How iron controls iron

The Non-Canonical Iron-Responsive Element of IRE-tvcp12 Hairpin Structure at the 3'-UTR of Trichomonas vaginalis TvCP12 mRNA That Binds TvHSP70 and TvACTN-3 Can Regulate mRNA Stability and Amount of Protein

Trichomonas vaginalis is one of the most common sexually transmitted parasites in humans. This protozoan has high iron requirements for growth, metabolism, and virulence. However, iron concentrations also differentially modulate T. vaginalis gene expression as in the genes encoding cysteine proteinases TvCP4 and TvCP12. Our goal was to identify the regulatory mechanism mediating the upregulation of tvcp12 under iron-restricted (IR) conditions. Here, we showed by RT-PCR, Western blot, and immunocytochemistry assays that IR conditions increase mRNA stability and amount of TvCP12. RNA electrophoretic mobility shift assay (REMSA), UV cross-linking, and competition assays demonstrated that a non-canonical iron-responsive element (IRE)-like structure at the 3'-untranslated region of the tvcp12 transcript (IRE-tvcp12) specifically binds to human iron regulatory proteins (IRPs) and to atypical RNA-binding cytoplasmic proteins from IR trichomonads, such as HSP70 and α-Actinin 3. These data were confirmed by REMSA supershift and Northwestern blot assays. Thus, our findings show that a positive gene expression regulation under IR conditions occurs at the posttranscriptional level possibly through RNA-protein interactions between atypical RNA-binding proteins and non-canonical IRE-like structures at the 3'-UTR of the transcript by a parallel mechanism to the mammalian IRE/IRP system that can be applied to other iron-regulated genes of T. vaginalis.

Iron, Myelin, and the Brain: Neuroimaging Meets Neurobiology

Although iron is crucial for neuronal functioning, many aspects of cerebral iron biology await clarification. The ability to quantify specific iron forms in the living brain would open new avenues for diagnosis, therapeutic monitoring, and understanding pathogenesis of diseases. A modality that allows assessment of brain tissue composition in vivo, in particular of iron deposits or myelin content on a submillimeter spatial scale, is magnetic resonance imaging (MRI). Multimodal strategies combining MRI with complementary analytical techniques ex vivo have emerged, which may lead to improved specificity. Interdisciplinary collaborations will be key to advance beyond simple correlative analyses in the biological interpretation of MRI data and to gain deeper insights into key factors leading to iron accumulation and/or redistribution associated with neurodegeneration.

High serum ferritin is associated with worse outcome of patients with decompensated cirrhosis

BACKGROUND

Studies in patients with decompensated cirrhosis showed a correlation between serum ferritin levels and patients' prognosis. Besides, red blood cell distribution width (RDW) and mean platelet volume (MPV) have been associated with the severity of hepatic function. The aim of this study was to evaluate the prognostic impact of serum ferritin and RDW/MPV in the outcome [survival, death, or liver transplantation (LT)] of patients with stable decompensated cirrhosis.

METHODS

Consecutive adult patients with stable decompensated cirrhosis admitted to our department between September 2010 and February 2016 were included. Serum ferritin, RDW and MPV were recorded in every patient. They were followed up and their outcome (alive, death, or LT) was evaluated.

RESULTS

192 consecutive patients with stable decompensated cirrhosis (142 men, age 54.2±12 years); at the end of follow up [12 (range: 1-64) months] 62 patients remained alive and 130 died or underwent LT. In multivariate analysis, serum ferritin (HR 1.001, 95%CI 1.00-1.002, P=0.005) and GFR (HR 0.96, 95%CI 0.92-0.99, P=0.035) were the only independent factors significantly associated with the outcome. Ferritin had low discriminative ability (AUC: 0.61) to the outcome yielding a sensitivity and specificity of 85.3% and 44.2%, respectively, at the best cut-off point (>55 ng/mL), while patients with ferritin >55 ng/mL (n=145) had a worse outcome compared to those with ferritin ≤55 ng/mL (n=47) (log rank P=0.001). RDW and MPV were not associated with the outcome.

CONCLUSION

High serum ferritin, but not RDW/MPV, is associated with worse outcome in patients with established decompensated cirrhosis. However, further studies are needed to elucidate better this issue.

A Novel Hybrid Iron Regulation Network Combines Features from Pathogenic and Nonpathogenic Yeasts

Iron is an essential micronutrient for both pathogens and their hosts, which restrict iron availability during infections in an effort to prevent microbial growth. Successful human pathogens like the yeast Candida glabrata have thus developed effective iron acquisition strategies. Their regulation has been investigated well for some pathogenic fungi and in the model organism Saccharomyces cerevisiae, which employs an evolutionarily derived system. Here, we show that C. glabrata uses a regulation network largely consisting of components of the S. cerevisiae regulon but also of elements of other pathogenic fungi. Specifically, similarly to baker's yeast, Aft1 is the main positive regulator under iron starvation conditions, while Cth2 degrades mRNAs encoding iron-requiring enzymes. However, unlike the case with S. cerevisiae, a Sef1 ortholog is required for full growth under iron limitation conditions, making C. glabrata an evolutionary intermediate to SEF1-dependent fungal pathogens. Therefore, C. glabrata has evolved an iron homeostasis system which seems to be unique within the pathogenic fungi.

