Iron regulatory protein-1 protects against mitoferrin-1-deficient porphyria

Crosstalk among mitophagy, pyroptosis, ferroptosis, and necroptosis in central nervous system injuries

Central nervous system injuries have a high rate of resulting in disability and mortality; however, at present, effective treatments are lacking. Programmed cell death, which is a genetically determined form of active and ordered cell death with many types, has recently attracted increasing attention due to its functions in determining the fate of cell survival. A growing number of studies have suggested that programmed cell death is involved in central nervous system injuries and plays an important role in the progression of brain damage. In this review, we provide an overview of the role of programmed cell death in central nervous system injuries, including the pathways involved in mitophagy, pyroptosis, ferroptosis, and necroptosis, and the underlying mechanisms by which mitophagy regulates pyroptosis, ferroptosis, and necroptosis. We also discuss the new direction of therapeutic strategies targeting mitophagy for the treatment of central nervous system injuries, with the aim to determine the connection between programmed cell death and central nervous system injuries and to identify new therapies to modulate programmed cell death following central nervous system injury. In conclusion, based on these properties and effects, interventions targeting programmed cell death could be developed as potential therapeutic agents for central nervous system injury patients.

Congenital asplenia impairs heme-iron recycling during erythropoiesis in zebrafish

The spleen is postulated to be a hematopoietic tissue in adult fish; however, clear evidence is still lacking to define its role in hematopoietic activity. In our previous study, a congenitally asplenic zebrafish was generated though gene editing, which provided a new perspective for studying the role of fish spleen in hematopoiesis. In this study, HSC-regulated and erythrocyte marker genes, such as gata1a, gata2, klf1, hbaa1, hbaa2, hbba1 and hbba2 were significantly reduced in congenitally asplenic zebrafish when compared with wild-type (WT). Subsequently, we conducted the transcriptome profiles of whole kidneys from WT and congenitally asplenic zebrafish to explore the possible molecular mechanisms underlying the impaired erythropoiesis caused by congenital asplenia. Our results demonstrated that congenital asplenia might impair heme-iron recycling during erythropoiesis, as evidenced by significant down-regulation of genes associated with iron acquisition (tfr1a, tfa, steap3 and slc25a37) and heme biosynthesis and transport (alas2, fech, uros, urod, copx, ppox and abcb10) in congenitally asplenic zebrafish. In addition, the down-regulation of hemopoiesis-related GO terms, including heme binding, tetrapyrrole binding, iron ion binding, heme metabolic process, heme biosynthetic process, erythrocyte differentiation, iron ion homeostasis and hemoglobin metabolic process confirmed the impaired erythropoiesis induced by congenital asplenia. Our study provides an in-depth understanding of spleen function in regulating heme-iron homeostasis during hematopoiesis, thereby providing valuable insights into pathological responses in splenectomized or congenitally asplenic patients.

Iron and mitochondria in the susceptibility, pathogenesis and progression of COPD

Chronic obstructive pulmonary disease (COPD) is a debilitating lung disease characterised by airflow limitation, chronic bronchitis, emphysema and airway remodelling. Cigarette smoke is considered the primary risk factor for the development of COPD; however, genetic factors, host responses and infection also play an important role. Accumulating evidence highlights a role for iron dyshomeostasis and cellular iron accumulation in the lung as a key contributing factor in the development and pathogenesis of COPD. Recent studies have also shown that mitochondria, the central players in cellular iron utilisation, are dysfunctional in respiratory cells in individuals with COPD, with alterations in mitochondrial bioenergetics and dynamics driving disease progression. Understanding the molecular mechanisms underlying the dysfunction of mitochondria and cellular iron metabolism in the lung may unveil potential novel investigational avenues and therapeutic targets to aid in the treatment of COPD.

Iron Metabolism in the Disorders of Heme Biosynthesis

Given its remarkable property to easily switch between different oxidative states, iron is essential in countless cellular functions which involve redox reactions. At the same time, uncontrolled interactions between iron and its surrounding milieu may be damaging to cells and tissues. Heme—the iron-chelated form of protoporphyrin IX—is a macrocyclic tetrapyrrole and a coordination complex for diatomic gases, accurately engineered by evolution to exploit the catalytic, oxygen-binding, and oxidoreductive properties of iron while minimizing its damaging effects on tissues. The majority of the body production of heme is ultimately incorporated into hemoglobin within mature erythrocytes; thus, regulation of heme biosynthesis by iron is central in erythropoiesis. Additionally, heme is a cofactor in several metabolic pathways, which can be modulated by iron-dependent signals as well. Impairment in some steps of the pathway of heme biosynthesis is the main pathogenetic mechanism of two groups of diseases collectively known as porphyrias and congenital sideroblastic anemias. In porphyrias, according to the specific enzyme involved, heme precursors accumulate up to the enzyme stop in disease-specific patterns and organs. Therefore, different porphyrias manifest themselves under strikingly different clinical pictures. In congenital sideroblastic anemias, instead, an altered utilization of mitochondrial iron by erythroid precursors leads to mitochondrial iron overload and an accumulation of ring sideroblasts in the bone marrow. In line with the complexity of the processes involved, the role of iron in these conditions is then multifarious. This review aims to summarise the most important lines of evidence concerning the interplay between iron and heme metabolism, as well as the clinical and experimental aspects of the role of iron in inherited conditions of altered heme biosynthesis.

