells. The SLC superfamily currently includes 458 transport proteins in 65 families that carry a wide variety of substances across cellular membranes. While members of this superfamily are found throughout cellular organelles, this review focuses on transporters expressed at the plasma membrane. At the cell surface, SLC proteins may be viewed as gatekeepers of the cellular milieu, dynamically responding to different metabolic states. With altered metabolism being one of the hallmarks of cancer, we also briefly review the roles that surface SLC proteins play in the development and progression of cancer through their influence on regulating metabolism and environmental conditions.

Molecular Mechanism of Nramp-Family Transition Metal Transport

The Natural resistance-associated macrophage protein (Nramp) family of transition metal transporters enables uptake and trafficking of essential micronutrients that all organisms must acquire to survive. Two decades after Nramps were identified as proton-driven, voltage-dependent secondary transporters, multiple Nramp crystal structures have begun to illustrate the fine details of the transport process and provide a new framework for understanding a wealth of preexisting biochemical data. Here we review the relevant literature pertaining to Nramps' biological roles and especially their conserved molecular mechanism, including our updated understanding of conformational change, metal binding and transport, substrate selectivity, proton transport, proton-metal coupling, and voltage dependence. We ultimately describe how the Nramp family has adapted the LeuT fold common to many secondary transporters to provide selective transition-metal transport with a mechanism that deviates from the canonical model of symport.

The new role of poly (rC)-binding proteins as iron transport chaperones: Proteins that could couple with inter-organelle interactions to safely traffic iron

BACKGROUND

Intracellular iron transport is mediated by iron chaperone proteins known as the poly(rC)-binding proteins (PCBPs), which were originally identified as RNA/DNA-binding molecules.

SCOPE OF REVIEW

PCBPs assume a role as not only as cytosolic iron carriers, but also as regulators of iron transport and recycling. PCBP1 is involved in the iron storage pathway that involves ferritin, while PCBP2 is involved in processes that include: iron transfer from the iron importer, divalent metal ion transporter 1; iron export mediated by ferroportin-1; and heme degradation via heme oxygenase 1.

MAJOR CONCLUSIONS

Both PCBP1 and PCBP2 possess iron-binding activity and form hetero/homo dimer complexes. These iron chaperones have a subset of non-redundant functions and regulate iron metabolism independently.

GENERAL SIGNIFICANCE

This intracellular iron chaperone system mediated by PCBPs provide a transport "gateway" of ferrous iron that may potentially link with dynamic, inter-organelle interactions to safely traffic intracellular iron.

How iron is handled in the course of heme catabolism: Integration of heme oxygenase with intracellular iron transport mechanisms mediated by poly (rC)-binding protein-2

Heme and iron are essential to almost all forms of life. The strict maintenance of heme and iron homeostasis is essential to prevent cellular toxicity and the existence of systemic and intracellular regulation is fundamental. Cytosolic heme can be catabolized and detoxified by heme oxygenases (HOs). Interestingly, free heme detoxification through HOs results in the production of free ferrous iron, which can be potentially hazardous for cells. Recently, the intracellular iron chaperone, poly (rC)-binding protein 2 (PCBP2), has been identified, which can be involved in accepting iron after heme catabolism as well as intracellular iron transport. In fact, HO1, NADPH-cytochrome P450 reductase, and PCBP2 form a functional unit that integrates the catabolism of heme with the binding and transport of iron by PCBP2. In this review, we provide an overview of our understanding of the iron chaperones and discuss the mechanism how iron chaperones bind iron released during the process of heme degradation.

DMT1 Expression and Iron Levels at the Crossroads Between Aging and Neurodegeneration

Iron homeostasis is an essential prerequisite for metabolic and neurological functions throughout the healthy human life, with a dynamic interplay between intracellular and systemic iron metabolism. The development of different neurodegenerative diseases is associated with alterations of the intracellular transport of iron and heavy metals, principally mediated by Divalent Metal Transporter 1 (DMT1), responsible for Non-Transferrin Bound Iron transport (NTBI). In addition, DMT1 regulation and its compartmentalization in specific brain regions play important roles during aging. This review highlights the contribution of DMT1 to the physiological exchange and distribution of body iron and heavy metals during aging and neurodegenerative diseases. DMT1 also mediates the crosstalk between central nervous system and peripheral tissues, by systemic diffusion through the Blood Brain Barrier (BBB), with the involvement of peripheral iron homeostasis in association with inflammation. In conclusion, a survey about the role of DMT1 and iron will illustrate the complex panel of interrelationship with aging, neurodegeneration and neuroinflammation.

DMT1 and iron transport

Many past and recent advances in the field of iron metabolism have relied upon the discovery of divalent metal transporter 1, DMT1 in 1997. DMT1 is the major iron transporter and contributes non-heme iron uptake in most types of cell. Each DMT1 isoform exhibits different expression patterns in cell-type specificity and distinct subcellular distribution, which enables cells to uptake both transferrin-bound and non-transferrin-bound irons efficiently. DMT1 expression is regulated by iron through the translational and degradation pathways to ensure iron homeostasis. It is considered that mammalian iron transporters including DMT1 cannot transport ferric iron but ferrous iron. Being reduced to ferrous state is likely to damage cells and tissues through the production of reactive oxygen species. Recently, iron chaperones have been identified, which can provide an answer to how ferrous iron is transported safely in cytosol. We summarize DMT1 expression depending on the types of cell or tissue and the function and mechanism of one of the iron chaperones, PCBP2.

