Abnormal mitoferrin-1 expression in patients with erythropoietic protoporphyria

Single-cell RNA seq analysis of erythroid cells reveals a specific sub-population of stress erythroid progenitors

BACKGROUND

: Erythroid cells play important roles in hemostasis and disease. However, there is still significant knowledge gap regarding stress erythropoiesis.

METHODS

: Two single-cell RNAseq datasets of erythroid cells on GEO with accession numbers GSE149938 and GSE184916 were obtained. The datasets from two sources, bone marrow and peripheral blood were analyzed using Seurat v4.1.1, and other tools in R. QC metrics were performed, data were normalized and scaled. Principal components that capture the variation of the data were determined. In clustering the cells, KNN graph was constructed and Louvain algorithm was applied to optimize the standard modularity function. Clusters were defined via differential expression of features.

RESULTS

We identified 9 different cell types, with a particular cluster representing the stress erythroids. The clusters showed differentially expressed genes as observed from the gene signature plot. The stress erythroid cluster differentially expressed some genes including ALAS2, HEMGN, and GUK1.

CONCLUSION

The erythroid population was found to be heterogeneous, with a distinct sub-cell type constituting the stress erythroids; this may have important implications for our knowledge of steady-state and stress erythropoiesis, and the markers found in this cluster may prove useful for future research into the dynamics of stress erythroid progenitor cell differentiation.

An implication of the mitochondrial carrier SLC25A3 as an oxidative stress modulator in NAFLD

Nonalcoholic fatty liver disease (NAFLD) is a type of steatosis not associated with excessive alcohol intake and includes nonalcoholic steatohepatitis (NASH), which can progress to advanced fibrosis and hepatocellular carcinoma. Mitochondrial dysfunction causes oxidative stress, triggering hepatocyte death and inflammation; therefore, the present study aimed to explore relationship between mitochondrial carriers and oxidative stress. Firstly, we established a high fat diet (HFD)-fed ICR mouse NAFLD model characterized by obesity with insulin resistance and found transcriptional upregulation of Slc25a17 and downregulation of Slc25a3 (isoform B) and Slc25a13 in their fatty liver. A mitochondrial phosphate and Cu carrier, SLC25A3, was further studied in wild-type (wt) and SLC25A3-defective HepG2 cells (C1 and C3). SLC25A3 deficiency had insignificant effect on mitochondrial membrane potential (MtMP) and oxygen consumption rate (OCR) in untreated cells but suppressed them when cells were exposed to oleic acid. C1 and C3 cells were prone to produce reactive oxygen species (ROS), and increased ROS was associated with reduced mRNA expression of glutathione peroxidase (GPX) 1 and glutathione disulfide reductase (GSX) in these cell lines. Interestingly, cytoplasmic and mitochondrial Cu accumulation significantly reduced in C1 cells, demonstrating a predominant contribution of SLC25A3 to Cu transport into mitochondrial matrix. Cytotoxicity of free fatty acids was unchanged between wt and SLC25A3-deficient cells. These results indicate that reduced expression of SLC25A3 in fatty liver contributes to electron leak from mitochondria by limiting Cu availability, rendering hepatocytes more susceptible to oxidative stress. This study provides evidence that SLC25A3 is a novel risk factor for developing NASH.

Mitoferrin, Cellular and Mitochondrial Iron Homeostasis

Iron is essential for many cellular processes, but cellular iron homeostasis must be maintained to ensure the balance of cellular signaling processes and prevent disease. Iron transport in and out of the cell and cellular organelles is crucial in this regard. The transport of iron into the mitochondria is particularly important, as heme and the majority of iron-sulfur clusters are synthesized in this organelle. Iron is also required for the production of mitochondrial complexes that contain these iron-sulfur clusters and heme. As the principal iron importers in the mitochondria of human cells, the mitoferrins have emerged as critical regulators of cytosolic and mitochondrial iron homeostasis. Here, we review the discovery and structure of the mitoferrins, as well as the significance of these proteins in maintaining cytosolic and mitochondrial iron homeostasis for the prevention of cancer and many other diseases.