Regulation of Heme Synthesis by Mitochondrial Homeostasis Proteins

Heme plays a central role in diverse, life-essential processes that range from ubiquitous, housekeeping pathways such as respiration, to highly cell-specific ones such as oxygen transport by hemoglobin. The regulation of heme synthesis and its utilization is highly regulated and cell-specific. In this review, we have attempted to describe how the heme synthesis machinery is regulated by mitochondrial homeostasis as a means of coupling heme synthesis to its utilization and to the metabolic requirements of the cell. We have focused on discussing the regulation of mitochondrial heme synthesis enzymes by housekeeping proteins, transport of heme intermediates, and regulation of heme synthesis by macromolecular complex formation and mitochondrial metabolism. Recently discovered mechanisms are discussed in the context of the model organisms in which they were identified, while more established work is discussed in light of technological advancements.

Iron in Porphyrias: Friend or Foe?

Iron is a trace element that is important for many vital processes, including oxygen transport, oxidative metabolism, cellular proliferation, and catalytic reactions. Iron supports these functions mainly as part of the heme molecule. Heme synthesis is an eight-step process which, when defective at the level of one of the eight enzymes involved, can cause the development of a group of diseases, either inherited or acquired, called porphyrias. Despite the strict link between iron and heme, the role of iron in the different types of porphyrias, particularly as a risk factor for disease development/progression or as a potential therapeutic target or molecule, is still being debated, since contrasting results have emerged from clinical observations, in vitro studies and animal models. In this review we aim to deepen such aspects by drawing attention to the current evidence on the role of iron in porphyrias and its potential implication. Testing for iron status and its metabolic pathways through blood tests, imaging techniques or genetic studies on patients affected by porphyrias can provide additional diagnostic and prognostic value to the clinical care, leading to a more tailored and effective management.

The immunometabolite itaconate inhibits heme synthesis and remodels cellular metabolism in erythroid precursors

The immunometabolite itaconate is taken up by erythroid precursors and converted to itaconyl-CoA by the CoA transferase SUGCT.

Itaconyl-CoA is a competitive inhibitor of ALAS2 and inhibits erythropoietic heme synthesis.

As part of the inflammatory response by macrophages, Irg1 is induced, resulting in millimolar quantities of itaconate being produced. This immunometabolite remodels the macrophage metabolome and acts as an antimicrobial agent when excreted. Itaconate is not synthesized within the erythron but instead may be acquired from central macrophages within the erythroid island. Previously, we reported that itaconate inhibits hemoglobinization of developing erythroid cells. Herein we show that this action is accomplished by inhibition of tetrapyrrole synthesis. In differentiating erythroid precursors, cellular heme and protoporphyrin IX synthesis are reduced by itaconate at an early step in the pathway. In addition, itaconate causes global alterations in cellular metabolite pools, resulting in elevated levels of succinate, 2-hydroxyglutarate, pyruvate, glyoxylate, and intermediates of glycolytic shunts. Itaconate taken up by the developing erythron can be converted to itaconyl–coenzyme A (CoA) by the enzyme succinyl-CoA:glutarate-CoA transferase. Propionyl-CoA, propionyl-carnitine, methylmalonic acid, heptadecanoic acid, and nonanoic acid, as well as the aliphatic amino acids threonine, valine, methionine, and isoleucine, are increased, likely due to the impact of endogenous itaconyl-CoA synthesis. We further show that itaconyl-CoA is a competitive inhibitor of the erythroid-specific 5-aminolevulinate synthase (ALAS2), the first and rate-limiting step in heme synthesis. These findings strongly support our hypothesis that the inhibition of heme synthesis observed in chronic inflammation is mediated not only by iron limitation but also by limitation of tetrapyrrole synthesis at the point of ALAS2 catalysis by itaconate. Thus, we propose that macrophage-derived itaconate promotes anemia during an inflammatory response in the erythroid compartment.

Effects of Iron and Zinc on Mitochondria: Potential Mechanisms of Glaucomatous Injury

Glaucoma is the most substantial cause of irreversible blinding, which is accompanied by progressive retinal ganglion cell damage. Retinal ganglion cells are energy-intensive neurons that connect the brain and retina, and depend on mitochondrial homeostasis to transduce visual information through the brain. As cofactors that regulate many metabolic signals, iron and zinc have attracted increasing attention in studies on neurons and neurodegenerative diseases. Here, we summarize the research connecting iron, zinc, neuronal mitochondria, and glaucomatous injury, with the aim of updating and expanding the current view of how retinal ganglion cells degenerate in glaucoma, which can reveal novel potential targets for neuroprotection.