Targeted LC-MS/MS Proteomics-Based Strategy To Characterize in Vitro Models Used in Drug Metabolism and Transport Studies

Subcellular fractionation of tissue homogenate provides enriched in vitro models (e.g., microsomes, cytosol, or membranes), which are routinely used in the drug metabolism or transporter activity and protein abundance studies. However, batch-to-batch or interlaboratory variability in the recovery, enrichment, and purity of the subcellular fractions can affect performance of in vitro models leading to inaccurate in vitro to in vivo extrapolation (IVIVE) of drug clearance. To evaluate the quality of subcellular fractions, we developed a simple, targeted, and sensitive LC-MS/MS proteomics-based strategy, which relies on determination of protein markers of various cellular organelles, i.e., plasma membrane, cytosol, nuclei, mitochondria, endoplasmic reticulum (ER), lysosomes, peroxisomes, cytoskeleton, and exosomes. Application of the quantitative proteomics method confirmed a significant effect of processing variables (i.e., homogenization method and centrifugation speed) on the recovery, enrichment, and purity of isolated proteins in microsomes and cytosol. Particularly, markers of endoplasmic reticulum lumen and mitochondrial lumen were enriched in the cytosolic fractions as a result of their release during homogenization. Similarly, the relative recovery and composition of the total membrane fraction isolated from cell vs tissue samples was quantitatively different and should be considered in IVIVE. Further, analysis of exosomes isolated from sandwich-cultured hepatocyte media showed the effect of culture duration on compositions of purified exosomes. Therefore, the quantitative proteomics-based strategy developed here can be applied for efficient and simultaneous determination of multiple protein markers of various cellular organelles when compared to antibody- or activity-based assays and can be used for quality control of subcellular fractionation procedures including in vitro model development for drug metabolism and transport studies.

Duplication and Sub/Neofunctionalization of Malvolio, an Insect Homolog of Nramp, in the Subsocial Beetle Nicrophorus vespilloides

With growing numbers of sequenced genomes, increasing numbers of duplicate genes are being uncovered. Here we examine Malvolio, a gene in the natural resistance-associated macrophage protein (Nramp) family, that has been duplicated in the subsocial beetle, Nicrophorus vespilloides, which exhibits advanced parental behavior. There is only one copy of Mvl in honey bees and Drosophila, whereas in vertebrates there are two copies that are subfunctionalized. We first compared amino acid sequences for Drosophila, beetles, mice, and humans. We found a high level of conservation between the different species, although there was greater variation in the C-terminal regions. A phylogenetic analysis across multiple insect orders suggested that Mvl has undergone several independent duplications. To examine the potential for different functions where it has been duplicated, we quantified expression levels of Mvl1 and Mvl2 in eight tissues in N. vespilloides We found that while Mvl1 was expressed ubiquitously, albeit at varying levels, expression of Mvl2 was limited to brain and midgut. Because Mvl has been implicated in behavior, we examined expression during different behavioral states that reflected differences in opportunity for social interactions and expression of parental care behaviors. We found differing expression patterns for the two copies, with Mvl1 increasing in expression during resource preparation and feeding offspring, and Mvl2 decreasing in these same states. Given these patterns of expression, along with the protein analysis, we suggest that Mvl in N. vespilloides has experienced sub/neofunctionalization following its duplication, and may be evolving differing and tissue-specific roles in behavior and physiology.

The iron chaperone poly(rC)-binding protein 2 forms a metabolon with the heme oxygenase 1/cytochrome P450 reductase complex for heme catabolism and iron transfer

Mammals incorporate a major proportion of absorbed iron as heme, which is catabolized by the heme oxygenase 1 (HO1)-NADPH-cytochrome P450 reductase (CPR) complex into biliverdin, carbon monoxide, and ferrous iron. Moreover, intestinal iron is incorporated as ferrous iron, which is transported via the iron importer, divalent metal transporter 1 (DMT1). Recently, we demonstrated that the iron chaperone poly(rC)-binding protein 2 (PCBP2) can directly receive ferrous iron from DMT1 or transfer iron to the iron exporter, ferroportin 1. To promote intracellular iron flux, an iron chaperone may be essential for receiving iron generated by heme catabolism, but this hypothesis is untested so far. Herein, we demonstrate that HO1 binds to PCBP2, but not to other PCBP family members, namely PCBP1, PCBP3, or PCBP4. Interestingly, HO1 formed a complex with either CPR or PCBP2, and it was demonstrated that PCBP2 competes with CPR for HO1 binding. Using PCBP2-deletion mutants, we demonstrated that the PCBP2 K homology 3 domain is important for the HO1/PCBP2 interaction. In heme-loaded cells, heme prompted HO1-CPR complex formation and decreased the HO1/PCBP2 interaction. Furthermore, in vitro reconstitution experiments with purified recombinant proteins indicated that HO1 could bind to PCBP2 in the presence of heme, whereas loading of PCBP2 with ferrous iron caused PCBP2 to lose its affinity for HO1. These results indicate that ferrous iron released from heme can be bound by PCBP2 and suggest a model for an integrated heme catabolism and iron transport metabolon.

Intra- or extra-exosomal secretion of HDGF isoforms: the extraordinary function of the HDGF-A N-terminal peptide

Hepatoma-derived growth factor (HDGF) is a protein with diverse intracellular functions. Moreover, after non-conventional secretion, extracellular HDGF is able to influence different signaling pathways, leading for example to induction of processes like epithelial-mesenchymal transition (EMT) and cell migration. Intriguingly, in recent proteome studies, HDGF was also found secreted by special microvesicles called exosomes. Recently, we demonstrated the existence of two new HDGF isoforms (B and C). These isoforms are involved in different cellular processes than HDGF-A. Along this line, in the present study we discovered that full length HDGF-A clearly is located inside of exosomes, whereas the isoforms HDGF-B and HDGF-C are found exclusively on the outer surface. Furthermore, while HDGF-B and HDGF-C seem to use exosomes mediated pathway exclusively, HDGF-A was found also as unbound protein in the conditioned media. The new finding of an intra- or extra-exosomal localisation of protein splice variants opens a fascinating new perspective concerning functional diversity of HDGF isoforms. Dysregulation of HDGF expression during cancer development and tumor progression is a commonly known fact. With our new findings, unraveling the potential functional impact according to physiological versus pathophysiologically altered levels and compositions of intra- and extra-exosomal HDGF has to be addressed in future studies.

