Brain Iron Transport Research Articles

Sex-Biased Associations of Circulating Ferroptosis Inhibitors with Reduced Lipid Peroxidation and Better Neurocognitive Performance in People with HIV.

Ferroptosis is implicated in viral neuropathogenesis and may underlie HIV-associated neurocognitive impairment (NCI). Emerging data also suggest differences in brain iron transport by sex. We hypothesized that circulating ferritins that inhibit ferroptosis associate with neurocognitive function and NCI in people with HIV (PWH) in a sex-biased manner. Serum ferritin heavy-chain-1 (FTH1), ferritin light-chain (FTL), and urinary F2-isoprostanes (uF2-isoPs, specific lipid peroxidation marker) were quantified in 324 PWH (including 61 women) with serial global (NPZ-4) and domain-specific neurocognitive testing. Biomarker associations with neurocognitive test scores and NCIs were evaluated by multivariable regression; correlations with uF2-isoPs were also assessed. Higher FTL and FTH1 levels were associated with less NCI in all PWH (adjusted odds ratios 0.53, 95% confidence interval (95% CI) 0.36-0.79 and 0.66, 95% CI 0.45-0.97, respectively). In women, higher FTL and FTH1 were also associated with better NPZ-4 (FTL adjusted beta (β) = 0.15, 95% CI 0.02-0.29; FTL-by-sex βinteraction = 0.32, p = 0.047) and domain-specific neurocognitive test scores. Effects on neurocognitive performance persisted for up to 5 years. Levels of both ferritins correlated inversely with uF2-isoPs in women (FTL: rho = -0.47, p < 0.001). Circulating FTL and FTH1 exert sustained, sex-biased neuroprotective effects in PWH, possibly by protecting against iron-mediated lipid peroxidation (ferroptosis). Larger studies are needed to confirm the observed sex differences and further delineate the underlying mechanisms.

Amyloid-β exposed astrocytes induce iron transport from endothelial cells at the blood-brain barrier by altering the ratio of apo- and holo-transferrin.

Excessive brain iron accumulation is observed early in the onset of Alzheimer's disease, notably prior to widespread proteinopathy. These findings suggest that increases in brain iron levels are due to a dysregulation of the iron transport mechanism at the blood-brain barrier. Astrocytes release signals (apo- and holo-transferrin) that communicate brain iron needs to endothelial cells in order to modulate iron transport. Here we use iPSC-derived astrocytes and endothelial cells to investigate how early-disease levels of amyloid-β disrupt iron transport signals secreted by astrocytes to stimulate iron transport from endothelial cells. We demonstrate that conditioned media from astrocytes treated with amyloid-β stimulates iron transport from endothelial cells and induces changes in iron transport pathway proteins. The mechanism underlying this response begins with increased iron uptake and mitochondrial activity by the astrocytes, which in turn increases levels of apo-transferrin in the amyloid-β conditioned astrocyte media leading to increased iron transport from endothelial cells. These novel findings offer a potential explanation for the initiation of excessive iron accumulation in early stages of Alzheimer's disease. What's more, these data provide the first example of how the mechanism of iron transport regulation by apo- and holo-transferrin becomes misappropriated in disease that can lead to iron accumulation. The clinical benefit from understanding early dysregulation in brain iron transport in AD cannot be understated. If therapeutics can target this early process, they could possibly prevent the detrimental cascade that occurs with excessive iron accumulation.

The role of transferrins and iron-related proteins in brain iron transport: applications to neurological diseases.

Iron transport in the central nervous system (CNS) is a highly regulated process in which several important proteins participate to ensure this important metal reaches its sites of action. However, iron accumulation has been shown to be a common factor in different neurological disorders such as Alzheimer's disease, Parkinson's disease, Huntington's disease, Multiple Sclerosis, and Sanfilippo syndrome. This review is divided into four parts. The first part describes brain iron transport in homeostasis, mentioning the main proteins involved, whereas the second part contrasts the consequences of iron dysregulation, elaborating on its role in the aforementioned neurodegenerative diseases. The third part details the functions of the main proteins involved in brain iron homeostasis and their role in neurodegeneration. In the fourth part, in order to highlight the importance of transport proteins, the focus is set on human serum transferrin, the main iron transport protein. This final part describes perspectives about the mechanisms and chemical properties of human transferrin for the development of potential targeted drug delivery systems across the blood-brain barrier (BBB) or enhancers for the treatment of neurological diseases.