Down the Iron Path: Mitochondrial Iron Homeostasis and Beyond

Cellular iron homeostasis and mitochondrial iron homeostasis are interdependent. Mitochondria must import iron to form iron–sulfur clusters and heme, and to incorporate these cofactors along with iron ions into mitochondrial proteins that support essential functions, including cellular respiration. In turn, mitochondria supply the cell with heme and enable the biogenesis of cytosolic and nuclear proteins containing iron–sulfur clusters. Impairment in cellular or mitochondrial iron homeostasis is deleterious and can result in numerous human diseases. Due to its reactivity, iron is stored and trafficked through the body, intracellularly, and within mitochondria via carefully orchestrated processes. Here, we focus on describing the processes of and components involved in mitochondrial iron trafficking and storage, as well as mitochondrial iron–sulfur cluster biogenesis and heme biosynthesis. Recent findings and the most pressing topics for future research are highlighted.

The SLC25 Carrier Family: Important Transport Proteins in Mitochondrial Physiology and Pathology

Members of the mitochondrial carrier family (SLC25) transport a variety of compounds across the inner membrane of mitochondria. These transport steps provide building blocks for the cell and link the pathways of the mitochondrial matrix and cytosol. An increasing number of diseases and pathologies has been associated with their dysfunction. In this review, the molecular basis of these diseases is explained based on our current understanding of their transport mechanism.

Inflaming the Brain with Iron

Iron accumulation and neuroinflammation are pathological conditions found in several neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD). Iron and inflammation are intertwined in a bidirectional relationship, where iron modifies the inflammatory phenotype of microglia and infiltrating macrophages, and in turn, these cells secrete diffusible mediators that reshape neuronal iron homeostasis and regulate iron entry into the brain. Secreted inflammatory mediators include cytokines and reactive oxygen/nitrogen species (ROS/RNS), notably hepcidin and nitric oxide (·NO). Hepcidin is a small cationic peptide with a central role in regulating systemic iron homeostasis. Also present in the cerebrospinal fluid (CSF), hepcidin can reduce iron export from neurons and decreases iron entry through the blood-brain barrier (BBB) by binding to the iron exporter ferroportin 1 (Fpn1). Likewise, ·NO selectively converts cytosolic aconitase (c-aconitase) into the iron regulatory protein 1 (IRP1), which regulates cellular iron homeostasis through its binding to iron response elements (IRE) located in the mRNAs of iron-related proteins. Nitric oxide-activated IRP1 can impair cellular iron homeostasis during neuroinflammation, triggering iron accumulation, especially in the mitochondria, leading to neuronal death. In this review, we will summarize findings that connect neuroinflammation and iron accumulation, which support their causal association in the neurodegenerative processes observed in AD and PD.

Systematic Surveys of Iron Homeostasis Mechanisms Reveal Ferritin Superfamily and Nucleotide Surveillance Regulation to be Modified by PINK1 Absence

Iron deprivation activates mitophagy and extends lifespan in nematodes. In patients suffering from Parkinson’s disease (PD), PINK1-PRKN mutations via deficient mitophagy trigger iron accumulation and reduce lifespan. To evaluate molecular effects of iron chelator drugs as a potential PD therapy, we assessed fibroblasts by global proteome profiles and targeted transcript analyses. In mouse cells, iron shortage decreased protein abundance for iron-binding nucleotide metabolism enzymes (prominently XDH and ferritin homolog RRM2). It also decreased the expression of factors with a role for nucleotide surveillance, which associate with iron-sulfur-clusters (ISC), and are important for growth and survival. This widespread effect included prominently Nthl1-Ppat-Bdh2, but also mitochondrial Glrx5-Nfu1-Bola1, cytosolic Aco1-Abce1-Tyw5, and nuclear Dna2-Elp3-Pold1-Prim2. Incidentally, upregulated Pink1-Prkn levels explained mitophagy induction, the downregulated expression of Slc25a28 suggested it to function in iron export. The impact of PINK1 mutations in mouse and patient cells was pronounced only after iron overload, causing hyperreactive expression of ribosomal surveillance factor Abce1 and of ferritin, despite ferritin translation being repressed by IRP1. This misregulation might be explained by the deficiency of the ISC-biogenesis factor GLRX5. Our systematic survey suggests mitochondrial ISC-biogenesis and post-transcriptional iron regulation to be important in the decision, whether organisms undergo PD pathogenesis or healthy aging.