The effect of amino acid deprivation on the transfer of iron through Caco-2 cell monolayers

Iron (Fe) metabolism is modified by many nutritional factors. Amino acids (AA) play a central role in various biological processes, such as protein synthesis and energy supply. However, the influence of AA status on iron metabolism has not been investigated. Here, we test whether AA alters iron metabolism in an intestinal cell model. Both Fe uptake and transfer across the cell monolayer were significantly increased by non-essential AA deficiency (both p<0.001) while only Fe transfer was increased by essential AA deficiency (p<0.0001). Both essential and non-essential AA deficiency decreased DMT1 (±IRE) exon1A mRNA expression (respectively p=0.0007 and p=0.006) and increased expression of ferritin heavy chain. DMT1+IRE (also expressing exon1A or 1B) mRNA levels were decreased by essential AA deficiency (p=0.012). The mRNA levels of total DMT1 were also decreased by essential, but not non-essential, AA deficiency (p=0.006). Hepcidin levels were increased significantly by non-essential amino acid deprivation (p=0.047). Protein levels of ferroportin and/or ferritin heavy chain were not altered by AA deficiency, suggesting that they had no effect on Fe efflux or storage in the cell, though iron content of ferritin could be increased. Our data demonstrate, for the first time, that AA status affects iron transport and the expression of genes related to iron metabolism in Caco-2 cells, although the changes observed are not sufficient to explain the alteration in iron transport. We hypothesise that the effect on Fe transfer is mediated through an increased movement across the cell layer, rather than transfer across the cell membranes.

Ferrous Iron Up-regulation in Fibroblasts of Patients with Beta Propeller Protein-Associated Neurodegeneration (BPAN)

Mutations in WDR45 gene, coding for a beta-propeller protein, have been found in patients affected by Neurodegeneration with Brain Iron Accumulation, NBIA5 (also known as BPAN). BPAN is a movement disorder with Non Transferrin Bound Iron (NTBI) accumulation in the basal ganglia as common hallmark between NBIA classes (Hayflick et al., 2013). WDR45 has been predicted to have a role in autophagy, while the impairment of iron metabolism in the different NBIA subclasses has not currently been clarified. We found the up-regulation of the ferrous iron transporter (-)IRE/Divalent Metal Transporter1 and down-regulation of Transferrin receptor in the fibroblasts of two BPAN affected patients with splicing mutations 235+1G>A (BPAN1) and 517_519ΔVal 173 (BPAN2). The BPAN patients showed a concomitant increase of intracellular ferrous iron after starvation. An altered pattern of iron transporters with iron overload is highlighted in BPAN human fibroblasts, supporting for a role of DMT1 in NBIA. We here present a novel element, about iron accumulation, to the existing knowledge in field of NBIA. Attention is focused to a starvation-dependent iron overload, possibly accounting for iron accumulation in the basal ganglia. Further investigation could clarify iron regulation in BPAN.

Cystinosin-LKG rescues cystine accumulation and decreases apoptosis rate in cystinotic proximal tubular epithelial cells

BACKGROUND

Nephropathic cystinosis is a lysosomal storage disease that is caused by mutations in the CTNS gene encoding a cystine/proton symporter cystinosin and an isoform cystinosin-LKG which is generated by an alternative splicing of exon 12. We have investigated the physiological role of the cystinosin-LKG that is widely expressed in epithelial tissues.

METHODS

We have analyzed the intracellular localization and the function of the cystinosin-LKG conjugated with DsRed (cystinosin-LKG-RFP) in Madin-Darby canine kidney cells (MDCK II) and in proximal tubular epithelial cells carrying a deletion of the CTNS gene (cystinotic PTEC), respectively.

RESULTS

Cystinosin-LKG-RFP colocalized with markers of lysosomes, late endosomes and was also expressed on the apical surface of polarized MDCK II cells. Moreover, immune-electron microscopy images of MDCK II cells overexpressing cystinosin-LKG-RFP showed stacked lamellar membranes inside perinuclear lysosomal structures. To study the role of LKG-isoform, we have investigated cystine accumulation and apoptosis that have been described in cystinotic cells. Cystinosin-LKG decreased cystine levels by approximately 10-fold similarly to cystinosin-RFP. The levels of TNFα- and actinomycin D-inducted apoptosis dropped in cystinotic cells expressing LKG-isoform. This effect was also similar to the main isoform.

CONCLUSION

Our results suggest that cystinosin-LKG and cystinosin move similar functional activities in cells.

Structural Mechanism for Cargo Recognition by the Retromer Complex

Retromer is a multi-protein complex that recycles transmembrane cargo from endosomes to the trans-Golgi network and the plasma membrane. Defects in retromer impair various cellular processes and underlie some forms of Alzheimer's disease and Parkinson's disease. Although retromer was discovered over 15 years ago, the mechanisms for cargo recognition and recruitment to endosomes have remained elusive. Here, we present an X-ray crystallographic analysis of a four-component complex comprising the VPS26 and VPS35 subunits of retromer, the sorting nexin SNX3, and a recycling signal from the divalent cation transporter DMT1-II. This analysis identifies a binding site for canonical recycling signals at the interface between VPS26 and SNX3. In addition, the structure highlights a network of cooperative interactions among the VPS subunits, SNX3, and cargo that couple signal-recognition to membrane recruitment.