Transferrin and H-ferritin involvement in brain iron acquisition during postnatal development: impact of sex and genotype.

Iron delivery to the developing brain is essential for energy and metabolic support needed for processes such as myelination and neuronal development. Iron deficiency, especially in the developing brain, can result in a number of long-term neurological deficits that persist into adulthood. There is considerable debate that excess access to iron during development may result in iron overload in the brain and subsequently predispose individuals to age-related neurodegenerative diseases. There is a significant gap in knowledge regarding how the brain acquires iron during development and how biological variables such as development, genetics, and sex impact brain iron status. In this study, we used a mouse model expressing a mutant form of the iron homeostatic regulator protein HFE, (Hfe H63D), the most common gene variant in Caucasians, to determine impact of the mutation on brain iron uptake. Iron uptake was assessed using 59 Fe bound to either transferrin or H-ferritin as the iron carrier proteins. We demonstrate that at postnatal day 22, mutant mice brains take up greater amounts of iron compared with wildtype. Moreover, we introduce H-ferritin as a key protein in brain iron transport during development and identify a sex and genotype effect demonstrating female mutant mice take up more iron by transferrin, whereas male mutant mice take up more iron from H-ferritin at PND22. Furthermore, we begin to elucidate the mechanism for uptake using immunohistochemistry to profile the regional distribution and temporal expression of transferrin receptor and T-cell immunoglobulin and mucin domain 2, the latter is the receptor for H-ferritin. These data demonstrate that sex and genotype have significant effects on iron uptake and that regional receptor expression may play a large role in the uptake patterns during development. Open Science: This manuscript was awarded with the Open Materials Badge For more information see: https://cos.io/our-services/open-science-badges/ Cover Image for this issue: doi: 10.1111/jnc.14731.

Brain iron transport.

Brain iron is a crucial participant and regulator of normal physiological activity. However, excess iron is involved in the formation of free radicals, and has been associated with oxidative damage to neuronal and other brain cells. Abnormally high brain iron levels have been observed in various neurodegenerative diseases, including neurodegeneration with brain iron accumulation, Alzheimer's disease, Parkinson's disease and Huntington's disease. However, the key question of why iron levels increase in the relevant regions of the brain remains to be answered. A full understanding of the homeostatic mechanisms involved in brain iron transport and metabolism is therefore critical not only for elucidating the pathophysiological mechanisms responsible for excess iron accumulation in the brain but also for developing pharmacological interventions to disrupt the chain of pathological events occurring in these neurodegenerative diseases. Numerous studies have been conducted, but to date no effort to synthesize these studies and ideas into a systematic and coherent summary has been made, especially concerning iron transport across the luminal (apical) membrane of the capillary endothelium and the membranes of different brain cell types. Herein, we review key findings on brain iron transport, highlighting the mechanisms involved in iron transport across the luminal (apical) as well as the abluminal (basal) membrane of the blood-brain barrier, the blood-cerebrospinal fluid barrier, and iron uptake and release in neurons, oligodendrocytes, astrocytes and microglia within the brain. We offer suggestions for addressing the many important gaps in our understanding of this important topic, and provide new insights into the potential causes of abnormally increased iron levels in regions of the brain in neurodegenerative disorders.

Mechanisms of Brain Iron Transport: Insight into Neurodegeneration and CNS Disorders

Trace metals such as iron, copper, zinc, manganese, and cobalt are essential cofactors for many cellular enzymes. Extensive research on iron, the most abundant transition metal in biology, has contributed to an increased understanding of the molecular machinery involved in maintaining its homeostasis in mammalian peripheral tissues. However, the cellular and intercellular iron transport mechanisms in the central nervous system (CNS) are still poorly understood. Accumulating evidence suggests that impaired iron metabolism is an initial cause of neurodegeneration, and several common genetic and sporadic neurodegenerative disorders have been proposed to be associated with dysregulated CNS iron homeostasis. This review aims to provide a summary of the molecular mechanisms of brain iron transport. Our discussion is focused on iron transport across endothelial cells of the blood-brain barrier and within the neuro- and glial-vascular units of the brain, with the aim of revealing novel therapeutic targets for neurodegenerative and CNS disorders.