Mutations in the iron-sulfur cluster biogenesis protein HSCB cause congenital sideroblastic anemia

The congenital sideroblastic anemias (CSAs) can be caused by primary defects in mitochondrial iron-sulfur (Fe-S) cluster biogenesis. HSCB (heat shock cognate B), which encodes a mitochondrial cochaperone, also known as HSC20 (heat shock cognate protein 20), is the partner of mitochondrial heat shock protein A9 (HSPA9). Together with glutaredoxin 5 (GLRX5), HSCB and HSPA9 facilitate the transfer of nascent 2-iron, 2-sulfur clusters to recipient mitochondrial proteins. Mutations in both HSPA9 and GLRX5 have previously been associated with CSA. Therefore, we hypothesized that mutations in HSCB could also cause CSA. We screened patients with genetically undefined CSA and identified a frameshift mutation and a rare promoter variant in HSCB in a female patient with non-syndromic CSA. We found that HSCB expression was decreased in patient-derived fibroblasts and K562 erythroleukemia cells engineered to have the patient-specific promoter variant. Furthermore, gene knockdown and deletion experiments performed in K562 cells, zebrafish, and mice demonstrate that loss of HSCB results in impaired Fe-S cluster biogenesis, a defect in RBC hemoglobinization, and the development of siderocytes and more broadly perturbs hematopoiesis in vivo. These results further affirm the involvement of Fe-S cluster biogenesis in erythropoiesis and hematopoiesis and define HSCB as a CSA gene.

Drosophila melanogaster Mitochondrial Carriers: Similarities and Differences with the Human Carriers

Mitochondrial carriers are a family of structurally related proteins responsible for the exchange of metabolites, cofactors and nucleotides between the cytoplasm and mitochondrial matrix. The in silico analysis of the Drosophila melanogaster genome has highlighted the presence of 48 genes encoding putative mitochondrial carriers, but only 20 have been functionally characterized. Despite most Drosophila mitochondrial carrier genes having human homologs and sharing with them 50% or higher sequence identity, D. melanogaster genes display peculiar differences from their human counterparts: (1) in the fruit fly, many genes encode more transcript isoforms or are duplicated, resulting in the presence of numerous subfamilies in the genome; (2) the expression of the energy-producing genes in D. melanogaster is coordinated from a motif known as Nuclear Respiratory Gene (NRG), a palindromic 8-bp sequence; (3) fruit-fly duplicated genes encoding mitochondrial carriers show a testis-biased expression pattern, probably in order to keep a duplicate copy in the genome. Here, we review the main features, biological activities and role in the metabolism of the D. melanogaster mitochondrial carriers characterized to date, highlighting similarities and differences with their human counterparts. Such knowledge is very important for obtaining an integrated view of mitochondrial function in D. melanogaster metabolism.

Iron-responsive-like elements and neurodegenerative ferroptosis

A set of common-acting iron-responsive 5′untranslated region (5′UTR) motifs can fold into RNA stem loops that appear significant to the biology of cognitive declines of Parkinson's disease dementia (PDD), Lewy body dementia (LDD), and Alzheimer's disease (AD). Neurodegenerative diseases exhibit perturbations of iron homeostasis in defined brain subregions over characteristic time intervals of progression. While misfolding of Aβ from the amyloid-precursor-protein (APP), alpha-synuclein, prion protein (PrP) each cause neuropathic protein inclusions in the brain subregions, iron-responsive-like element (IRE-like) RNA stem–loops reside in their transcripts. APP and αsyn have a role in iron transport while gene duplications elevate the expression of their products to cause rare familial cases of AD and PDD. Of note, IRE-like sequences are responsive to excesses of brain iron in a potential feedback loop to accelerate neuronal ferroptosis and cognitive declines as well as amyloidosis. This pathogenic feedback is consistent with the translational control of the iron storage protein ferritin. We discuss how the IRE-like RNA motifs in the 5′UTRs of APP, alpha-synuclein and PrP mRNAs represent uniquely folded drug targets for therapies to prevent perturbed iron homeostasis that accelerates AD, PD, PD dementia (PDD) and Lewy body dementia, thus preventing cognitive deficits. Inhibition of alpha-synuclein translation is an option to block manganese toxicity associated with early childhood cognitive problems and manganism while Pb toxicity is epigenetically associated with attention deficit and later-stage AD. Pathologies of heavy metal toxicity centered on an embargo of iron export may be treated with activators of APP and ferritin and inhibitors of alpha-synuclein translation.

MiR-7 mediates mitochondrial impairment to trigger apoptosis and necroptosis in Rhabdomyosarcoma

BACKGROUND

Rhabdomyosarcoma (RMS) is a pediatric cancer with rhabdomyoblastic phenotype and mitochondria act as pivotal regulators of its growth and progression. While miR-7-5p (miR-7) is reported to have a tumor-suppressive role, little is yet known about its antitumor activity in RMS.

METHODS

The effects of miR-7 on RMS were analyzed both in vitro and in vivo. Cell death modalities induced by miR-7 were identified. Influence on mitochondria was evaluated through RNA sequencing data, morphological observation and mitochondrial functional assays, including outer membrane permeability, bioenergetics and redox balance. Dual-luciferase assay and phenotype validation after transient gene silencing were performed to identify miR-7 targets in RMS.