Iron transport in the kidney: implications for physiology and cadmium nephrotoxicity

The kidney has recently emerged as an organ with a significant role in systemic iron (Fe) homeostasis. Substantial amounts of Fe are filtered by the kidney, which have to be reabsorbed to prevent Fe deficiency. Accordingly Fe transporters and receptors for protein-bound Fe are expressed in the nephron that may also function as entry pathways for toxic metals, such as cadmium (Cd), by way of "ionic and molecular mimicry". Similarities, but also differences in handling of Cd by these transport routes offer rationales for the propensity of the kidney to develop Cd toxicity. This critical review provides a comprehensive update on Fe transport by the kidney and its relevance for physiology and Cd nephrotoxicity. Based on quantitative considerations, we have also estimated the in vivo relevance of the described transport pathways for physiology and toxicology. Under physiological conditions all segments of the kidney tubules are likely to utilize Fe for cellular Fe requiring processes for metabolic purposes and also to contribute to reabsorption of free and bound forms of Fe into the circulation. But Cd entering tubule cells disrupts metabolic pathways and is unable to exit. Furthermore, our quantitative analyses contest established models linking chronic Cd nephrotoxicity to proximal tubular uptake of metallothionein-bound Cd. Hence, Fe transport by the kidney may be beneficial by preventing losses from the body. But increased uptake of Fe or Cd that cannot exit tubule cells may lead to kidney injury, and Fe deficiency may facilitate renal Cd uptake.

Iron Export through the Transporter Ferroportin 1 Is Modulated by the Iron Chaperone PCBP2

Ferroportin 1 (FPN1) is an iron export protein found in mammals. FPN1 is important for the export of iron across the basolateral membrane of absorptive enterocytes and across the plasma membrane of macrophages. The expression of FPN1 is regulated by hepcidin, which binds to FPN1 and then induces its degradation. Previously, we demonstrated that divalent metal transporter 1 (DMT1) interacts with the intracellular iron chaperone protein poly(rC)-binding protein 2 (PCBP2). Subsequently, PCBP2 receives iron from DMT1 and then disengages from the transporter. In this study, we investigated the function of PCBP2 in iron export. Mammalian genomes encode four PCBPs (i.e. PCBP1-4). Here, for the first time, we demonstrated using both yeast and mammalian cells that PCBP2, but not PCBP1, PCBP3, or PCBP4, binds with FPN1. Importantly, iron-loaded, but not iron-depleted, PCBP2 interacts with FPN1. The PCBP2-binding domain of FPN1 was identified in its C-terminal cytoplasmic region. The silencing of PCBP2 expression suppressed FPN1-dependent iron export from cells. These results suggest that FPN1 exports iron received from the iron chaperone PCBP2. Therefore, it was found that PCBP2 modulates cellular iron export, which is an important physiological process.

ACBD2/ECI2-Mediated Peroxisome-Mitochondria Interactions in Leydig Cell Steroid Biosynthesis

Fatty acid metabolism and steroid biosynthesis are 2 major pathways shared by peroxisomes and mitochondria. Both organelles are in close apposition to the endoplasmic reticulum, with which they communicate via interorganelle membrane contact sites to promote cellular signaling and the exchange of ions and lipids. To date, no convincing evidence of the direct contact between peroxisomes and mitochondria was reported in mammalian cells. Hormone-induced, tightly controlled steroid hormone biosynthesis requires interorganelle interactions. Using immunofluorescent staining and live-cell imaging, we found that dibutyryl-cAMP treatment of MA-10 mouse tumor Leydig cells rapidly induces peroxisomes to approach mitochondria and form peroxisome-mitochondrial contact sites/fusion, revealed by the subcellular distribution of the endogenous acyl-coenzyme A-binding domain (ACBD)2/ECI2 isoform A generated by alternative splicing, and further validated using a proximity ligation assay. This event occurs likely via a peroxisome-like structure, which is mediated by peroxisomal and mitochondrial matrix protein import complexes: peroxisomal import receptor peroxisomal biogenesis factor 5 (PEX5), and the mitochondrial import receptor subunit translocase of outer mitochondrial membrane 20 homolog (yeast) protein. Similar results were obtained using the mLTC-1 mouse tumor Leydig cells. Ectopic expression of the ACBD2/ECI2 isoform A in MA-10 cells led to increased basal and hormone-stimulated steroid formation, indicating that ACBD2/ECI2-mediated peroxisomes-mitochondria interactions favor in the exchange of metabolites and/or macromolecules between these 2 organelles in support of steroid biosynthesis. Considering the widespread occurrence of the ACBD2/ECI2 protein, we propose that this protein might serve as a tool to assist in understanding the contact between peroxisomes and mitochondria.

Endometrial expression and in vitro modulation of the iron transporter divalent metal transporter-1: implications for endometriosis

OBJECTIVE

To evaluate divalent metal transporter-1 (DMT1) expression in healthy women's and endometriosis patients' endometrium and to analyze DMT1 and ferritin light chain (Fn-L) expression modulation by iron overload and IL-1β in endometrial stromal cells (ESCs).

DESIGN

Observational and experimental study.

SETTING

University hospital research laboratory.

PATIENT(S)

Thirty-one healthy women and 24 endometriosis patients.

INTERVENTION(S)

Menstrual, proliferative, and secretory endometrial biopsies. Isolated ESCs from seven endometrial biopsies incubated with IL-1β or FeSO4 overload for 24 hours.

MAIN OUTCOME MEASURE(S)

Divalent metal transporter-1 endometrial protein expression assessed by immunohistochemistry and Western blot. Divalent metal transporter-1 and Fn-L proteins expression in stimulated ESCs evaluated by Western blot.

RESULT(S)

Divalent metal transporter-1 is expressed throughout the menstrual cycle in human endometrium. Four endometrial DMT1 variants were identified accordingly to their molecular weight: DMT-80, -65, -55, and -50. Endometrial expression of DMT-80 and -55 is higher in endometriosis patients than in healthy women. In ESCs, iron overload induces an overexpression of DMT-80, DMT-50, and Fn-L, whereas IL-1β increases DMT-80 and -50 expressions and decreases Fn-L expression.

