Transport of Non-Transferrin Bound Iron to the Brain: Implications for Alzheimer's Disease

Mechanism of ferroptosis regulating ischemic stroke and pharmacologically inhibiting ferroptosis in treatment of ischemic stroke

Ferroptosis is a newly discovered form of programmed cell death that is non-caspase-dependent and is characterized by the production of lethal levels of iron-dependent lipid reactive oxygen species (ROS). In recent years, ferroptosis has attracted great interest in the field of cerebral infarction because it differs morphologically, physiologically, and genetically from other forms of cell death such as necrosis, apoptosis, autophagy, and pyroptosis. In addition, ROS is considered to be an important prognostic factor for ischemic stroke, making it a promising target for stroke treatment. This paper summarizes the induction and defense mechanisms associated with ferroptosis, and explores potential treatment strategies for ischemic stroke in order to lay the groundwork for the development of new neuroprotective drugs.

Iron Overload in Brain: Transport Mismatches, Microbleeding Events, and How Nanochelating Therapies May Counteract Their Effects

Iron overload in many brain regions is a common feature of aging and most neurodegenerative diseases. In this review, the causes, mechanisms, mathematical models, and possible therapies are summarized. Indeed, physiological and pathological conditions can be investigated using compartmental models mimicking iron trafficking across the blood-brain barrier and the Cerebrospinal Fluid-Brain exchange membranes located in the choroid plexus. In silico models can investigate the alteration of iron homeostasis and simulate iron concentration in the brain environment, as well as the effects of intracerebral iron chelation, determining potential doses and timing to recover the physiological state. Novel formulations of non-toxic nanovectors with chelating capacity are already tested in organotypic brain models and could be available to move from in silico to in vivo experiments.

TauSTED super-resolution imaging of labile iron in primary hippocampal neurons

Iron dyshomeostasis is involved in many neurological disorders, particularly neurodegenerative diseases where iron accumulates in various brain regions. Identifying mechanisms of iron transport in the brain is crucial for understanding the role of iron in healthy and pathological states. In neurons, it has been suggested that iron can be transported by the axon to different brain regions in the form of labile iron; a pool of reactive and exchangeable intracellular iron. Here we report a novel approach to imaging labile ferrous iron, Fe(II), in live primary hippocampal neurons using confocal and TauSTED (stimulated emission depletion) microscopy. TauSTED is based on super-resolution STED nanoscopy, which combines high spatial resolution imaging (<40 nm) with fluorescence lifetime information, thus reducing background noise and improving image quality. We applied TauSTED imaging utilizing biotracker FerroFarRed Fe(II) and found that labile iron was present as submicrometric puncta in dendrites and axons. Some of these iron-rich structures are mobile and move along neuritic pathways, arguing for a labile iron transport mechanism in neurons. This super-resolution imaging approach offers a new perspective for studying the dynamic mechanisms of axonal and dendritic transport of iron at high spatial resolution in living neurons. In addition, this methodology could be transposed to the imaging of other fluorescent metal sensors.

The (Bio)Chemistry of Non-Transferrin-Bound Iron

In healthy individuals, virtually all blood plasma iron is bound by transferrin. However, in several diseases and clinical conditions, hazardous non-transferrin-bound iron (NTBI) species occur. NTBI represents a potentially toxic iron form, being a direct cause of oxidative stress in the circulating compartment and tissue iron loading. The accumulation of these species can cause cellular damage in several organs, namely, the liver, spleen, and heart. Despite its pathophysiological relevance, the chemical nature of NTBI remains elusive. This has precluded its use as a clinical biochemical marker and the development of targeted therapies. Herein, we make a critical assessment of the current knowledge of NTBI speciation. The currently accepted hypotheses suggest that NTBI is mostly iron bound to citric acid and iron bound to serum albumin, but the chemistry of this system remains fuzzy. We explore the complex chemistry of iron complexation by citric acid and its implications towards NTBI reactivity. Further, the ability of albumin to bind iron is revised and the role of protein post-translational modifications on iron binding is discussed. The characterization of the NTBI species structure may be the starting point for the development of a standardized analytical assay, the better understanding of these species’ reactivity or the identification of NTBI uptake mechanisms by different cell types, and finally, to the development of new therapies.