IMPORTANCE

The fungus Candida glabrata represents an evolutionarily close relative of the well-studied and benign baker's yeast and model organism Saccharomyces cerevisiae On the other hand, C. glabrata is an important opportunistic human pathogen causing both superficial and systemic infections. The ability to acquire trace metals, in particular, iron, and to tightly regulate this process during infection is considered an important virulence attribute of a variety of pathogens. Importantly, S. cerevisiae uses a highly derivative regulatory system distinct from those of other fungi. Until now, the regulatory mechanism of iron homeostasis in C. glabrata has been mostly unknown. Our study revealed a hybrid iron regulation network that is unique to C. glabrata and is placed at an evolutionary midpoint between those of S. cerevisiae and related fungal pathogens. We thereby show that, in the host, even a successful human pathogen can rely largely on a strategy normally found in nonpathogenic fungi from a terrestrial environment.

RNA-Binding Proteins in Trichomonas vaginalis: Atypical Multifunctional Proteins

Iron homeostasis is highly regulated in vertebrates through a regulatory system mediated by RNA-protein interactions between the iron regulatory proteins (IRPs) that interact with an iron responsive element (IRE) located in certain mRNAs, dubbed the IRE-IRP regulatory system. Trichomonas vaginalis, the causal agent of trichomoniasis, presents high iron dependency to regulate its growth, metabolism, and virulence properties. Although T. vaginalis lacks IRPs or proteins with aconitase activity, possesses gene expression mechanisms of iron regulation at the transcriptional and posttranscriptional levels. However, only one gene with iron regulation at the transcriptional level has been described. Recently, our research group described an iron posttranscriptional regulatory mechanism in the T. vaginalis tvcp4 and tvcp12 cysteine proteinase mRNAs. The tvcp4 and tvcp12 mRNAs have a stem-loop structure in the 5'-coding region or in the 3'-UTR, respectively that interacts with T. vaginalis multifunctional proteins HSP70, α-Actinin, and Actin under iron starvation condition, causing translation inhibition or mRNA stabilization similar to the previously characterized IRE-IRP system in eukaryotes. Herein, we summarize recent progress and shed some light on atypical RNA-binding proteins that may participate in the iron posttranscriptional regulation in T. vaginalis.

Iron regulatory proteins and their role in controlling iron metabolism

Cellular iron homeostasis is regulated by post-transcriptional feedback mechanisms, which control the expression of proteins involved in iron uptake, release and storage. Two cytoplasmic proteins with mRNA-binding properties, iron regulatory proteins 1 and 2 (IRP1 and IRP2) play a central role in this regulation. Foremost, IRPs regulate ferritin H and ferritin L translation and thus iron storage, as well as transferrin receptor 1 (TfR1) mRNA stability, thereby adjusting receptor expression and iron uptake via receptor-mediated endocytosis of iron-loaded transferrin. In addition splice variants of iron transporters for import and export at the plasma-membrane, divalent metal transporter 1 (DMT1) and ferroportin are regulated by IRPs. These mechanisms have probably evolved to maintain the cytoplasmic labile iron pool (LIP) at an appropriate level. In certain tissues, the regulation exerted by IRPs influences iron homeostasis and utilization of the entire organism. In intestine, the control of ferritin expression limits intestinal iron absorption and, thus, whole body iron levels. In bone marrow, erythroid heme biosynthesis is coordinated with iron availability through IRP-mediated translational control of erythroid 5-aminolevulinate synthase mRNA. Moreover, the translational control of HIF2α mRNA in kidney by IRP1 coordinates erythropoietin synthesis with iron and oxygen supply. Besides IRPs, body iron absorption is negatively regulated by hepcidin. This peptide hormone, synthesized and secreted by the liver in response to high serum iron, downregulates ferroportin at the protein level and thereby limits iron absorption from the diet. Hepcidin will not be discussed in further detail here.

Trichomonas vaginalis Cysteine Proteinases: Iron Response in Gene Expression and Proteolytic Activity

We focus on the iron response of Trichomonas vaginalis to gene family products such as the cysteine proteinases (CPs) involved in virulence properties. In particular, we examined the effect of iron on the gene expression regulation and function of cathepsin L-like and asparaginyl endopeptidase-like CPs as virulence factors. We addressed some important aspects about CPs genomic organization and we offer possible explanations to the fact that only few members of this large gene family are expressed at the RNA and protein levels and the way to control their proteolytic activity. We also summarized all known iron regulations of CPs at transcriptional, posttranscriptional, and posttranslational levels along with new insights into the possible epigenetic and miRNA processes.