CONCLUSION(S)

Divalent metal transporter-1 overexpression in endometriosis patients' endometrium can increase iron influx to endometrial cells, inducing oxidative stress-mediated proinflammatory signaling. In turn, endometriosis-related conditions, as iron overload and inflammation (IL-1β), enhance endometriosis patients endometrial DMT1 expression, creating a vicious circle on DMT-1-modulated pathways.

Constant change: dynamic regulation of membrane transport by calcium signalling networks keeps plants in tune with their environment

Despite substantial variation and irregularities in their environment, plants must conform to spatiotemporal demands on the molecular composition of their cytosol. Cell membranes are the major interface between organisms and their environment and the basis for controlling the contents and intracellular organization of the cell. Membrane transport proteins (MTPs) govern the flow of molecules across membranes, and their activities are closely monitored and regulated by cell signalling networks. By continuously adjusting MTP activities, plants can mitigate the effects of environmental perturbations, but effective implementation of this strategy is reliant on precise coordination among transport systems that reside in distinct cell types and membranes. Here, we examine the role of calcium signalling in the coordination of membrane transport, with an emphasis on potassium transport. Potassium is an exceptionally abundant and mobile ion in plants, and plant potassium transport has been intensively studied for decades. Classic and recent studies have underscored the importance of calcium in plant environmental responses and membrane transport regulation. In reviewing recent advances in our understanding of the coding and decoding of calcium signals, we highlight established and emerging roles of calcium signalling in coordinating membrane transport among multiple subcellular locations and distinct transport systems in plants, drawing examples from the CBL-CIPK signalling network. By synthesizing classical studies and recent findings, we aim to provide timely insights on the role of calcium signalling networks in the modulation of membrane transport and its importance in plant environmental responses.

Iron mitigates DMT1-mediated manganese cytotoxicity via the ASK1-JNK signaling axis: Implications of iron supplementation for manganese toxicity

Manganese (Mn²⁺) neurotoxicity from occupational exposure is well documented to result in a Parkinson-like syndrome. Although the understanding of Mn²⁺ cytotoxicity is still incomplete, both Mn²⁺ and Fe²⁺ can be transported via the divalent metal transporter 1 (DMT1), suggesting that competitive uptake might disrupt Fe²⁺ homeostasis. Here, we found that DMT1 overexpression significantly enhanced Mn²⁺ cytoplasmic accumulation and JNK phosphorylation, leading to a reduction in cell viability. Although a robust activation of autophagy was observed alongside these changes, it did not trigger autophagic cell death, but was instead shown to be essential for the degradation of ferritin, which normally sequesters labile Fe²⁺. Inhibition of ferritin degradation through the neutralization of lysosomal pH resulted in increased ferritin and enhanced cytoplasmic Fe²⁺ depletion. Similarly, direct Fe²⁺ chelation also resulted in aggravated Mn²⁺-mediated JNK phosphorylation, while Fe²⁺ repletion protected cells, and this occurs via the ASK1-thioredoxin pathway. Taken together, our study presents the novel findings that Mn²⁺ cytotoxicity involves the depletion of the cytoplasmic Fe²⁺ pool, and the increase in autophagy-lysosome activity is important to maintain Fe²⁺ homeostasis. Thus, Fe²⁺ supplementation could have potential applications in the prevention and treatment of Mn²⁺-mediated toxicity.

Homeostatic regulation of trace mineral transport by ubiquitination of membrane transporters

Post-translational modification is a critical mechanism by which trace mineral transporters rapidly adapt to their environment to homeostatically regulate ion transport. Recently, a novel pathway was described whereby iron stimulates the ubiquitination and proteasomal degradation of the trace mineral transporter ZIP14. Discovery of this pathway suggests the proteasome as a potential therapeutic target for regulation of iron storage. Moreover, these findings contribute to a theoretical framework that can be applied to other ubiquitinated trace mineral transporters. This review will detail the current state of knowledge regarding the ubiquitination of trace mineral transporters, focusing on iron and zinc transporters, and the potential utility of post-translational modification of trace mineral transporters in the treatment of disease.

Intestinal DMT1 is critical for iron absorption in the mouse but is not required for the absorption of copper or manganese

Divalent metal-ion transporter-1 (DMT1) is a widely expressed iron-preferring membrane-transport protein that serves a critical role in erythroid iron utilization. We have investigated its role in intestinal metal absorption by studying a mouse model lacking intestinal DMT1 (i.e., DMT1(int/int)). DMT1(int/int) mice exhibited a profound hypochromic-microcytic anemia, splenomegaly, and cardiomegaly. That the anemia was due to iron deficiency was demonstrated by the following observations in DMT1(int/int) mice: 1) blood iron and tissue nonheme-iron stores were depleted; 2) mRNA expression of liver hepcidin (Hamp1) was depressed; and 3) intraperitoneal iron injection corrected the anemia, and reversed the changes in blood iron, nonheme-iron stores, and hepcidin expression levels. We observed decreased total iron content in multiple tissues from DMT1(int/int) mice compared with DMT1(+/+) mice but no meaningful change in copper, manganese, or zinc. DMT1(int/int) mice absorbed (64)Cu and (54)Mn from an intragastric dose to the same extent as did DMT1(+/+) mice but the absorption of (59)Fe was virtually abolished in DMT1(int/int) mice. This study reveals a critical function for DMT1 in intestinal nonheme-iron absorption for normal growth and development. Further, this work demonstrates that intestinal DMT1 is not required for the intestinal transport of copper, manganese, or zinc.