Upregulation of brain hepcidin in prion diseases

Accumulation of redox-active iron in human sporadic Creutzfeldt-Jakob disease (sCJD) brain tissue and scrapie-infected mouse brains has been demonstrated previously. Here, we explored whether upregulation of local hepcidin secreted within the brain is the underlying cause of iron accumulation and associated toxicity. Using scrapie-infected mouse brains, we demonstrate transcriptional upregulation of hepcidin relative to controls. As a result, ferroportin (Fpn), the downstream effector of hepcidin and the only known iron export protein was downregulated, and ferritin, an iron storage protein was upregulated, suggesting increased intracellular iron. A similar transcriptional and translational upregulation of hepcidin, and decreased expression of Fpn and an increase in ferritin expression was observed in sCJD brain tissue. Further evaluation in human neuroblastoma cells (M17) exposed to synthetic mini-hepcidin showed downregulation of Fpn, upregulation of ferritin, and an increase in reactive oxygen species (ROS), resulting in cytotoxicity in a dose-dependent manner. Similar effects were noted in primary neurons isolated from mouse brain. As in M17 cells, primary neurons accumulated ferritin and ROS, and showed toxicity at five times lower concentration of mini-hepcidin. These observations suggest that upregulation of brain hepcidin plays a significant role in iron accumulation and associated neurotoxicity in human and animal prion disorders.

Release of Iron-Loaded Ferritin in Sodium Iodate-Induced Model of Age Related Macular Degeneration: An In-Vitro and In-Vivo Study

To evaluate the role of iron in sodium iodate (NaIO3)-induced model of age-related macular degeneration (AMD) in ARPE-19 cells in-vitro and in mouse models in-vivo. ARPE-19 cells, a human retinal pigment epithelial cell line, was exposed to 10 mM NaIO3 for 24 h, and the expression and localization of major iron modulating proteins was evaluated by Western blotting (WB) and immunostaining. Synthesis and maturation of cathepsin-D (cat-D), a lysosomal enzyme, was evaluated by quantitative reverse-transcriptase polymerase chain reaction (RT-qPCR) and WB, respectively. For in-vivo studies, C57BL/6 mice were injected with 40 mg/kg mouse body weight of NaIO3 intraperitoneally, and their retina was evaluated after 3 weeks as above. NaIO3 induced a 10-fold increase in ferritin in ARPE-19 cells, which co-localized with LC3II, an autophagosomal marker, and LAMP-1, a lysosomal marker. A similar increase in ferritin was noted in retinal lysates and retinal sections of NaIO3-injected mice by WB and immunostaining. Impaired synthesis and maturation of cat-D was also noted. Accumulated ferritin was loaded with iron, and released from retinal pigmented epithelial (RPE) cells in Perls' and LAMP-1 positive vesicles. NaIO3 impairs lysosomal degradation of ferritin by decreasing the transcription and maturation of cat-D in RPE cells. Iron-loaded ferritin accumulates in lysosomes and is released in lysosomal membrane-enclosed vesicles to the extracellular milieu. Accumulation of ferritin in RPE cells and fusion of ferritin-containing vesicles with adjacent photoreceptor cells is likely to create an iron overload, compromising their viability. Moreover, reduced activity of cat-D is likely to promote accumulation of other cellular debris in lysosomal vesicles, contributing to AMD-like pathology.