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, Heme Synthesis and Erythropoietic Porphyrias: A Complex Interplay

Erythropoietic porphyrias are caused by enzymatic dysfunctions in the heme biosynthetic pathway, resulting in porphyrins accumulation in red blood cells. The porphyrins deposition in tissues, including the skin, leads to photosensitivity that is present in all erythropoietic porphyrias. In the bone marrow, heme synthesis is mainly controlled by intracellular labile iron by post-transcriptional regulation: translation of ALAS2 mRNA, the first and rate-limiting enzyme of the pathway, is inhibited when iron availability is low. Moreover, it has been shown that the expression of ferrochelatase (FECH, an iron-sulfur cluster enzyme that inserts iron into protoporphyrin IX to form heme), is regulated by intracellular iron level. Accordingly, there is accumulating evidence that iron status can mitigate disease expression in patients with erythropoietic porphyrias. This article will review the available clinical data on how iron status can modify the symptoms of erythropoietic porphyrias. We will then review the modulation of heme biosynthesis pathway by iron availability in the erythron and its role in erythropoietic porphyrias physiopathology. Finally, we will summarize what is known of FECH interactions with other proteins involved in iron metabolism in the mitochondria.

RNA m6A Demethylase ALKBH5 Protects Against Pancreatic Ductal Adenocarcinoma via Targeting Regulators of Iron Metabolism

Although RNA m6A regulators have been implicated in the tumorigenesis of several different types of tumors, including pancreatic cancer, their clinical relevance and intrinsic regulatory mechanism remain elusive. This study analyzed eight m6A regulators (METTL3, METTL14, WTAP, FTO, ALKBH5, and YTHDF1-3) in pancreatic ductal adenocarcinoma (PDAC) and found that only RNA m6A demethylase ALKBH5 serves as an independent favorable prognostic marker for this tumor. To better understand the molecular mechanism underlying the protective effect conferred by ALKBH5 against pancreatic tumorigenesis, we performed a transcriptome-wide analysis of m6A methylation, gene expression, and alternative splicing (AS) using the MIA PaCa-2 stable cell line with ALKBH5 overexpression. We demonstrated that ALKBH5 overexpression induced a reduction in RNA m6A levels globally. Furthermore, mRNAs encoding ubiquitin ligase FBXL5, and mitochondrial iron importers SLC25A28 and SLC25A37, were identified as substrates of ALKBH5. Mechanistically, the RNA stabilities of FBXL5 and SLC25A28, and the AS of SLC25A37 were affected, which led to their upregulation in pancreatic cancer cell line. Particularly, we observed that downregulation of FBXL5 in tumor samples correlated with shorter survival time of patients. Owing to FBXL5-mediated degradation, ALKBH5 overexpression incurred a significant reduction in iron-regulatory protein IRP2 and the modulator of epithelial-mesenchymal transition (EMT) SNAI1. Notably, ALKBH5 overexpression led to a significant reduction in intracellular iron levels as well as cell migratory and invasive abilities, which could be rescued by knocking down FBXL5. Overall, our results reveal a previously uncharacterized mechanism of ALKBH5 in protecting against PDAC through modulating regulators of iron metabolism and underscore the multifaceted role of m6A in pancreatic cancer.

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.

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.

Strong correlation of ferrochelatase enzymatic activity with Mitoferrin-1 mRNA in lymphoblasts of patients with protoporphyria

Accumulation of protoporphyrin IX (PPIX) and Zn-PPIX, are the clinical hallmarks of protoporphyria. Phenotypic expression of protoporphyria is due to decreased activity of ferrochelatase (FECH) or to increased activity of aminolevulinic acid synthase (ALAS) in red blood cells. Other genetic defects have been shown to contribute to disease severity including loss of function mutations in the mitochondrial AAA-ATPase, CLPX and mutations in the Iron-responsive element binding protein 2 (IRP2), in mice. It is clear that multiple paths lead to a common phenotype of excess plasma PPIX that causes a phototoxic reaction on sun exposed areas. In this study we examined the association between mitochondrial iron acquisition and utilization with activity of FECH. Our data show that there is a metabolic link between the activity FECH and levels of MFRN1 mRNA. We examined the correlation between FECH activity and MFRN1 mRNA in cell lines established from patients with the classical protoporphyria, porphyria due to defects in ALAS2 mutations. Our data confirm MFRN1 message levels positively correlated with FECH enzymatic activity in all cell types.

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.

Mitochondrial Iron in Human Health and Disease

Mitochondria are an iconic distinguishing feature of eukaryotic cells. Mitochondria encompass an active organellar network that fuses, divides, and directs a myriad of vital biological functions, including energy metabolism, cell death regulation, and innate immune signaling in different tissues. Another crucial and often underappreciated function of these dynamic organelles is their central role in the metabolism of the most abundant and biologically versatile transition metals in mammalian cells, iron. In recent years, cellular and animal models of mitochondrial iron dysfunction have provided vital information in identifying new proteins that have elucidated the pathways involved in mitochondrial homeostasis and iron metabolism. Specific signatures of mitochondrial iron dysregulation that are associated with disease pathogenesis and/or progression are becoming increasingly important. Understanding the molecular mechanisms regulating mitochondrial iron pathways will help better define the role of this important metal in mitochondrial function and in human health and disease.