Conditional deletion of ferritin h in mice reduces B and T lymphocyte populations

The immune system and iron availability are intimately linked as appropriate iron supply is needed for cell proliferation, while excess iron, as observed in hemochromatosis, may reduce subsets of lymphocytes. We have tested the effects of a ferritin H gene deletion on lymphocytes. Mx-Cre mediated conditional deletion of ferritin H in bone marrow reduced the number of mature B cells and peripheral T cells in all lymphoid organs. FACS analysis showed an increase in the labile iron pool, enhanced reactive oxygen species formation and mitochondrial depolarization. The findings were confirmed by a B-cell specific deletion using Fth^lox/lox; CD19-Cre mice. Mature B cells were strongly under-represented in bone marrow and spleen of the deleted mice, whereas pre-B and immature B cells were not affected. Bone marrow B cells showed increased proliferation as judged by the number of cells in S and G2/M phase as well as BrdU incorporation. Upon in vitro culture with B-cell activating factor of the tumor necrosis factor family (BAFF), ferritin H-deleted spleen B cells showed lower survival rates than wild type cells. This was partially reversed with iron-chelator deferiprone. The loss of T cells was also confirmed by a T cell-specific deletion in Fth^lox/lox;CD4-Cre mice. Our data show that ferritin H is required for B and T cell survival by actively reducing the labile iron pool. They further suggest that natural B and T cell maturation is influenced by intracellular iron levels and possibly deregulated in iron excess or deprivation.

Mitochondrial iron metabolism and its role in neurodegeneration

In addition to their well-established role in providing the cell with ATP, mitochondria are the source of iron-sulfur clusters (ISCs) and heme - prosthetic groups that are utilized by proteins throughout the cell in various critical processes. The post-transcriptional system that mammalian cells use to regulate intracellular iron homeostasis depends, in part, upon the synthesis of ISCs in mitochondria. Thus, proper mitochondrial function is crucial to cellular iron homeostasis. Many neurodegenerative diseases are marked by mitochondrial impairment, brain iron accumulation, and oxidative stress - pathologies that are inter-related. This review discusses the physiological role that mitochondria play in cellular iron homeostasis and, in so doing, attempts to clarify how mitochondrial dysfunction may initiate and/or contribute to iron dysregulation in the context of neurodegenerative disease. We review what is currently known about the entry of iron into mitochondria, the ways in which iron is utilized therein, and how mitochondria are integrated into the system of iron homeostasis in mammalian cells. Lastly, we turn to recent advances in our understanding of iron dysregulation in two neurodegenerative diseases (Alzheimer's disease and Parkinson's disease), and discuss the use of iron chelation as a potential therapeutic approach to neurodegenerative disease.

Cytosolic iron-sulfur cluster assembly (CIA) system: factors, mechanism, and relevance to cellular iron regulation

FeS cluster biogenesis is an essential process in virtually all forms of life. Complex protein machineries that are conserved from bacteria through higher eukaryotes facilitate assembly of the FeS cofactor in proteins. In the last several years, significant strides have been made in our understanding of FeS cluster assembly and the functional overlap of this process with cellular iron homeostasis. This minireview summarizes the present understanding of the cytosolic iron-sulfur cluster assembly (CIA) system in eukaryotes, with a focus on information gained from studies in budding yeast and mammalian systems.

Accumulation of oxidative DNA damage in brain mitochondria in mouse model of hereditary ferritinopathy

Tissue iron content is strictly regulated to concomitantly satisfy specialized metabolic requirements and avoid toxicity. Ferritin, a multi-subunit iron storage protein, is central to maintenance of iron homeostasis in the brain. Mutations in the ferritin light chain (FTL)-encoding gene underlie the autosomal dominant, neurodegenerative disease, neuroferritinopathy/hereditary ferritinopathy (HF). HF is characterized by progressive accumulation of ferritin and iron. To gain insight into mechanisms by which FTL mutations promote neurodegeneration, a transgenic mouse, expressing human mutant form of FTL, was recently generated. The FTL mouse exhibits buildup of iron in the brain and presents manifestations of oxidative stress reminiscent of the human disease. Here, we asked whether oxidative DNA damage accumulates in the FTL mouse brain. Long-range PCR (L-PCR) amplification-mediated DNA damage detection assays revealed that the integrity of mitochondrial DNA (mtDNA) in the brain was significantly compromised in the 12- but not 6-month-old FTL mice. Furthermore, L-PCR employed in conjunction with DNA modifying enzymes, which target specific DNA adducts, revealed the types of oxidative adducts accumulating in mtDNA in the FTL brain. Consistently with DNA damage predicted to form under conditions of excessive oxidative stress, detected adducts include, oxidized guanines, abasic sites and strand breaks. Elevated mtDNA damage may impair mitochondrial function and brain energetics and in the long term contribute to neuronal loss and exacerbate neurodegeneration in HF.