Alteration of Iron Concentration in Alzheimer's Disease as a Possible Diagnostic Biomarker Unveiling Ferroptosis

Proper functioning of all organs, including the brain, requires iron. It is present in different forms in biological fluids, and alterations in its distribution can induce oxidative stress and neurodegeneration. However, the clinical parameters normally used for monitoring iron concentration in biological fluids (i.e., serum and cerebrospinal fluid) can hardly detect the quantity of circulating iron, while indirect measurements, e.g., magnetic resonance imaging, require further validation. This review summarizes the mechanisms involved in brain iron metabolism, homeostasis, and iron imbalance caused by alterations detectable by standard and non-standard indicators of iron status. These indicators for iron transport, storage, and metabolism can help to understand which biomarkers can better detect iron imbalances responsible for neurodegenerative diseases.

Non-transferrin-bound iron transporters

Most cells in the body acquire iron via receptor-mediated endocytosis of transferrin, the circulating iron transport protein. When cellular iron levels are sufficient, the uptake of transferrin decreases to limit further iron assimilation and prevent excessive iron accumulation. In iron overload conditions, such as hereditary hemochromatosis and thalassemia major, unregulated iron entry into the plasma overwhelms the carrying capacity of transferrin, resulting in non-transferrin-bound iron (NTBI), a redox-active, potentially toxic form of iron. Plasma NTBI is rapidly cleared from the circulation primarily by the liver and other organs (e.g., pancreas, heart, and pituitary) where it contributes significantly to tissue iron overload and related pathology. While NTBI is usually not detectable in the plasma of healthy individuals, it does appear to be a normal constituent of brain interstitial fluid and therefore likely serves as an important source of iron for most cell types in the CNS. A growing body of literature indicates that NTBI uptake is mediated by non-transferrin-bound iron transporters such as ZIP14, L-type and T-type calcium channels, DMT1, ZIP8, and TRPC6. This review provides an overview of NTBI uptake by various tissues and cells and summarizes the evidence for and against the roles of individual transporters in this process.

Deciphering the Iron Side of Stroke: Neurodegeneration at the Crossroads Between Iron Dyshomeostasis, Excitotoxicity, and Ferroptosis

In general, iron represents a double-edged sword in metabolism in most tissues, especially in the brain. Although the high metabolic demands of brain cells require iron as a redox-active metal for ATP-producing enzymes, the brain is highly vulnerable to the devastating consequences of excessive iron-induced oxidative stress and, as recently found, to ferroptosis as well. The blood-brain barrier (BBB) protects the brain from fluctuations in systemic iron. Under pathological conditions, especially in acute brain pathologies such as stroke, the BBB is disrupted, and iron pools from the blood gain sudden access to the brain parenchyma, which is crucial in mediating stroke-induced neurodegeneration. Each brain cell type reacts with changes in their expression of proteins involved in iron uptake, efflux, storage, and mobilization to preserve its internal iron homeostasis, with specific organelles such as mitochondria showing specialized responses. However, during ischemia, neurons are challenged with excess extracellular glutamate in the presence of high levels of extracellular iron; this causes glutamate receptor overactivation that boosts neuronal iron uptake and a subsequent overproduction of membrane peroxides. This glutamate-driven neuronal death can be attenuated by iron-chelating compounds or free radical scavenger molecules. Moreover, vascular wall rupture in hemorrhagic stroke results in the accumulation and lysis of iron-rich red blood cells at the brain parenchyma and the subsequent presence of hemoglobin and heme iron at the extracellular milieu, thereby contributing to iron-induced lipid peroxidation and cell death. This review summarizes recent progresses made in understanding the ferroptosis component underlying both ischemic and hemorrhagic stroke subtypes.