Glycosylation of solute carriers: mechanisms and functional consequences

Solute carriers (SLCs) are one of the largest groups of multi-spanning membrane proteins in mammals and include ubiquitously expressed proteins as well as proteins with highly restricted tissue expression. A vast number of studies have addressed the function and organization of SLCs as well as their posttranslational regulation, but only relatively little is known about the role of SLC glycosylation. Glycosylation is one of the most abundant posttranslational modifications of animal proteins and through recent advances in our understanding of protein-glycan interactions, the functional roles of SLC glycosylation are slowly emerging. The purpose of this review is to provide a concise overview of the aspects of glycobiology most relevant to SLCs, to discuss the roles of glycosylation in the regulation and function of SLCs, and to outline the major open questions in this field, which can now be addressed given major technical advances in this and related fields of study in recent years.

Iron-induced reactive oxygen species mediate transporter DMT1 endocytosis and iron uptake in intestinal epithelial cells

Recent evidence shows that iron induces the endocytosis of the iron transporter dimetal transporter 1 (DMT1) during intestinal absorption. We, and others, have proposed that iron-induced DMT1 internalization underlies the mucosal block phenomena, a regulatory response that downregulates intestinal iron uptake after a large oral dose of iron. In this work, we investigated the participation of reactive oxygen species (ROS) in the establishment of this response. By means of selective surface protein biotinylation of polarized Caco-2 cells, we determined the kinetics of DMT1 internalization from the apical membrane after an iron challenge. The initial decrease in DMT1 levels in the apical membrane induced by iron was followed at later times by increased levels of DMT1. Addition of Fe(2+), but not of Cd(2+), Zn(2+), Cu(2+), or Cu(1+), induced the production of intracellular ROS, as detected by 2',7'-dichlorofluorescein (DCF) fluorescence. Preincubation with the antioxidant N-acetyl-l-cysteine (NAC) resulted in increased DMT1 at the apical membrane before and after addition of iron. Similarly, preincubation with the hydroxyl radical scavenger dimethyl sulfoxide (DMSO) resulted in the enhanced presence of DMT1 at the apical membrane. The decrease of DMT1 levels at the apical membrane induced by iron was associated with decreased iron uptake rates. A kinetic mathematical model based on operational rate constants of DMT1 endocytosis and exocytosis is proposed. The model qualitatively captures the experimental observations and accurately describes the effect of iron, NAC, and DMSO on the apical distribution of DMT1. Taken together, our data suggest that iron uptake induces the production of ROS, which modify DMT1 endocytic cycling, thus changing the iron transport activity at the apical membrane.

An overview of molecular basis of iron metabolism regulation and the associated pathologies

Iron is essential for several vital biological processes. Its deficiency or overload drives to the development of several pathologies. To maintain iron homeostasis, the organism controls the dietary iron absorption by enterocytes, its recycling by macrophages and storage in hepatocytes. These processes are mainly controlled by hepcidin, a liver-derived hormone which synthesis is regulated by iron levels, inflammation, infection, anemia and erythropoiesis. Besides the systemic regulation of iron metabolism mediated by hepcidin, cellular regulatory processes also occur. Cells are able to regulate themselves the expression of the iron metabolism-related genes through different post-transcriptional mechanisms, such as the alternative splicing, microRNAs, the IRP/IRE system and the proteolytic cleavage. Whenever those mechanisms are disturbed, due to genetic or environmental factors, iron homeostasis is disrupted and iron related pathologies may arise.

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.

Role of NADPH oxidase in the regulation of autophagy in cardiomyocytes

In the past several years, it has been demonstrated that the reactive oxygen species (ROS) may act as intracellular signalling molecules to activate or inhibit specific signalling pathways and regulate physiological cellular functions. It is now well-established that ROS regulate autophagy, an intracellular degradation process. However, the signalling mechanisms through which ROS modulate autophagy in a regulated manner have only been minimally clarified. NADPH oxidase (Nox) enzymes are membrane-bound enzymatic complexes responsible for the dedicated generation of ROS. Different isoforms of Nox exist with different functions. Recent studies demonstrated that Nox-derived ROS can promote autophagy, with Nox2 and Nox4 representing the isoforms of Nox implicated thus far. Nox2- and Nox4-dependent autophagy plays an important role in the elimination of pathogens by phagocytes and in the regulation of vascular- and cancer-cell survival. Interestingly, we recently found that Nox is also important for autophagy regulation in cardiomyocytes. We found that Nox4, but not Nox2, promotes the activation of autophagy and survival in cardiomyocytes in response to nutrient deprivation and ischaemia through activation of the PERK (protein kinase RNA-like endoplasmic reticulum kinase) signalling pathway. In the present paper, we discuss the importance of Nox family proteins and ROS in the regulation of autophagy, with a particular focus on the role of Nox4 in the regulation of autophagy in the heart.

Chaperone protein involved in transmembrane transport of iron

DMT1 (divalent metal transporter 1) is the main iron importer found in animals, and ferrous iron is taken up by cells via DMT1. Once ferrous iron reaches the cytosol, it is subjected to subcellular distribution and delivered to various sites where iron is required for a variety of biochemical reactions in the cell. Until now, the mechanism connecting the transporter and cytosolic distribution had not been clarified. In the present study, we have identified PCBP2 [poly(rC)-binding protein 2] as a DMT1-binding protein. The N-terminal cytoplasmic region of DMT1 is the binding domain for PCBP2. An interaction between DMT1 and PCBP1, which is known to be a paralogue of PCBP2, could not be demonstrated in vivo or in vitro. Iron uptake and subsequent ferritin expression were suppressed by either DMT1 or PCBP2 knockdown. Iron-associated DMT1 could interact with PCBP2 in vitro, whereas iron-chelated DMT1 could not. These results indicate that ferrous iron imported by DMT1 is transferred directly to PCBP2. Moreover, we demonstrated that PCBP2 could bind to ferroportin, which exports ferrous iron out of the cell. These findings suggest that PCBP2 can transfer ferrous iron from DMT1 to the appropriate intracellular sites or ferroportin and could function as an iron chaperone.