RESULTS

MiR-7 executed anti-tumor effect in RMS beyond proliferation inhibition. Morphologic features and molecular characteristics with apoptosis and necroptosis were found in miR-7-transfected RMS cells. Chemical inhibitors of apoptosis and necroptosis were able to prevent miR-7-induced cell death. Further, we identified that mitochondrial impairment mainly contributed to these phenomena and mitochondrial proteins SLC25A37 and TIMM50 were crucial targets for miR-7 to induce cell death in RMS.

CONCLUSION

Our results extended the mechanism of miR-7 antitumor role in rhabdomyosarcoma cancer, and provided potential implications for its therapy.

Cellular iron sensing and regulation: Nuclear IRP1 extends a classic paradigm

The classic view is that iron regulatory proteins operate at the post-transcriptional level. Iron Regulatory Protein 1 (IRP1) shifts between an apo-form that binds mRNAs and a holo-form that harbors a [4Fe4S] cluster. The latter form is not considered relevant to iron regulation, but rather thought to act as a non-essential cytosolic aconitase. Recent work in Drosophila, however, shows that holo-IRP1 can also translocate to the nucleus, where it appears to downregulate iron metabolism genes, preparing the cell for a decline in iron uptake. The shifting of IRP1 between states requires a functional mitoNEET pathway that includes a glycogen branching enzyme for the repair or disassembly of IRP1's oxidatively damaged [3Fe4S] cluster. The new findings add to the notion that glucose metabolism is modulated by iron metabolism. Furthermore, we propose that ferritin ferroxidase activity participates in the repair of the IRP1 [3Fe4S] cluster leading to the hypothesis that cytosolic ferritin directly contributes to cellular iron sensing.

From Synthesis to Utilization: The Ins and Outs of Mitochondrial Heme

Heme is a ubiquitous and essential iron containing metallo-organic cofactor required for virtually all aerobic life. Heme synthesis is initiated and completed in mitochondria, followed by certain covalent modifications and/or its delivery to apo-hemoproteins residing throughout the cell. While the biochemical aspects of heme biosynthetic reactions are well understood, the trafficking of newly synthesized heme—a highly reactive and inherently toxic compound—and its subsequent delivery to target proteins remain far from clear. In this review, we summarize current knowledge about heme biosynthesis and trafficking within and outside of the mitochondria.

The Role of Iron Regulation in Immunometabolism and Immune-Related Disease

Immunometabolism explores how the intracellular metabolic pathways in immune cells can regulate their function under different micro-environmental and (patho-)-physiological conditions (Pearce, 2010; Buck et al., 2015; O'Neill and Pearce, 2016). In the last decade great advances have been made in studying and manipulating metabolic programs in immune cells. Immunometabolism has primarily focused on glycolysis, the TCA cycle and oxidative phosphorylation (OXPHOS) as well as free fatty acid synthesis and oxidation. These pathways are important for providing the energy needs of cell growth, membrane rigidity, cytokine production and proliferation. In this review, we will however, highlight the specific role of iron metabolism at the cellular and organismal level, as well as how the bioavailability of this metal orchestrates complex metabolic programs in immune cell homeostasis and inflammation. We will also discuss how dysregulation of iron metabolism contributes to alterations in the immune system and how these novel insights into iron regulation can be targeted to metabolically manipulate immune cell function under pathophysiological conditions, providing new therapeutic opportunities for autoimmunity and cancer.

Zebrafish as a model system to delineate the role of heme and iron metabolism during erythropoiesis

Coordination of iron acquisition and heme synthesis is required for effective erythropoiesis. The small teleost zebrafish (Danio rerio) is an ideal vertebrate animal model to replicate various aspects of human physiology and provides an efficient and cost-effective way to model human pathophysiology. Importantly, zebrafish erythropoiesis largely resembles mammalian erythropoiesis. Gene discovery by large-scale forward mutagenesis screening has identified key components in heme and iron metabolism. Reverse genetic screens, using morpholino-knockdown and CRISPR/Cas9, coupled with the genetic tractability of the developing embryo have further accelerated functional studies. Ultimately, the ex utero development of zebrafish embryos combined with their transparency and developmental plasticity could provide a deeper understanding of the role of iron and heme metabolism during early vertebrate embryonic development.

Feasibility of cellular bioenergetics as a biomarker in porphyria patients

Porphyria is a group of metabolic disorders due to altered enzyme activities within the heme biosynthetic pathway. It is a systemic disease with multiple potential contributions to mitochondrial dysfunction and oxidative stress. Recently, it has become possible to measure mitochondrial function from cells isolated from peripheral blood (cellular bioenergetics) using the XF96 analyzer (Seahorse Bioscience). Mitochondrial respiration in these cells is measured with the addition of activators and inhibitors of respiration. The output is measured as the O₂ consumption rate (OCR) at basal conditions, ATP linked, proton leak, maximal, reserve capacity, non-mitochondrial, and oxidative burst. We performed cellular bioenergetics on 22 porphyria (12 porphyria cutanea tarda (PCT), seven acute hepatic porphyria (AHP), and three erythropoietic protoporphyria (EPP)) patients and 18 age and gender matched healthy controls. Of porphyria cases, eight were active (2 PCT, 1 EPP, and 5 AHP) and 14 in biochemical remission. The OCR were decreased in patients compared to healthy controls. The bioenergetic profile was significantly lower when measuring proton leak and the non-mitochondrial associated OCR in the eight active porphyria patients when compared to 18 healthy controls. In conclusion, we demonstrate that the bioenergetic profile and mitochondrial activities assessed in porphyria patients and is different than in healthy control individuals. Further, our novel preliminary findings suggest the existence of a mitochondrial dysfunction in porphyria and this may be used as potential non-invasive biomarker for disease activity. This needs to be assessed with a systematic examination in a larger patient cohort. Studies are also suggested to examine mitochondrial metabolism as basis to understand mechanisms of these findings and deriving mitochondrial based therapies for porphyria.