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.

Iron-sulfur cluster biosynthesis and trafficking - impact on human disease conditions

Iron-sulfur clusters (Fe-S) are one of the most ancient, ubiquitous and versatile classes of metal cofactors found in nature. Proteins that contain Fe-S clusters constitute one of the largest families of proteins, with varied functions that include electron transport, regulation of gene expression, substrate binding and activation, radical generation, and, more recently discovered, DNA repair. Research during the past two decades has shown that mitochondria are central to the biogenesis of Fe-S clusters in eukaryotic cells via a conserved cluster assembly machinery (ISC assembly machinery) that also controls the synthesis of Fe-S clusters of cytosolic and nuclear proteins. Several key steps for synthesis and trafficking have been determined for mitochondrial Fe-S clusters, as well as the cytosol (CIA - cytosolic iron-sulfur protein assembly), but detailed mechanisms of cluster biosynthesis, transport, and exchange are not well established. Genetic mutations and the instability of certain steps in the biosynthesis and maturation of mitochondrial, cytosolic and nuclear Fe-S cluster proteins affects overall cellular iron homeostasis and can lead to severe metabolic, systemic, neurological and hematological diseases, often resulting in fatality. In this review we briefly summarize the current molecular understanding of both mitochondrial ISC and CIA assembly machineries, and present a comprehensive overview of various associated inborn human disease states.

Using the Zebrafish as an Approach to Examine the Mechanisms of Vertebrate Erythropoiesis

The zebrafish, Danio rerio, is a powerful model for the study of erythropoiesis and defining the genetic basis of hematological diseases. The mechanisms of erythroid differentiation are highly conserved in the zebrafish, permitting translational research studies and the modeling of erythropoiesis in higher vertebrates. An advantage of the system is the ability to manipulate gene expression and observe the effect on erythroid development in vivo, with relative ease and rapidity. The production of optically transparent embryos also makes it an attractive tool for visual analysis of circulating erythrocytes that can be used to study erythropoiesis. Through large-scale chemical mutagenesis screens, a variety of zebrafish blood mutants have been identified that are used for gene discoveries and the recapitulation of human diseases. Experimental techniques including in situ hybridization, o-dianisidine staining, flow cytometry, and microinjection are now commonly employed to study red blood cell biochemistry and erythropoiesis in the zebrafish. These techniques have been applied for identifying novel genes required for the hemoglobin synthesis, isolating blood cell lineages, visualizing genetic expression within erythroid tissues, and characterizing the phenotype of blood disorders. The applications of zebrafish methodology to the study of erythropoiesis and optimized step-by-step protocols are discussed in this chapter.

RNA splicing during terminal erythropoiesis

PURPOSE OF REVIEW

Erythroid progenitors must accurately and efficiently splice thousands of pre-mRNAs as the cells undergo extensive changes in gene expression and cellular remodeling during terminal erythropoiesis. Alternative splicing choices are governed by interactions between RNA binding proteins and cis-regulatory binding motifs in the RNA. This review will focus on recent studies that define the genome-wide scope of splicing in erythroblasts and discuss what is known about its regulation.

RECENT FINDINGS

RNA-seq analysis of highly purified erythroblast populations has revealed an extensive program of alternative splicing of both exons and introns. During normal erythropoiesis, stage-specific splicing transitions alter the structure and abundance of protein isoforms required for optimized red cell production. Mutation or deficiency of splicing regulators underlies hematopoietic disease in myelopdysplasia syndrome patients via disrupting the splicing program.

SUMMARY

Erythroid progenitors execute an elaborate alternative splicing program that modulates gene expression posttranscriptionally, ultimately regulating the structure and function of the proteome in a differentiation stage-specific manner during terminal erythropoiesis. This program helps drive differentiation and ensure synthesis of the proper protein isoforms required to produce mechanically stable red cells. Mutation or deficiency of key splicing regulatory proteins disrupts the splicing program to cause disease.

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.