Brain iron homeostasis, the choroid plexus, and localization of iron transport proteins

Maintenance of appropriate iron homeostasis in the brain is important, but the mechanisms involved in brain iron uptake are incompletely understood. Here, we have analyzed where messenger RNAs that encode iron transport proteins are expressed in the brain, using the Allen Brain atlas, and we conclude that several important iron transporters are highly expressed in the choroid plexus. Based on recent estimates of the surface area of the choroid plexus and on MRI imaging studies of manganese uptake in the brain, we propose that the choroid plexus may have a much greater role than has been previously appreciated in brain iron transport.

Effects of Intracerebroventricular Injection of Iron Dextran on the Iron Concentration and Divalent Metal Transporter 1 Expression in the Caudate Putamen and Substantia Nigra of Rats

The cellular localization of DMT1 and its functional characterization suggest that DMT1 may play an important role in the physiological brain iron transport. But the regulation of DMT1 expression by iron in the brain is still not clearly understood. In this study, both the contents of ferric and ferrous iron as well as DMT1 expression were evaluated in CPu and SN after ICV of 500 microg iron dextran/rat/day for 3 or 7 days. It was found that the iron levels in CPu and SN were not altered obviously until ICV for 7 days. Immunohistochemistry results indicated that the expression of DMT1 (-IRE) in CPu and SN was not altered significantly after 3 days of ICV. Whereas the expression of DMT1 (-IRE) decreased significantly after 7 days of ICV when ferrous iron was increased significantly. Contrary to that of DMT1 (-IRE) in the same regions, there were no significant alterations in DMT1 (+IRE) expression in CPu and SN in spite of the existence of the altered iron levels, compared with that of control groups. The results demonstrate that DMT1 (-IRE) expression was correlated probably with brain iron levels; especially, its regulation was correlated with ferrous iron (not ferric iron) in CPu and SN in adult rats, compared with those of saline-injected control rats. The effect of ferrous iron on the expression of DMT1 (-IRE) in the brain also suggests that it might play a major physiological role in brain iron uptake and transport, but further studies are needed to clarify these issues.

Ceruloplasmin expression and its role in iron transport in C6 cells

Ceruloplasmin (CP) is essential for brain iron homeostasis. However, little is known about the effect of iron on CP expression in the brain. Also, the role of CP in brain iron transport has not been well determined. In this study, we investigated the effects of iron on CP expression and the role of CP in iron transport in the C6 rat glioma cells. Our data showed that treatment of the cells with iron (cell iron overload) or iron chelators (cell iron deficiency) did not induce a significant change in the expression of CP mRNA. However, western blotting analysis demonstrated that cell iron overload induced a significant decrease in CP protein content in the cells and that treatment with iron chelators led to a significant increase in CP protein level in the cells. These findings suggest a translational regulation of CP expression by iron in the cells. We also examined the effects of CP on iron transport in the cells. We found that glycosylphosphatidylinositol-anchored CP did not have any impact on iron uptake by normal iron or iron-deficient cells nor on iron release from normal iron or iron-sufficient cells. However, low concentrations of soluble CP (2–8 μg/ml) increased iron uptake by iron-deficient C6 glioma cells, while the same concentrations of CP had no effect on iron uptake by normal iron cells and iron release from normal iron and iron-sufficient cells. The possible reason for the difference between our results in vitro and those obtained from in vivo studies was discussed.

Role of soluble ceruloplasmin in iron uptake by midbrain and hippocampus neurons

Ceruloplasmin (CP) is essential for brain iron homeostasis. However, its precise function in brain iron transport has not been definitely determined. In this study, we investigated the effects of soluble CP on iron influx and efflux in primary neuronal culture from the midbrain (the substantia nigra and striatum) and the hippocampus. Our data showed that low concentrations of CP (2, 4, 8 microg/ml) can promote iron influx into iron-deficient neurons, but not the neurons with normal iron status. The same concentrations of CP had no effect on iron efflux from iron-sufficient and normal-iron neurons. Contrary to our expectation, we did not find any regional difference in the effects of CP on iron influx as well as efflux in neurons. The changes in quenching (iron influx) and also dequenching (iron efflux) of intracellular fluorescence, induced by the addition of CP with iron, in the midbrain neurons were no different from those in the hippocampus neurons. The data showed that soluble CP has a role in iron uptake by iron-deficient brain neurons under our experimental conditions. The physiological significance of the results forms the focus for future work.

Regulation of the profile of iron-management proteins in brain microvasculature.

