Iron Responsive Element-Mediated Responses to Iron Dyshomeostasis in Alzheimer’s Disease

Background: Iron trafficking and accumulation is associated with Alzheimer’s disease (AD) pathogenesis. However, the role of iron dyshomeostasis in early disease stages is uncertain. Currently, gene expression changes indicative of iron dyshomeostasis are not well characterized, making it difficult to explore these in existing datasets. Objective: To identify sets of genes predicted to contain iron responsive elements (IREs) and use these to explore possible iron dyshomeostasis-associated gene expression responses in AD. Methods: Comprehensive sets of genes containing predicted IRE or IRE-like motifs in their 3′ or 5′ untranslated regions (UTRs) were identified in human, mouse, and zebrafish reference transcriptomes. Further analyses focusing on these genes were applied to a range of cultured cell, human, mouse, and zebrafish gene expression datasets. Results: IRE gene sets are sufficiently sensitive to distinguish not only between iron overload and deficiency in cultured cells, but also between AD and other pathological brain conditions. Notably, changes in IRE transcript abundance are among the earliest observable changes in zebrafish familial AD (fAD)-like brains, preceding other AD-typical pathologies such as inflammatory changes. Unexpectedly, while some IREs in the 3′ untranslated regions of transcripts show significantly increased stability under iron deficiency in line with current assumptions, many such transcripts instead display decreased stability, indicating that this is not a generalizable paradigm. Conclusion: Our results reveal IRE gene expression changes as early markers of the pathogenic process in fAD and are consistent with iron dyshomeostasis as an important driver of this disease. Our work demonstrates how differences in the stability of IRE-containing transcripts can be used to explore and compare iron dyshomeostasis-associated gene expression responses across different species, tissues, and conditions.

Conservation in the Iron Responsive Element Family

Iron responsive elements (IREs) are mRNA stem-loop targets for translational control by the two iron regulatory proteins IRP1 and IRP2. They are found in the untranslated regions (UTRs) of genes that code for proteins involved in iron metabolism. There are ten “classic” IRE types that define the conserved secondary and tertiary structure elements necessary for proper IRP binding, and there are 83 published “IRE-like” sequences, most of which depart from the established IRE model. Here are structurally-guided discussions regarding the essential features of an IRE and what is important for IRE family membership.

Iron enhances the binding rates and translational efficiency of iron responsive elements (IREs) mRNA with initiation factor eIF4F

Interaction of iron responsive elements (IRE) mRNA with the translational machinery is an early step critical in the initiation of protein synthesis. To investigate the binding specificity of IRE mRNA for eIF4F, kinetic rates for the eIF4F·IRE RNA interactions were determined and correlated with the translational efficiency. The observed rate of eIF4F·FRT IRE RNA interactions was 2-fold greater as compared to eIF4F·ACO2 IRE RNA binding. Addition of iron enhanced the association rates and lowered the dissociation rates for the eIF4F binding to both IRE RNAs, with having higher preferential binding to the FRT IRE RNA. The binding rates of both eIF4F·IRE RNA complexes correlated with the enhancement of protein synthesis in vitro. Presence of iron and eIF4F in the depleted WGE significantly enhanced translation for both IRE RNAs. This suggests that iron promotes translation by enhancing the binding rates of the eIF4F∙IRE RNA complex. eIF4F·IRE RNA binding is temperature-dependent; raising the temperature from 5 to 25°C, enhanced the binding rates of eIF4F·FRT IRE (4-fold) and eIF4F·ACO2 IRE (5-fold). Presence of Fe2+ caused reduction in the activation energy for the binding of FRT IRE and ACO2 IRE to eIF4F, suggesting a more stable platform for initiating protein synthesis. In the presence of iron, lowered energy barrier has leads to the faster association rate and slower rate of dissociation for the protein-RNA complex, thus favoring efficient protein synthesis. Our results correlate well with the observed translational efficiency of IRE RNA, thereby suggesting that the presence of iron leads to a rapid, favorable, and stable complex formation that directs regulatory system to respond efficiently to cellular iron levels.

Functional analysis of Cti6 core domain responsible for recruitment of epigenetic regulators Sin3, Cyc8 and Tup1

Mapping of effective protein domains is a demanding stride to disclose the functional relationship between regulatory complexes. Domain analysis of protein interactions is requisite for understanding the pleiotropic responses of the respective partners. Cti6 is a multifunctional regulator for which we could show recruitment of co-repressors Sin3, Cyc8 and Tup1. However, the responsible core domain tethering Cti6 to these co-repressors is poorly understood. Here, we report the pivotal domain of Cti6 that is indispensable for co-repressor recruitment. We substantiated that amino acids 450–506 of Cti6 bind PAH2 of Sin3. To analyse this Cti6–Sin3 Interaction Domain (CSID) in more detail, selected amino acids within CSID were replaced by alanine. It is revealed that hydrophobic amino acids V467, L481 and L491 L492 L493 are important for Cti6–Sin3 binding. In addition to PAH2 of Sin3, CSID also binds to tetratricopeptide repeats (TPR) of Cyc8. Indeed, we could demonstrate Cti6 recruitment to promoters of genes, such as RNR3 and SMF3, containing iron-responsive elements (IRE). Importantly, Sin3 is also recruited to these promoters but only in the presence of functional Cti6. Our findings provide novel insights toward the critical interaction domain in the co-regulator Cti6, which is a component of regulatory complexes that are closely related to chromatin architecture and the epigenetic status of genes that are regulated by pleiotropic co-repressors.