Identification of SLC25A37 as a major depressive disorder risk gene

Major depressive disorder (MDD) is one of the most prevalent and disabling mental disorders, but the genetic etiology remains largely unknown. We performed a meta-analysis (14,543 MDD cases and 14,856 controls) through combining the GWAS data from the Major Depressive Disorder Working Group of the Psychiatric GWAS Consortium and the CONVERGE consortium and identified seven SNPs (four of them located in the downstream of SCL25A37) that showed suggestive associations (P < 5.0 × 10^-7) with MDD. Systematic integration (Sherlock integrative analysis) of brain eQTL and GWAS meta-analysis identified SCL25A37 as a novel MDD risk gene (P = 2.22 × 10^-6). A cis SNP (rs6983724, ∼28 kb downstream of SCL25A37) showed significant association with SCL25A37 expression (P = 1.19 × 10^-9) and suggestive association with MDD (P = 1.65 × 10^-7). We validated the significant association between rs6983724 and SCL25A37 expression in independent expression datasets. Finally, we found that SCL25A37 is significantly down-regulated in hippocampus and blood of MDD patients (P = 3.49 × 10^-3 and P = 2.66 × 10^-13, respectively). Our findings implicate that the SCL25A37 is a MDD susceptibility gene whose expression may influence MDD risk. The consistent down-regulation of SCL25A37 in MDD patients in three independent samples suggest that SCL25A37 may be used as a potential biomarker for MDD diagnosis. Further functional characterization of SCL25A37 may provide a potential target for future therapeutics and diagnostics.

Oxidative folding in the mitochondrial intermembrane space: A regulated process important for cell physiology and disease

Mitochondria are fundamental organelles with a complex internal architecture that fulfill important diverse functions including iron–sulfur cluster assembly and cell respiration. Intense work for more than 30 years has identified the key protein import components and the pathways involved in protein targeting and assembly. More recently, oxidative folding has been discovered as one important mechanism for mitochondrial proteostasis whilst several human disorders have been linked to this pathway. We describe the molecular components of this pathway in view of their putative redox regulation and we summarize available evidence on the connections of these pathways to human disorders.

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Mitochondria are the cell center of iron–sulfur cluster assembly and cell respiration.

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The MIA pathway has recently been linked to Fe/S pathways, Ca^2 + uptake and apoptosis.

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Mitochondria along with the ER and peroxisomes are major sources of ROS.

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Many diseases have been linked to mitochondrial dysfunction.

A Trypanosomatid Iron Transporter that Regulates Mitochondrial Function Is Required for Leishmania amazonensis Virulence

Iron, an essential co-factor of respiratory chain proteins, is critical for mitochondrial function and maintenance of its redox balance. We previously reported a role for iron uptake in differentiation of Leishmania amazonensis into virulent amastigotes, by a mechanism that involves reactive oxygen species (ROS) production and is independent of the classical pH and temperature cues. Iron import into mitochondria was proposed to be essential for this process, but evidence supporting this hypothesis was lacking because the Leishmania mitochondrial iron transporter was unknown. Here we describe MIT1, a homolog of the mitochondrial iron importer genes mrs3 (yeast) and mitoferrin-1 (human) that is highly conserved among trypanosomatids. MIT1 expression was essential for the survival of Trypanosoma brucei procyclic but not bloodstream forms, which lack functional respiratory complexes. L. amazonensis LMIT1 null mutants could not be generated, suggesting that this mitochondrial iron importer is essential for promastigote viability. Promastigotes lacking one LMIT1 allele (LMIT1/Δlmit1) showed growth defects and were more susceptible to ROS toxicity, consistent with the role of iron as the essential co-factor of trypanosomatid mitochondrial superoxide dismutases. LMIT1/Δlmit1 metacyclic promastigotes were unable to replicate as intracellular amastigotes after infecting macrophages or cause cutaneous lesions in mice. When induced to differentiate axenically into amastigotes, LMIT1/Δlmit1 showed strong defects in iron content and function of mitochondria, were unable to upregulate the ROS-regulatory enzyme FeSOD, and showed mitochondrial changes suggestive of redox imbalance. Our results demonstrate the importance of mitochondrial iron uptake in trypanosomatid parasites, and highlight the role of LMIT1 in the iron-regulated process that orchestrates differentiation of L. amazonensis into infective amastigotes.

Leishmaniasis is a serious parasitic disease that affects 12 million people worldwide, with clinical manifestations ranging from self-healing cutaneous lesions to deadly visceralizing disease. A vaccine is not available, and new and less toxic drugs against this protozoan parasite are urgently needed. Following introduction into vertebrate hosts during a sand fly blood meal, Leishmania parasites undergo extensive changes in morphology and metabolism that are critical for adaptation to life inside host macrophages and replication as amastigotes. Earlier studies identified major events that occur during amastigote differentiation, but the signaling mechanism initiating this process remained poorly understood. Previously we demonstrated a novel role for the reactive oxygen species (ROS) H₂O₂ in initiating amastigote differentiation, a process proposed to be dependent on iron availability inside the parasite’s mitochondria. In this study we identify LMIT1, a Leishmania transmembrane protein that functions as a mitochondrial iron transporter and is conserved in other trypanosomatid protozoan parasites. Reduced LMIT1 expression impairs mitochondrial function in the infective amastigote stage, abolishing parasite virulence. Our findings identify LMIT1 as a promising new drug target, and support the conclusion that iron-dependent ROS signals generated in the mitochondria regulate differentiation of virulent Leishmania amastigotes.