FAM210B is an erythropoietin target and regulates erythroid heme synthesis by controlling mitochondrial iron import and ferrochelatase activity

Erythropoietin (EPO) signaling is critical to many processes essential to terminal erythropoiesis. Despite the centrality of iron metabolism to erythropoiesis, the mechanisms by which EPO regulates iron status are not well-understood. To this end, here we profiled gene expression in EPO-treated 32D pro-B cells and developing fetal liver erythroid cells to identify additional iron regulatory genes. We determined that FAM210B, a mitochondrial inner-membrane protein, is essential for hemoglobinization, proliferation, and enucleation during terminal erythroid maturation. Fam210b deficiency led to defects in mitochondrial iron uptake, heme synthesis, and iron-sulfur cluster formation. These defects were corrected with a lipid-soluble, small-molecule iron transporter, hinokitiol, in Fam210b-deficient murine erythroid cells and zebrafish morphants. Genetic complementation experiments revealed that FAM210B is not a mitochondrial iron transporter but is required for adequate mitochondrial iron import to sustain heme synthesis and iron-sulfur cluster formation during erythroid differentiation. FAM210B was also required for maximal ferrochelatase activity in differentiating erythroid cells. We propose that FAM210B functions as an adaptor protein that facilitates the formation of an oligomeric mitochondrial iron transport complex, required for the increase in iron acquisition for heme synthesis during terminal erythropoiesis. Collectively, our results reveal a critical mechanism by which EPO signaling regulates terminal erythropoiesis and iron metabolism.

Intracellular iron and heme trafficking and metabolism in developing erythroblasts

Vertebrate red blood cells (RBCs) arise from erythroblasts in the human bone marrow through a process known as erythropoiesis. Iron uptake is a crucial hallmark, essential for heme biosynthesis in the differentiating erythroblasts, which are dedicated to producing hemoglobin. Erythropoiesis is facilitated by a network of intracellular transport proteins, chaperones, and circulating hormones. Intracellular iron is targeted to the mitochondria for incorporation into a porphyrin ring to form heme and cytosolic iron-sulfur proteins, including Iron Regulatory Protein 1 (IRP1). These processes are tightly regulated to prevent both excess and insufficient levels of iron and heme precursors. Crosstalk between the heme and iron-sulfur synthesizing pathways has been demonstrated to serve as a regulatory feedback mechanism. The activity of δ-aminolevulinic acid synthase (ALAS), the first and rate-limiting enzyme of heme biosynthesis, is a fundamental node of this regulation. Recently, the mitochondrial unfoldase, ClpX, has received attention as a novel key player that modulates this step in heme biogenesis, implicating a role in the pathophysiology of anemic diseases. This chapter reviews the canonical pathways in intracellular iron and heme trafficking and recent findings of iron and heme metabolism in vertebrate red cells. A discussion of the molecular approaches to studying iron and heme transport is provided to highlight opportunities for revealing therapeutic targets.

Iron-sulfur clusters: from metals through mitochondria biogenesis to disease

Iron–sulfur clusters are ubiquitous inorganic co-factors that contribute to a wide range of cell pathways including the maintenance of DNA integrity, regulation of gene expression and protein translation, energy production, and antiviral response. Specifically, the iron–sulfur cluster biogenesis pathways include several proteins dedicated to the maturation of apoproteins in different cell compartments. Given the complexity of the biogenesis process itself, the iron–sulfur research area constitutes a very challenging and interesting field with still many unaddressed questions. Mutations or malfunctions affecting the iron–sulfur biogenesis machinery have been linked with an increasing amount of disorders such as Friedreich’s ataxia and various cardiomyopathies. This review aims to recap the recent discoveries both in the yeast and human iron–sulfur cluster arena, covering recent discoveries from chemistry to disease.