Links Between Iron and Lipids: Implications in Some Major Human Diseases

Maintenance of iron homeostasis is critical to cellular health as both its excess and insufficiency are detrimental. Likewise, lipids, which are essential components of cellular membranes and signaling mediators, must also be tightly regulated to hinder disease progression. Recent research, using a myriad of model organisms, as well as data from clinical studies, has revealed links between these two metabolic pathways, but the mechanisms behind these interactions and the role these have in the progression of human diseases remains unclear. In this review, we summarize literature describing cross-talk between iron and lipid pathways, including alterations in cholesterol, sphingolipid, and lipid droplet metabolism in response to changes in iron levels. We discuss human diseases correlating with both iron and lipid alterations, including neurodegenerative disorders, and the available evidence regarding the potential mechanisms underlying how iron may promote disease pathogenesis. Finally, we review research regarding iron reduction techniques and their therapeutic potential in treating patients with these debilitating conditions. We propose that iron-mediated alterations in lipid metabolic pathways are involved in the progression of these diseases, but further research is direly needed to elucidate the mechanisms involved.

Lower Expression of Ndfip1 Is Associated With Alzheimer Disease Pathogenesis Through Decreasing DMT1 Degradation and Increasing Iron Influx

We have previously reported that high expression of divalent metal transporter 1 (DMT1) plays a crucial role in iron dyshomeostasis and β-amyloid (Aβ) peptide generation in the brain of Alzheimer’s disease (AD). Recent studies have shown that Nedd4 family interacting protein 1 (Ndfip1) can degrade DMT1 through ubiquitination pathway and reduce the accumulation of intracellular iron. The present study aims to evaluate whether Ndfip1 is involved in AD pathogenesis through mediating DMT1 degradation and iron metabolism. β-amyloid precursor protein/presenilin 1 (APP/PS1) transgenic mouse and Ndfip1 transfected SH-SY5Y cells were used in this study. Immunohistochemistry and Western blot were performed to examine the distribution and expression levels of Ndfip1 and DMT1. In addition, ELISA and calcein fluorescence were carried out for analyzing the levels of Aβ peptide and iron influx, respectively. The results showed that Ndfip1 immunoreactivity was decreased in the cortex and hippocampus of APP/PS1 mice, compared with wild type (WT) controls. Colocalization of Ndfip1 and Aβ within senile plaques could be observed. Immunoblot analyses showed that low expression of Ndfip1 and high expression of DMT1 proteins were detected in APP/PS1 mouse brain, compared with age-matched WT animals. Overexpression of Ndfip1 down-regulated DMT1 expression, and reduced iron influx and Aβ secretion in SH-SY5Y cells. Further, overexpressed Ndfip1 significantly attenuated iron-induced cell damage in Ndfip1 transfected cells. The present study suggests that lower expression of Ndfip1 might be associated with the pathogenesis of AD, through decreasing DMT1 degradation and increasing iron accumulation in the brain.

Identification of a novel Na+-coupled Fe3+-citrate transport system, distinct from mammalian INDY, for uptake of citrate in mammalian cells

NaCT is a Na⁺-coupled transporter for citrate expressed in hepatocytes and neurons. It is the mammalian ortholog of INDY (I’m Not Dead Yet), a transporter which modifies lifespan in Drosophila. Here we describe a hitherto unknown transport system for citrate in mammalian cells. When liver and mammary epithelial cells were pretreated with the iron supplement ferric ammonium citrate (FAC), uptake of citrate increased >10-fold. Iron chelators abrogated the stimulation of citrate uptake in FAC-treated cells. The iron exporter ferroportin had no role in this process. The stimulation of citrate uptake also occurred when Fe³⁺ was added during uptake without pretreatment. Similarly, uptake of Fe³⁺ was enhanced by citrate. The Fe³⁺-citrate uptake was coupled to Na⁺. This transport system was detectable in primary hepatocytes and neuronal cell lines. The functional features of this citrate transport system distinguish it from NaCT. Loss-of-function mutations in NaCT cause early-onset epilepsy and encephalopathy; the newly discovered Na⁺-coupled Fe³⁺-citrate transport system might offer a novel treatment strategy for these patients to deliver citrate into affected neurons independent of NaCT. It also has implications to iron-overload conditions where circulating free iron increases, which would stimulate cellular uptake of citrate and consequently affect multiple metabolic pathways.