Homeostasis of iron and hepcidin in erythropoietic protoporphyria

BACKGROUND

Erythropoietic protoporphyria (EPP) and X-linked protoporphyria (XLP) are genetic abnormalities of heme synthesis that result in excess production of protoporphyrin and that manifest as severe photosensitivity. These disorders are often associated with iron deficiency anaemia (IDA). Our aim was to determine whether hepcidin is increased in EPP/XLP patients, resulting in decreased enteral iron absorption and IDA.

MATERIAL AND METHODS

Eight subjects with EPP, one with XLP and nine controls had baseline blood and urine samples collected, and thereafter were given oral ferrous sulphate (660 mg). Post-iron blood and urine samples were collected at 2, 4, 6 and 8 h. Blood counts, serum cytokines, ferritin and iron studies were analysed at baseline. Serum iron studies, serum and urine hepcidin, and erythropoietin (Epo) were analysed at baseline and subsequent time points.

RESULTS

At baseline, EPP-XLP subjects had lower mean blood haemoglobin (13·9/15·3 g/dL) and serum ferritin (31·6/115 ng/mL) than controls. Serum iron levels increased markedly in both cohorts. Mean serum and urine hepcidin levels were significantly lower in the EPP-XLP group at 4 and 8 h post-iron (serum - 4 h, 3·79/26·6, 8 h, 5·79/34·6 nM; urine - 4 h, 0·85/2·50, 8 h, 1·44/6·63 nM/mM creatinine). Serum cytokines and Epo were normal and not different between groups.

CONCLUSIONS

We conclude that serum and urine hepcidin are not inappropriately increased in EPP/XLP subjects at baseline and do not increase over time as serum iron increases after oral ferrous sulphate. Levels of serum cytokines and Epo are normal in EPP/XLP. The molecular basis for the iron-deficient phenotype in EPP/XLP remains unknown.

[Erythropoietic protoporphyria. A rare differential diagnosis among photosensitive diseases]

A 31-year-old woman presented with recurrent swelling, burning, prickling and itching of the arms and the hands which appeared after UV exposure in the summer. Simultaneously she often had chills, malaise, nausea and circulatory problems lasting for hours. She did not have erythema or wheals. She had been seen by a number of specialties but to no avail. Her deceased mother suffered from the same symptoms. On presentation she had no skin lesions. Laboratory testing showed a dramatic increase in total protoporphyrin, enabling us to diagnose erythropoietic protoporphyria. This is a rare, genetic metabolic disturbance in hematopoiesis, whose true prevalence is probably underestimated. Because of abnormal or absent function of ferrochelatase, protoporphyrin accumulates in blood, erythrocytes and tissue usually causing photosensitivity as the first clinical symptom. The prognosis depends on the severity of liver involvement. Because of the marked restrictions on activity, especially in avoiding UV exposition, the quality of life of the patients and their families are strongly influenced. The odyssey of our patient and her mother in finding a diagnosis demonstrates impressively that the EPP is an important photosensitizing disease which must not be forgotten.

Repairing quite swimmingly: advances in regenerative medicine using zebrafish

Regenerative medicine has the promise to alleviate morbidity and mortality caused by organ dysfunction, longstanding injury and trauma. Although regenerative approaches for a few diseases have been highly successful, some organs either do not regenerate well or have no current treatment approach to harness their intrinsic regenerative potential. In this Review, we describe the modeling of human disease and tissue repair in zebrafish, through the discovery of disease-causing genes using classical forward-genetic screens and by modulating clinically relevant phenotypes through chemical genetic screening approaches. Furthermore, we present an overview of those organ systems that regenerate well in zebrafish in contrast to mammalian tissue, as well as those organs in which the regenerative potential is conserved from fish to mammals, enabling drug discovery in preclinical disease-relevant models. We provide two examples from our own work in which the clinical translation of zebrafish findings is either imminent or has already proven successful. The promising results in multiple organs suggest that further insight into regenerative mechanisms and novel clinically relevant therapeutic approaches will emerge from zebrafish research in the future.

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.