A role for amyloid precursor protein translation to restore iron homeostasis and ameliorate lead (Pb) neurotoxicity

Iron supplementation ameliorates the neurotoxicity of the environmental contaminant lead (Pb); however, the mechanism remains undefined. Iron is an essential nutrient but high levels are toxic due to the catalytic generation of destructive hydroxyl radicals. Using human neuroblastoma SH-SY5Y cells to model human neurons, we investigated the effect of Pb on proteins of iron homeostasis: the Alzheimer's amyloid precursor protein (APP), which stabilizes the iron exporter ferroportin 1; and, the heavy subunit of the iron-storage protein, ferritin (FTH). Lead (Pb(II) and Pb(IV) inhibited APP translation and raised cytosolic iron(II). Lead also increased iron regulatory protein-1 binding to the cognate 5'untranslated region-specific iron-responsive element (IRE) of APP and FTH mRNAs. Concurrent iron treatment rescued cells from Pb toxicity by specifically restoring APP synthesis, i.e. levels of the APP-related protein, APLP-2, were unchanged. Significantly, iron/IRE-independent over-expression of APP695  protected SH-SY5Y cells from Pb toxicity, demonstrating that APP plays a key role in maintaining safe levels of intracellular iron. Overall, our data support a model of neurotoxicity where Pb enhances iron regulatory protein/IRE-mediated repression of APP and FTH translation. We propose novel treatment options for Pb poisoning to include chelators and the use of small molecules to maintain APP and FTH translation. We propose the following cascade for Lead (Pb) toxicity to neurons; by targeting the interaction between Iron regulatory protein-1 and Iron-responsive elements, Pb caused translational repression of proteins that control intracellular iron homeostasis, including the Alzheimer's amyloid precursor protein (APP) that stabilizes the iron exporter ferroportin, and the ferroxidase heavy subunit of the iron-storage protein, ferritin. When unregulated, IRE-independent over-expression of APP695 protected SH-SY5Y neurons from Pb toxicity. There is a novel and key role for APP in maintaining safe levels of intracellular iron pertinent to lead toxicity.

A Red Carpet for Iron Metabolism

200 billion red blood cells (RBCs) are produced every day, requiring more than 2 × 10¹⁵ iron atoms every second to maintain adequate erythropoiesis. These numbers translate into 20 mL of blood being produced each day, containing 6 g of hemoglobin and 20 mg of iron. These impressive numbers illustrate why the making and breaking of RBCs is at the heart of iron physiology, providing an ideal context to discuss recent progress in understanding the systemic and cellular mechanisms that underlie the regulation of iron homeostasis and its disorders.

Restored iron transport by a small molecule promotes absorption and hemoglobinization in animals

Multiple human diseases ensue from a hereditary or acquired deficiency of iron-transporting protein function that diminishes transmembrane iron flux in distinct sites and directions. Because other iron-transport proteins remain active, labile iron gradients build up across the corresponding protein-deficient membranes. Here we report that a small-molecule natural product, hinokitiol, can harness such gradients to restore iron transport into, within, and/or out of cells. The same compound promotes gut iron absorption in DMT1-deficient rats and ferroportin-deficient mice, as well as hemoglobinization in DMT1- and mitoferrin-deficient zebrafish. These findings illuminate a general mechanistic framework for small molecule-mediated site- and direction-selective restoration of iron transport. They also suggest that small molecules that partially mimic the function of missing protein transporters of iron, and possibly other ions, may have potential in treating human diseases.

Studying disorders of vertebrate iron and heme metabolism using zebrafish

Iron is a crucial component of heme- and iron-sulfur clusters, involved in vital cellular functions such as oxygen transport, DNA synthesis, and respiration. Both excess and insufficient levels of iron and heme-precursors cause human disease, such as iron-deficiency anemia, hemochromatosis, and porphyrias. Hence, their levels must be tightly regulated, requiring a complex network of transporters and feedback mechanisms. The use of zebrafish to study these pathways and the underlying genetics offers many advantages, among others their optical transparency, ex-vivo development and high genetic and physiological conservations. This chapter first reviews well-established methods, such as large-scale mutagenesis screens that have led to the initial identification of a series of iron and heme transporters and the generation of a variety of mutant lines. Other widely used techniques are based on injection of RNA, including complementary morpholino knockdown and gene overexpression. In addition, we highlight several recently developed approaches, most notably endonuclease-based gene knockouts such as TALENs or the CRISPR/Cas9 system that have been used to study how loss of function can induce human disease phenocopies in zebrafish. Rescue by chemical complementation with iron-based compounds or small molecules can subsequently be used to confirm causality of the genetic defect for the observed phenotype. All together, zebrafish have proven to be - and will continue to serve as an ideal model to advance our understanding of the pathogenesis of human iron and heme-related diseases and to develop novel therapies to treat these conditions.

Cellular iron uptake, trafficking and metabolism: Key molecules and mechanisms and their roles in disease

Iron is a crucial transition metal for virtually all life. Two major destinations of iron within mammalian cells are the cytosolic iron-storage protein, ferritin, and mitochondria. In mitochondria, iron is utilized in critical anabolic pathways, including: iron-storage in mitochondrial ferritin, heme synthesis, and iron-sulfur cluster (ISC) biogenesis. Although the pathways involved in ISC synthesis in the mitochondria and cytosol have begun to be characterized, many crucial details remain unknown. In this review, we discuss major aspects of the journey of iron from its initial cellular uptake, its modes of trafficking within cells, to an overview of its downstream utilization in the cytoplasm and within mitochondria. The understanding of mitochondrial iron processing and its communication with other organelles/subcellular locations, such as the cytosol, has been elucidated by the analysis of certain diseases e.g., Friedreich's ataxia. Increased knowledge of the molecules and their mechanisms of action in iron processing pathways (e.g., ISC biogenesis) will shape the investigation of iron metabolism in human health and disease.