Iron Responsive Element (IRE)-mediated responses to iron dyshomeostasis in Alzheimer’s disease

BackgroundIron trafficking and accumulation has been associated with Alzheimer’s disease (AD) pathogenesis. However, the role of iron dyshomeostasis in early disease stages is uncertain. Currently, gene expression changes indicative of iron dyshomeostasis are not well characterised, making it difficult to explore these in existing datasets.ResultsWe identified sets of genes predicted to contain Iron Responsive Elements (IREs), and used these to explore iron dyshomeostasis responses in transcript datasets involving (1) cultured cells under iron overload and deficiency treatments, (2) post-mortem brain tissues from AD and other neuropathologies, (3) 5XFAD transgenic mice modelling AD pathologies, and (4) a zebrafish knock-in model of early-onset, familial AD (fAD). IRE gene sets were sufficiently sensitive to distinguish not only between iron overload and deficiency in cultured cells, but also between AD and other pathological brain conditions. Notably, we see changes in 3’ IRE transcript abundance as amongst the earliest observable in zebrafish fAD-like brains and preceding other AD-typical pathologies such as inflammatory changes. Unexpectedly, while some 3’ IRE transcripts show significantly increased stability under iron deficiency in line with current assumptions, many such transcripts instead show decreased stability, indicating that this is not a generalizable paradigm.ConclusionsOur results reveal iron dyshomeostasis as a likely early driver of fAD and as able to distinguish AD from other brain pathologies. Our work demonstrates how differences in the stability of IRE-containing transcripts can be used to explore and compare iron dyshomeostasis responses in different species, tissues, and conditions.

Relevance of the iron-responsive element (IRE) pseudotriloop structure for IRP1/2 binding and validation of IRE-like structures using the yeast three-hybrid system

Iron-responsive-elements (IREs) are ~35-nucleotide (nt) stem-loop RNA structures located in 5′ or 3′ untranslated regions (UTRs) of mRNAs, and mediate post-transcriptional regulation by their association with IRE-binding proteins (IRPs). IREs are characterized by their apical 6-nt loop motif 5′-CAGWGH-3′ (W = A or U and H= A, C or U), the so-called pseudotriloop, of which the loop nts C1 and G5 are paired, and the none-paired C between the two stem regions. In this study, the yeast three-hybrid (Y3H) system was used to investigate the relevance of the pseudotriloop structure of ferritin light chain (FTL) for the IRE-IRP interaction and the binding affinities between variant IRE(-like) structures and the two IRP isoforms, IRP1 and 2. Mutational analysis of FTL IRE showed that deletion of the bulged-out U6 of the pseudotriloop does not significantly affect its binding to either IRP1 or 2, but substitution with C enhances binding of both IRPs. In addition, IRP1 was found more sensitive toward changes in the pseudotriloop-stabilizing C1-G5 base pair than IRP2, while mutation of the conserved G3 was lowering the binding of both IRPs. In comparison to FTL IRE other variant IREs, IRE of 5′-aminolevulinate synthase 2 (ALAS2), SLC40A1 (also known as Ferroportin-1), and endothelial PAS domain protein 1 (EPAS1) mRNA showed slightly higher, similar, and slightly weaker affinity for IRPs, respectively, while SLC11A2 IRE exhibited very weak binding to IRP1 and medium binding to IRP2, indicating the different binding modes of IRP1 and 2. Notably, α-Synuclein IRE showed no detectable binding to either IRP1 or 2. Our results indicate that Y3H represents a bona fide system to characterize binding between IRPs and various IRE-like structures.

The iron regulatory proteins are defective in repressing translation via exogenous 5′ iron responsive elements despite their relative abundance in leukemic cellular models

Endogenous IRP outnumbers IRE-containing mRNA but fails to bind to transfected exogenous ones.

Hereditary Hyperferritinemia-cataract Syndrome

Hereditary hyperferritinemia-cataract syndrome (HHCS) is a rare autosomal dominant disorder associated with high plasma ferritin concentration without iron overload and early-onset bilateral cataract. The deregulation of ferritin production in HHCS is caused by mutations in the iron-responsive elements (IREs) of the ferritin L-subunit gene (FTLgene) – interfering with the high-affinity interaction between IREs and iron regulatory proteins (IRPs), disturbing the negative regulatory control of ferritin synthesis and resulting in excessive production of L-ferritin. We report a 44-year-old woman initially suspected of having hereditary haemochromatosis and later together with family members diagnosed with HHCS. Genetic analysis showed heterozygosity for a G32T point mutation (Paris 2 mutation) in the IRE located in the 5' untranslated region (UTR) of theFTLgene. The differential diagnosis of hereditary haemochromatosis and HHCS together with the rarity and the versatile phenotype in HHCS obscures the diagnostic process, which emphasises the importance of the correct diagnosis of HHCS in order to prevent unnecessary phlebotomy.