Modelling inborn errors of metabolism in zebrafish

The majority of human inborn errors of metabolism are fatal multisystem disorders that lack proper treatment and have a poorly understood mechanistic basis. Novel technologies are required to address this issue, and the use of zebrafish to model these diseases is an emerging field. Here we present the published zebrafish models of inborn metabolic diseases, discuss their validity, and review the novel mechanistic insights that they have provided. We also review the available methods for creating and studying zebrafish disease models, advantages and disadvantages of using this model organism, and successful examples of the use of zebrafish for drug discovery and development. Using a zebrafish to model inborn errors of metabolism in vivo, although still in its infancy, shows promise for a deeper understanding of disease pathomechanisms, onset, and progression, and also for the development of specific therapies.

Distinct iron architecture in SF3B1-mutant myelodysplastic syndrome patients is linked to an SLC25A37 splice variant with a retained intron

Perturbation in iron homeostasis is a hallmark of some hematologic diseases. Abnormal sideroblasts with accumulation of iron in the mitochondria are named ring sideroblasts (RS). RS is a cardinal feature of refractory anemia with RS (RARS) and RARS with marked thrombocytosis (RARS/-T). Mutations in SF3B1, a member of the RNA splicing machinery are frequent in RARS/-T and defects of this gene were linked to RS formation. Here we showcase the differences in iron architecture of SF3B1-mutant and wild-type (WT) RARS/-T and provide new mechanistic insights by which SF3B1 mutations lead to differences in iron. We found higher iron levels in SF3B1 mutant vs WT RARS/-T by transmission electron microscopy/spectroscopy/flow cytometry. SF3B1 mutations led to increased iron without changing the valence as shown by the presence of Fe(2+) in mutant and WT. Reactive oxygen species and DNA damage were not increased in SF3B1-mutant patients. RNA-sequencing and Reverse transcriptase PCR showed higher expression of a specific isoform of SLC25A37 in SF3B1-mutant patients, a crucial importer of Fe(2+) into the mitochondria. Our studies suggest that SF3B1 mutations contribute to cellular iron overload in RARS/-T by deregulating SLC25A37.

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

Mitochondrial iron is essential for the biosynthesis of heme and iron-sulfur ([Fe-S]) clusters in mammalian cells. In developing erythrocytes, iron is imported into the mitochondria by MFRN1 (mitoferrin-1, SLC25A37). Although loss of MFRN1 in zebrafish and mice leads to profound anemia, mutant animals showed no overt signs of porphyria, suggesting that mitochondrial iron deficiency does not result in an accumulation of protoporphyrins. Here, we developed a gene trap model to provide in vitro and in vivo evidence that iron regulatory protein-1 (IRP1) inhibits protoporphyrin accumulation. Mfrn1(+/gt);Irp1(-/-) erythroid cells exhibit a significant increase in protoporphyrin levels. IRP1 attenuates protoporphyrin biosynthesis by binding to the 5'-iron response element (IRE) of alas2 mRNA, inhibiting its translation. Ectopic expression of alas2 harboring a mutant IRE, preventing IRP1 binding, in Mfrn1(gt/gt) cells mimics Irp1 deficiency. Together, our data support a model whereby impaired mitochondrial [Fe-S] cluster biogenesis in Mfrn1(gt/gt) cells results in elevated IRP1 RNA-binding that attenuates ALAS2 mRNA translation and protoporphyrin accumulation.

Zebrafish as a model system for mitochondrial biology and diseases

Animal models for studying human disease are essential to the continuing evolution of medicine. Rodent models are attractive for the obvious similarities in development and genetic makeup compared with humans, but have cost and technical limitations. The zebrafish (Danio rerio) represents an ideal alternative vertebrate model of human disease because of its high conservation of genetic information and physiological processes, inexpensive maintenance, and optical clarity facilitating direct observation. This review highlights recent advances in understanding genetic disease states associated with the dynamic organelle, the mitochondrion, using the zebrafish. Mitochondrial diseases that have been replicated in the zebrafish include those affecting the nervous and cardiovascular systems, as well as red blood cell function. Gene silencing techniques, including morpholino knockdown and transcription activator-like (TAL)-effector endonucleases, have been exploited to demonstrate how loss of function can induce human disease-like states in zebrafish. Moreover, modeling mitochondrial diseases has been facilitated greatly by the creation of transgenic fish with fluorescently labeled mitochondria for in vivo visualization of these structures. In addition, behavioral assays have been developed to examine changes in motor activity and sensory responses, particularly in larval stages. Zebrafish are poised to advance our understanding of the pathogenesis of human mitochondrial diseases beyond the current state of knowledge and provide a key tool in the development of novel therapeutic approaches to treat these conditions.