The distribution of brain iron is heterogeneous, but the mechanism by which these regional differences are achieved and maintained is unknown. In this study, the authors test two hypotheses related to brain iron transport. The first is that there is regional variability in the profile of proteins associated with iron transport and storage in the brain microvasculature. The second hypothesis is that the iron status of the brain will dictate the response of the protein profile in the microvasculature to changes in systemic iron status. The profile analysis consists of transferrin (iron transport), ferritin (iron storage), transferrin receptor (iron uptake), and divalent metal transporter 1 (release of iron from endosomes). An additional protein involved in cellular iron efflux, ferroportin, was not detected in brain microvasculature. The results show that there are significantly higher levels of these proteins in the microvasculature from each area of the brain compared to a whole brain homogenate, but no regional differences within the microvasculature. The levels of ferritin observed in the microvasculature indicate that the microvascular endothelial cells have significant iron storage capacity. There are no significant changes in the regional protein profiles in response to systemic iron manipulation when brain iron status was normal. In contrast, in Belgrade rats, whose brain is iron deficient, the expression of both divalent metal transporter 1 and transferrin receptor was increased compared with control in almost all brain regions examined, but not transferrin or ferritin. These findings indicate that regional brain iron heterogeneity is not maintained by differences in microvascular iron-management protein levels. The results also indicate that brain iron status dictates the response of the microvascular protein profile to systemic iron manipulation.

The significance of the mutated divalent metal transporter (DMT1) on iron transport into the Belgrade rat brain.

Brain iron transport and distributional pattern of divalent metal transporter I (DMT1) were studied in homozygous Belgrade rats (b/b) which suffer from a mutation in the DMT1 gene. In adult rats, brain uptake of transferrin-bound iron injected intravenously (i.v.) was significantly lower compared with that in heterozygous Belgrade (+/b) and Wistar rats, whereas transferrin uptake was identical. The difference in iron uptake was not apparent until 30 min after injection. The brain iron concentration was lower, and neuronal transferrin receptor-immunoreactivity higher, in adult b/b rats, thus confirming their iron-deficient stage. Antibodies targeting different sites on the DMT1 molecule consistently detected DMT1 in neurones and choroid plexus at the same level irrespective of strain, but failed to detect DMT1 in brain capillary endothelial cells (BCECs), or macro- or microglial cells. The absence of DMT1 in BCECs was confirmed in immunoblots of purified BCECs. DMT1 was virtually undetectable in neurones of rats aged 18 post-natal days irrespective of strain. Neuronal expression of transferrin receptors and DMT1 in adult rats implies that neurones at this age acquire iron by receptor-mediated endocytosis of transferrin followed by iron transport out of endosomes mediated by DMT1. The existence of the mutated DMT1 molecule in neurones suggests that the low cerebral iron uptake in b/b rats derives from a reduced neuronal uptake rather than an impaired iron transport through the blood-brain barrier.

Iron transport proteins in the diseased brain

The significance of delivery of normal amounts of iron to the brain in a timely manner is manifested in the number of devastating neurological sequelae associated with too little or too much iron. Brain iron deficiency during development results in chronic neurological impairment [1]. More recently, an adult-onset syndrome associated with brain iron deficiency known as “restless-legs-syndrome” has been identified [2]. In addition to insufficient amounts of iron, excess brain iron is a consistent observation in common neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease, and less common disorders such as Huntington's chorea and Hallervorden–Spatz syndrome [3]. Thus, elucidation of the mechanism by which the brain acquires iron and, as importantly, the regulation of that mechanism will likely provide insights and opportunities to manipulate the brain iron acquisition system therapeutically. Even with the limited understanding of brain iron uptake, attempts have been made to exploit the system to deliver trophic factors across the blood–brain barrier (BBB) (4–6). The clinical relevance of these advances in knowledge will benefit individuals across the entire human life span. This discussion will focus on the known mechanisms of brain iron transport and how it is manipulated in a region-specific manner. A number of in vivo studies have been conducted to demonstrate that iron uptake into the brain increases in animals fed with an iron deficient diet [4,5]. Although the mechanism of this adaptation is unknown, the use of sham proteins demonstrated that the increase uptake in response to iron deficiency are selective to iron uptake and not a generalized response.