Iron-sulfur cluster biogenesis in mammalian cells: New insights into the molecular mechanisms of cluster delivery

Iron-sulfur (Fe-S) clusters are ancient, ubiquitous cofactors composed of iron and inorganic sulfur. The combination of the chemical reactivity of iron and sulfur, together with many variations of cluster composition, oxidation states and protein environments, enables Fe-S clusters to participate in numerous biological processes. Fe-S clusters are essential to redox catalysis in nitrogen fixation, mitochondrial respiration and photosynthesis, to regulatory sensing in key metabolic pathways (i.e. cellular iron homeostasis and oxidative stress response), and to the replication and maintenance of the nuclear genome. Fe-S cluster biogenesis is a multistep process that involves a complex sequence of catalyzed protein-protein interactions and coupled conformational changes between the components of several dedicated multimeric complexes. Intensive studies of the assembly process have clarified key points in the biogenesis of Fe-S proteins. However several critical questions still remain, such as: what is the role of frataxin? Why do some defects of Fe-S cluster biogenesis cause mitochondrial iron overload? How are specific Fe-S recipient proteins recognized in the process of Fe-S transfer? This review focuses on the basic steps of Fe-S cluster biogenesis, drawing attention to recent advances achieved on the identification of molecular features that guide selection of specific subsets of nascent Fe-S recipients by the cochaperone HSC20. Additionally, it outlines the distinctive phenotypes of human diseases due to mutations in the components of the basic pathway. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

TMEM14C is required for erythroid mitochondrial heme metabolism

The transport and intracellular trafficking of heme biosynthesis intermediates are crucial for hemoglobin production, which is a critical process in developing red cells. Here, we profiled gene expression in terminally differentiating murine fetal liver-derived erythroid cells to identify regulators of heme metabolism. We determined that TMEM14C, an inner mitochondrial membrane protein that is enriched in vertebrate hematopoietic tissues, is essential for erythropoiesis and heme synthesis in vivo and in cultured erythroid cells. In mice, TMEM14C deficiency resulted in porphyrin accumulation in the fetal liver, erythroid maturation arrest, and embryonic lethality due to profound anemia. Protoporphyrin IX synthesis in TMEM14C-deficient erythroid cells was blocked, leading to an accumulation of porphyrin precursors. The heme synthesis defect in TMEM14C-deficient cells was ameliorated with a protoporphyrin IX analog, indicating that TMEM14C primarily functions in the terminal steps of the heme synthesis pathway. Together, our data demonstrate that TMEM14C facilitates the import of protoporphyrinogen IX into the mitochondrial matrix for heme synthesis and subsequent hemoglobin production. Furthermore, the identification of TMEM14C as a protoporphyrinogen IX importer provides a genetic tool for further exploring erythropoiesis and congenital anemias.

The physiological functions of iron regulatory proteins in iron homeostasis - an update

Iron regulatory proteins (IRPs) regulate the expression of genes involved in iron metabolism by binding to RNA stem-loop structures known as iron responsive elements (IREs) in target mRNAs. IRP binding inhibits the translation of mRNAs that contain an IRE in the 5′untranslated region of the transcripts, and increases the stability of mRNAs that contain IREs in the 3′untranslated region of transcripts. By these mechanisms, IRPs increase cellular iron absorption and decrease storage and export of iron to maintain an optimal intracellular iron balance. There are two members of the mammalian IRP protein family, IRP1 and IRP2, and they have redundant functions as evidenced by the embryonic lethality of the mice that completely lack IRP expression (Irp1^-/-/Irp2^-/- mice), which contrasts with the fact that Irp1^-/- and Irp2^-/- mice are viable. In addition, Irp2^-/- mice also display neurodegenerative symptoms and microcytic hypochromic anemia, suggesting that IRP2 function predominates in the nervous system and erythropoietic homeostasis. Though the physiological significance of IRP1 had been unclear since Irp1^-/- animals were first assessed in the early 1990s, recent studies indicate that IRP1 plays an essential function in orchestrating the balance between erythropoiesis and bodily iron homeostasis. Additionally, Irp1^-/- mice develop pulmonary hypertension, and they experience sudden death when maintained on an iron-deficient diet, indicating that IRP1 has a critical role in the pulmonary and cardiovascular systems. This review summarizes recent progress that has been made in understanding the physiological roles of IRP1 and IRP2, and further discusses the implications for clinical research on patients with idiopathic polycythemia, pulmonary hypertension, and neurodegeneration.