Mitochondrial iron-sulfur protein biogenesis and human disease

Work during the past 14 years has shown that mitochondria are the primary site for the biosynthesis of iron-sulfur (Fe/S) clusters. In fact, it is this process that renders mitochondria essential for viability of virtually all eukaryotes, because they participate in the synthesis of the Fe/S clusters of key nuclear and cytosolic proteins such as DNA polymerases, DNA helicases, and ABCE1 (Rli1), an ATPase involved in protein synthesis. As a consequence, mitochondrial function is crucial for nuclear DNA synthesis and repair, ribosomal protein synthesis, and numerous other extra-mitochondrial pathways including nucleotide metabolism and cellular iron regulation. Within mitochondria, the synthesis of Fe/S clusters and their insertion into apoproteins is assisted by 17 proteins forming the ISC (iron-sulfur cluster) assembly machinery. Biogenesis of mitochondrial Fe/S proteins can be dissected into three main steps: First, a Fe/S cluster is generated de novo on a scaffold protein. Second, the Fe/S cluster is dislocated from the scaffold and transiently bound to transfer proteins. Third, the latter components, together with specific ISC targeting factors insert the Fe/S cluster into client apoproteins. Disturbances of the first two steps impair the maturation of extra-mitochondrial Fe/S proteins and affect cellular and systemic iron homeostasis. In line with the essential function of mitochondria, genetic mutations in a number of ISC genes lead to severe neurological, hematological and metabolic diseases, often with a fatal outcome in early childhood. In this review we briefly summarize our current functional knowledge on the ISC assembly machinery, and we present a comprehensive overview of the various Fe/S protein assembly diseases.

Copper import into the mitochondrial matrix in Saccharomyces cerevisiae is mediated by Pic2, a mitochondrial carrier family protein

Saccharomyces cerevisiae must import copper into the mitochondrial matrix for eventual assembly of cytochrome c oxidase. This copper is bound to an anionic fluorescent molecule known as the copper ligand (CuL). Here, we identify for the first time a mitochondrial carrier family protein capable of importing copper into the matrix. In vitro transport of the CuL into the mitochondrial matrix was saturable and temperature-dependent. Strains with a deletion of PIC2 grew poorly on copper-deficient non-fermentable medium supplemented with silver and under respiratory conditions when challenged with a matrix-targeted copper competitor. Mitochondria from pic2Δ cells had lower total mitochondrial copper and exhibited a decreased capacity for copper uptake. Heterologous expression of Pic2 in Lactococcus lactis significantly enhanced CuL transport into these cells. Therefore, we propose a novel role for Pic2 in copper import into mitochondria.

Loss-of-function ferrochelatase and gain-of-function erythroid-specific 5-aminolevulinate synthase mutations causing erythropoietic protoporphyria and x-linked protoporphyria in North American patients reveal novel mutations and a high prevalence of X-linked protoporphyria

Erythropoietic protoporphyria (EPP) and X-linked protoporphyria (XLP) are inborn errors of heme biosynthesis with the same phenotype but resulting from autosomal recessive loss-of-function mutations in the ferrochelatase (FECH) gene and gain-of-function mutations in the X-linked erythroid-specific 5-aminolevulinate synthase (ALAS2) gene, respectively. The EPP phenotype is characterized by acute, painful, cutaneous photosensitivity and elevated erythrocyte protoporphyrin levels. We report the FECH and ALAS2 mutations in 155 unrelated North American patients with the EPP phenotype. FECH sequencing and dosage analyses identified 140 patients with EPP: 134 with one loss-of-function allele and the common IVS3-48T>C low expression allele, three with two loss-of-function mutations and three with one loss-of-function mutation and two low expression alleles. There were 48 previously reported and 23 novel FECH mutations. The remaining 15 probands had ALAS2 gain-of-function mutations causing XLP: 13 with the previously reported deletion, c.1706_1709delAGTG, and two with novel mutations, c.1734delG and c.1642C>T(p.Q548X). Notably, XLP represented ~10% of EPP phenotype patients in North America, two to five times more than in Western Europe. XLP males had twofold higher erythrocyte protoporphyrin levels than EPP patients, predisposing to more severe photosensitivity and liver disease. Identification of XLP patients permits accurate diagnosis and counseling of at-risk relatives and asymptomatic heterozygotes.