Experimental stress‐induced changes in trace element levels of various tissues in rats

AbstractIn this study, we investigated the effects of acute and chronic immobilization stress on the Zn, Cu, and Fe levels of the temporal lobe, brain stem, spleen, and liver tissues in rats. The animals in the acute stress group were put in the cages, one time only for 120 min. For the chronic stress groups (2h and 4h), the rats were kept in the cages daily for 2 and 4 h, respectively, for 5 consecutive days. Controls and immobilized rats were decapitated, and then tissue samples were taken. Zn, Cu, and Fe levels in the temporal lobe, brain stem, spleen, and liver were measured by flame atomic absorption spectrophotometer. Our results showed that acute immobilization stress causes endogenous Zn and Cu release from the brain tissues. In the 2h chronic stress group, Fe levels markedly increase in the temporal lobe and brain stem whereas they decrease in the spleen and liver. In the 4h chronic stress group, Fe levels increase in the temporal lobe and brain stem while Zn and Cu levels increase in the spleen and liver. In the acute and chronic immobilization stress groups, mobilization of Zn and Cu can be related to the induction of metallothionein (MT) in the liver and spleen but not in the brain. On the other hand, excess Fe in the temporal lobe and brain stem causes us to believe think that the brain iron transport proteins may be involved, and enhanced, by immobilization stress. J. Trace Elem. Exp. Med. 16:55–60, 2003. © 2003 Wiley‐Liss, Inc.

Distribution of divalent metal transporter 1 and metal transport protein 1 in the normal and Belgrade rat.

Iron accumulation in the brain occurs in a number of neurodegenerative diseases. Two new iron transport proteins have been identified that may help elucidate the mechanism of abnormal iron accumulation. The Divalent Metal Transporter 1 (DMT1), is responsible for iron uptake from the gut and transport from endosomes. The Metal Transport Protein 1 (MTP1) promotes iron export. In this study we determined the cellular and regional expression of these two transporters in the brains of normal adult and Belgrade rats. Belgrade rats have a defect in DMT1 that is associated with lower levels of iron in the brain. In the normal rat, DMT1 expression is highest in neurons in the striatum, cerebellum, thalamus, ependymal cells lining the third ventricle, and vascular cells throughout the brain. The staining in the ependymal cells and endothelial cells suggests that DMT1 has an important role in iron transport into the brain. In Belgrade rats, there is generalized decrease in immunodetectable DMT1 compared to normal rats except in the ependymal cells. This decrease in immunoreactivity, however, was absent on immunoblots. The immunoblot analysis indicates that this protein did not upregulate to compensate for the chronic defect in iron transport. MTP1 staining is found in most brain regions. MTP1 expression in the brain is robust in pyramidal neurons of the cerebral cortex but is not detected in the vascular endothelial cells and ependymal cells. MTP1 staining in Belgrade rats was decreased compared to normal, but similar to DMT1 this decrease was not corroborated by immunoblotting. These results indicate that DMT1 and MTP1 are involved in brain iron transport and this involvement is regionally and cellularly specific.

Homozygosity mapping of Hallervorden-Spatz syndrome to chromosome 20p12.3-p13.

Hallervorden-Spatz syndrome (HSS) (OMIM #234200) is a rare, autosomal recessive neurode-generative disorder with brain iron accumulation as a prominent finding. Clinical features include extrapyramidal dysfunction, onset in childhood, and a relentlessly progressive course. Histologic study reveals massive iron deposits in the basal ganglia. Systemic and cerebrospinal fluid iron levels are normal, as are plasma levels of ferritin, transferrin and ceruloplasmin. Conversely, in disorders of systemic iron overload, such as haemochromatosis, brain iron is not increased, which suggests that fundamental differences exist between brain and systemic iron metabolism and transport. In normal brain, non-haem iron accumulates regionally and is highest in basal ganglia. Pathologic brain iron accumulation is seen in common disorders, including Parkinson's disease, Alzheimer's disease and Huntington disease. In order to gain insight into normal and abnormal brain iron transport, metabolism and function, our approach was to map the gene for HSS. A primary genome scan was performed using samples from a large, consanguineous family (HS1) (see Fig. 1). While this family was immensely powerful for mapping, the region demonstrating homozygosity in all affected members spans only 4 cM, requiring very close markers in order to detect linkage. The HSS gene maps to an interval flanked by D20S906 and D20S116 on chromosome 20p12.3-p13. Linkage was confirmed in nine additional families of diverse ethnic backgrounds.