Genome-wide RNAi screen identifies SEC61A and VCP as conserved regulators of Sindbis virus entry

Alphaviruses are a large class of insect-borne human pathogens and little is known about the host-factor requirements for infection. To identify such factors, we performed a genome-wide RNAi screen using model Drosophila cells and validated 94 genes that impacted infection of Sindbis virus (SINV), the prototypical alphavirus. We identified a conserved role for SEC61A and valosin-containing protein (VCP) in facilitating SINV entry in insects and mammals. SEC61A and VCP selectively regulate trafficking of the entry receptor NRAMP2, and loss or pharmacological inhibition of these proteins leads to altered NRAMP2 trafficking to lysosomal compartments and proteolytic digestion within lysosomes. NRAMP2 is the major iron transporter in cells, and loss of NRAMP2 attenuates intracellular iron transport. Thus, this study reveals genes and pathways involved in both infection and iron homeostasis that may serve as targets for antiviral therapeutics or for iron-imbalance disorders.

Mitosis-coupled, microtubule-dependent clustering of endosomal vesicles around centrosomes

Upon cell division, not only cells themselves but also their organelles undergo drastic shape changes, although the behaviors of organelles other than the Golgi apparatus remain poorly understood. We followed the spatiotemporal changes in the localization of transferrin receptor (TfnR) and other proteins. In early mitotic phases, a population of proteins cycling through the endocytic recycling compartment (ERC) exhibits a distinct spatiotemporal change from that of Golgi proteins. In prophase/prometaphase, when the cell surface-to-volume ratio is reaching its minimum, the ERC proteins are transiently assembled around the centrated centrosome in a microtubule- and dynein-dependent manner, and soon separated polewards into two clusters concomitant with separation of duplicated centrosomes. Electron microscopic analysis revealed that endosomal vesicles containing endocytosed transferrin cluster tightly around centrosomes without fusing with one another. As cytokinesis proceeds, the clusters gradually collapse, and the ERC proteins reassemble around the furrowing equatorial region. FRAP (fluorescence recovery after photobleaching) analyses of EGFP-TfnR-expressing cells revealed minimal membrane exchange between the endosomal clusters and other cellular compartments until anaphase/telophase, when membrane traffic resumes. Our observations indicate that ERC clustering around centrosomes plays a fundamental role in restricting membrane delivery to the plasma membrane during early mitotic phases, when the cell surface-to-volume ratio reaches its minimum.

Genetically programmed superparamagnetic behavior of mammalian cells

Although magnetic fields and paramagnetic inorganic materials were abundant on planet earth during the entire evolution of living species the interaction of organisms with these physical forces remains a little-understood phenomenon. Interestingly, rather than being genetically encoded, organisms seem to accumulate and take advantage of inorganic nanoparticles to sense or react to magnetic fields. Using a synthetic biology-inspired approach we have genetically programmed mammalian cells to show superparamagnetic behavior. The combination of ectopic production of the human ferritin heavy chain 1 (hFTH1), engineering the cells for expression of an iron importer, the divalent metal ion transferase 1 (DMT1) and the design of an iron-loading culture medium to maximize cellular iron uptake enabled efficient iron mineralization in intracellular ferritin particles and conferred superparamagnetic behavior to the entire cell. When captured by a magnetic field the superparamagnetic cells reached attraction velocities of up to 30 μm/s and could be efficiently separated from complex cell mixtures using standard magnetic cell separation equipment. Technology that enables magnetic separation of genetically programmed superparamagnetic cells in the absence of inorganic particles could foster novel opportunities in diagnostics and cell-based therapies.

Rab11 regulates exocytosis of recycling vesicles at the plasma membrane

Rab11 is known to associate primarily with perinuclear recycling endosomes and regulate recycling of endocytosed proteins. However, the recycling step in which Rab11 participates remains unknown. We show here that, in addition to causing tubulation of recycling endosomes, Rab11 depletion gives rise to accumulation of recycling carriers containing endocytosed transferrin and transferrin receptor beneath the plasma membrane. We also show that the carriers are transported from perinuclear recycling endosomes to the cell periphery along microtubules. Total internal reflection fluorescence microscopy of cells expressing EGFP-tagged transferrin receptor revealed that Rab11 depletion inhibits tethering and fusion of recycling carriers to the plasma membrane. Depletion of Sec15, which interacts with Rab11, or Exo70, both components of the exocyst tethering complex, leads to essentially the same phenotypes as those of Rab11 depletion. Thus, in addition to its role in recycling processes at perinuclear recycling endosomes, Rab11 is transported along microtubules to the cell periphery through association with recycling carriers, and directly regulates vesicle exocytosis at the plasma membrane in concert with the exocyst.

1B/(-)IRE DMT1 expression during brain ischemia contributes to cell death mediated by NF-κB/RelA acetylation at Lys310