Molecular and functional analysis of the C-terminal region of human erythroid-specific 5-aminolevulinic synthase associated with X-linked dominant protoporphyria (XLDPP)

Frameshift mutations in the last coding exon of the 5-aminolevulinate synthase (ALAS) 2 gene were described to activate the enzyme causing increased levels of zinc- and metal-free protoporphyrin in patients with X-linked dominant protoporphyria (XLDPP). Only two such so-called gain-of-function mutations have been reported since the description of XLDPP in 2008. In this study of four newly identified XLDPP families, we identified two novel ALAS2 gene mutations, a nonsense p.Q548X and a frameshift c.1651-1677del26bp, along with a known mutation (delAGTG) found in two unrelated families. Of relevance, a de novo somatic and germinal mosaicism was present in a delAGTG family. Such a phenomenon may explain the high proportion of this mutation in XLDPP worldwide. Enhancements of over 3- and 14-fold in the catalytic rate and specificity constant of purified recombinant XLDPP variants in relation to those of wild-type ALAS2 confirmed the gain of function ascribed to these enzymes. The fact that both p.Q548X and c.1651-1677del26bp are located in close proximity and upstream from the two previously described mutations led us to propose the presence of a large gain-of-function domain within the C-terminus of ALAS2. To test this hypothesis, we generated four additional nonsense mutants (p.A539X, p.G544X, p.G576X and p.V583X) surrounding the human XLDPP mutations and defined an ALAS2 gain-of-function domain with a minimal size of 33 amino acids. The identification of this gain-of-function domain provides important information on the enzymatic activity of ALAS2, which was proposed to be constitutively inhibited, either directly or indirectly, through its own C-terminus.

Heme metabolism and erythropoiesis

PURPOSE OF REVIEW

Heme biosynthesis requires a series of enzymatic reactions that take place in the cytosol and the mitochondria as well as the proper intercellular and intracellular trafficking of iron. Heme can also be acquired by intestinal absorption and intercellular transport. The purpose of this review is to highlight recent work on heme and iron transport with an emphasis on their relevance in erythropoiesis.

RECENT FINDINGS

Whereas the enzymes responsible for heme biosynthesis have been identified, transport mechanisms for iron, heme, or heme synthesis intermediates are only emerging. Recent studies have shed light on how these molecules are transported among various cellular compartments, as well as tissues. Much of this progress can be attributed to the use of model organisms such as S. cerevisiae, C. elegans, D. rerio, and M. musculus. Genetic studies in these models have led to the identification of several new genes involved in heme metabolism. Although our understanding has greatly improved, it is highly likely that other regulators exist and additional work is required to characterize the pathways by which heme and iron are transported within the erythron.

SUMMARY

The identification of heme and iron transport mechanisms will improve our understanding of blood development and provide new insight into human blood disorders.

The long history of iron in the Universe and in health and disease

BACKGROUND

Not long after the Big Bang, iron began to play a central role in the Universe and soon became mired in the tangle of biochemistry that is the prima essentia of life. Since life's addiction to iron transcends the oxygenation of the Earth's atmosphere, living things must be protected from the potentially dangerous mix of iron and oxygen. The human being possesses grams of this potentially toxic transition metal, which is shuttling through his oxygen-rich humor. Since long before the birth of modern medicine, the blood-vibrant red from a massive abundance of hemoglobin iron-has been a focus for health experts.

SCOPE OF REVIEW

We describe the current understanding of iron metabolism, highlight the many important discoveries that accreted this knowledge, and describe the perils of dysfunctional iron handling.

GENERAL SIGNIFICANCE

Isaac Newton famously penned, "If I have seen further than others, it is by standing upon the shoulders of giants". We hope that this review will inspire future scientists to develop intellectual pursuits by understanding the research and ideas from many remarkable thinkers of the past.

MAJOR CONCLUSIONS

The history of iron research is a long, rich story with early beginnings, and is far from being finished. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Cellular and mitochondrial iron homeostasis in vertebrates

Iron plays an essential role in cellular metabolism and biological processes. However, due to its intrinsic redox activity, free iron is a potentially toxic molecule in cellular biochemistry. Thus, organisms have developed sophisticated ways to import, sequester, and utilize iron. The transferrin cycle is a well-studied iron uptake pathway that is important for most vertebrate cells. Circulating iron can also be imported into cells by mechanisms that are independent of transferrin. Once imported into erythroid cells, iron is predominantly consumed by the mitochondria for the biosynthesis of heme and iron sulfur clusters. This review focuses on canonical transferrin-mediated and the newly discovered, non-transferrin mediated iron uptake pathways, as well as, mitochondrial iron homeostasis in higher eukaryotes. This article is part of a Special Issue entitled: Cell Biology of Metals.