The molecular mechanisms responsible for increasing iron and neurodegeneration in brain ischemia are an interesting area of research which could open new therapeutic approaches. Previous evidence has shown that activation of nuclear factor kappa B (NF-κB) through RelA acetylation on Lys310 is the prerequisite for p50/RelA-mediated apoptosis in cellular and animal models of brain ischemia. We hypothesized that the increase of iron through a NF-κB-regulated 1B isoform of the divalent metal transporter-1 (1B/DMT1) might contribute to post-ischemic neuronal damage. Both in mice subjected to transient middle cerebral artery occlusion (MCAO) and in neuronally differentiated SK-N-SH cells exposed to oxygen-glucose-deprivation (OGD), 1A/DMT1 was only barely expressed while the 1B/DMT1 without iron-response-element (−IRE) protein and mRNA were early up-regulated. Either OGD or over-expression of 1B/(−)IRE DMT1 isoform significantly increased iron uptake, as detected by total reflection X-ray fluorescence, and iron-dependent cell death. Iron chelation by deferoxamine treatment or (−)IRE DMT1 RNA silencing displayed significant neuroprotection against OGD which concomitantly decreased intracellular iron levels. We found evidence that 1B/(−)IRE DMT1 was a target gene for RelA activation and acetylation on Lys310 residue during ischemia. Chromatin immunoprecipitation analysis of the 1B/DMT1 promoter showed there was increased interaction with RelA and acetylation of H3 histone during OGD exposure of cortical neurons. Over-expression of wild-type RelA increased 1B/DMT1 promoter-luciferase activity, the (−)IRE DMT1 protein, as well as neuronal death. Expression of the acetylation-resistant RelA-K310R construct, which carried a mutation from lysine 310 to arginine, but not the acetyl-mimic mutant RelA-K310Q, down-regulated the 1B/DMT1 promoter, consequently offering neuroprotection. Our data showed that 1B/(−)IRE DMT1 expression and intracellular iron influx are early downstream responses to NF-κB/RelA activation and acetylation during brain ischemia and contribute to the pathogenesis of stroke-induced neuronal damage.

DMT1 (IRE) expression in intestinal and erythroid cells is regulated by peripheral benzodiazepine receptor-associated protein 7

The divalent metal transporter 1 (DMT1) is essential for cellular uptake of iron, mediating iron absorption across the duodenal brush border membrane. We have previously shown that with iron feeding DMT1 in the brush border membrane undergoes endocytosis into the subapical compartment of enterocytes. To understand the mechanisms of iron-induced endocytosis of DMT1, we used the yeast two-hybrid system to find proteins that interact with DMT1 and isolated from a rat duodenal cDNA library a protein that interacts specifically with the IRE containing isoform of DMT1 {DMT1 [iron-responsive element (IRE)]}. The protein (Genbank AY336075) is 97.5% identical with peripheral benzodiazepine receptor-associated protein 7 (PAP7), a protein that interacts with the peripheral benzodiazepine receptor. PAP7 is ubiquitously expressed in the rat and in multiple cell lines with consensus sequences including a nuclear localization signal and a Golgi dynamic domain. PAP7, expressed on the brush border of rat duodenum, copurified with DMT1 in brush border membrane vesicles, and following iron feeding, was internalized in parallel with the internalization of DMT1. To determine if PAP7 plays a role in cellular iron metabolism, we downregulated PAP7 expression in K562 cells with small interfering RNA. Following the decrease in PAP7 protein, DMT1 (IRE) protein but not mRNA was significantly downregulated but without effect on DMT1 (non-IRE), transferin (Tf)R1, or ferritin expression. Lowered levels of PAP7 resulted also in decreased cell proliferation and G(1) cell cycle arrest. These data are consistent with PAP7 interacting with DMT1 (IRE) and regulating DMT1 (IRE) expression in K562 cells by modulating expression of DMT1 (IRE) protein.

H(+)-coupled divalent metal-ion transporter-1: functional properties, physiological roles and therapeutics

Divalent metal-ion transporter-1 (DMT1) is a widely expressed, iron-preferring membrane transport protein. Animal models establish that DMT1 plays indispensable roles in intestinal nonheme-iron absorption and iron acquisition by erythroid precursor cells. Rare mutations in human DMT1 result in severe microcytic-hypochromic anemia. When we express DMT1 in RNA-injected Xenopus oocytes, we observe rheogenic Fe(2+) transport that is driven by the proton electrochemical potential gradient. In that same preparation, DMT1 also transports cadmium and manganese but not copper. Whether manganese metabolism relies upon DMT1 remains unclear but DMT1 contributes to the effects of overexposure to cadmium and manganese in some tissues. There exist at least four DMT1 isoforms that arise from variant transcription of the SLC11A2 gene. Whereas these isoforms display identical functional properties, N- and C-terminal variations contain cues that direct the cell-specific targeting of DMT1 isoforms to discrete subcellular compartments (plasma membrane, endosomes, and lysosomes). An iron-responsive element (IRE) in the mRNA 3'-untranslated region permits the regulation of some isoforms by iron status, and additional mechanisms by which DMT1 is regulated are emerging. Natural-resistance-associated macrophage protein-1 (NRAMP1)-the only other member of the mammalian SLC11 gene family-contributes to antimicrobial function by extruding from the phagolysosome divalent metal ions (e.g. Mn(2+)) that may be essential cofactors for bacteria-derived enzymes or required for bacterial growth. The principal or only intestinal nonheme-iron transporter, DMT1 is a validated therapeutic target in hereditary hemochromatosis (HHC) and other iron-overload disorders.

Expression of divalent metal transporter 1 in primary hippocampal neurons: reconsidering its role in non-transferrin-bound iron influx

The divalent metal transporter 1 (DMT1) is the best characterized Fe²⁺ transporter involved in cellular iron uptake in mammals. Four possible isoforms have been identified as a result of alternative promoter (DMT1-1A and DMT1-1B) and alternative splicing involving the C-terminus and producing transcripts with or without an iron responsive element [DMT1-IRE⁺ and DMT1-IRE⁻, respectively]. Despite the general importance of DMT1 in controlling iron homeostasis, the distribution and the role of the transporter in the CNS is still controversial. In this study, we characterize the expression of DMT1 in hippocampal neurons and astrocytes. We found that the main isoform endogenously expressed is DMT1-1B/IRE⁺, which shows cytoplasmic distribution, colocalization with late endosome/lysosome markers and iron regulation, as expected from the presence of an iron responsive element. Our results also show that DMT1-1B/IRE⁺ isoform does not sustain iron entry, even after its neuronal over-expression. Overall, our results argue against a physiological role of the endogenous DMT1 in neuronal iron uptake but do not exclude that, under pathological conditions, the expression of other DMT1 isoforms might contribute to iron overload.