Brain iron acquisition depends on age and sex in iron-deficient mice.

Adequate and timely delivery of iron is essential for brain development. The uptake of transferrin-bound (Tf) iron into the brain peaks at the time of myelination, whereas the recently discovered H-ferritin (FTH1) transport of iron into the brain continues to increase beyond the peak in myelination. Here, we interrogate the impact of dietary iron deficiency (ID) on the uptake of FTH1- and Tf-bound iron. In the present study, we used C57BL/6J male and female mice at a developing (post-natal day (PND) 15) and adult age (PND 85). In developing mice, ID results in increased iron delivery from both FTH1 and Tf for both males and females. The amount of iron uptake from FTH1 was higher than the Tf and this difference between the iron delivery was much greater in females. In contrast, in the adult model, ID was associated with increased brain iron uptake by both FTH1 and Tf but only in the males. There was no increased uptake from either protein in the females. Moreover, transferrin receptor expression on the microvasculature as well as whole brain iron, and H and L ferritin levels revealed the male brains became iron deficient but not the female brains. Last, under normal dietary conditions, 55 Fe uptake was higher in the developing group from both delivery proteins than in the adult group. These results indicate that there are differences in iron acquisition between the developing and adult brain for FTH1 and Tf during nutritional ID and demonstrate a level of regulation of brain iron uptake that is age and sex-dependent.

Amyloid-ß 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-ß disrupt iron transport signals secreted by astrocytes to stimulate iron transport from endothelial cells. We demonstrate that conditioned media from astrocytes treated with amyloid-ß 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-ß 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.

Amyloid-ß 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 in 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-ß disrupt iron transport signals secreted by astrocytes to stimulate iron transport from endothelial cells. We demonstrate that conditioned media from astrocytes treated with amyloid-ß stimulates iron transport from endothelial cells and induces changes in iron transport pathway protein levels. 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-ß 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 to detrimental ends. 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. Significance Statement: Excessive brain iron accumulation is hallmark pathology of Alzheimer's disease that occurs early in the disease staging and before widespread proteinopathy deposition. This overabundance of brain iron has been implicated to contribute to disease progression, thus understandingthe mechanism of early iron accumulation has significant therapeutic potential to slow to halt disease progression. Here, we show that in response to low levels of amyloid-ß exposure, astrocytes increase their mitochondrial activity and iron uptake, resulting in iron deficient conditions. Elevated levels of apo (iron free)-transferrin stimulate iron release from endothelial cells. These data are the first to propose a mechanism for the initiation of iron accumulation and the misappropriation of iron transport signaling leading to dysfunctional brain iron homeostasis and resultant disease pathology.

Apo- and holo-transferrin differentially interact with hephaestin and ferroportin in a novel mechanism of cellular iron release regulation.

BACKGROUND: Apo- (iron free) and holo- (iron bound) transferrin (Tf) participate in precise regulation of brain iron uptake at endothelial cells of the blood-brain barrier. Apo-Tf indicates an iron-deficient environment and stimulates iron release, while holo-Tf indicates an iron sufficient environment and suppresses additional iron release. Free iron is exported through ferroportin, with hephaestin as an aid to the process. Until now, the molecular mechanisms of apo- and holo-Tf influence on iron release was largely unknown. METHODS: Here we use a variety of cell culture techniques, including co-immunoprecipitation and proximity ligation assay, in iPSC-derived endothelial cells and HEK 293 cells to investigate the mechanism by which apo- and holo-Tf influence cellular iron release. Given the established role of hepcidin in regulating cellular iron release, we further explored the relationship of hepcidin to transferrin in this model. RESULTS: We demonstrate that holo-Tf induces the internalization of ferroportin through the established ferroportin degradation pathway. Furthermore, holo-Tf directly interacts with ferroportin, whereas apo-Tf directly interacts with hephaestin. Only pathophysiological levels of hepcidin disrupt the interaction between holo-Tf and ferroportin, but similar hepcidin levels are unable to interfere with the interaction between apo-Tf and hephaestin. The disruption of the holo-Tf and ferroportin interaction by hepcidin is due to hepcidin's ability to more rapidly internalize ferroportin compared to holo-Tf. CONCLUSIONS: These novel findings provide a molecular mechanism for apo- and holo-Tf regulation of iron release from endothelial cells. They further demonstrate how hepcidin impacts these protein-protein interactions, and offer a model for how holo-Tf and hepcidin cooperate to suppress iron release. These results expand on our previous reports on mechanisms mediating regulation of brain iron uptake to provide a more thorough understanding of the regulatory mechanisms mediating cellular iron release in general.

Brain iron acquisition: An overview of homeostatic regulation and disease dysregulation.

Brain iron homeostasis is crucial for neurological health, with pathological fluctuations in brain iron levels associated with a variety of neurological disorders. Low levels are connected to cognitive impairment and restless legs syndrome, while high levels are connected to Alzheimer's disease, Parkinson's disease, and other neurodegenerative diseases. Given the detrimental effects unrestricted iron can have, regulated entry into the brain via transferrin and H-ferritin is critical. Endothelial cells of the blood-brain barrier are the site of iron transport regulation. The movement of iron through endothelial cells into the brain can be divided into three distinct processes: uptake, transcytosis, and release. Each process possesses external and internal influences on the regulation at each stage. This review discusses the mechanisms of iron uptake, transcytosis, and release at the blood-brain barrier, as well as the elements that contribute to regulation. Additionally, we explore the dysregulation of brain iron in Alzheimer's disease, Parkinson's disease, and restless legs syndrome.

Exosomes are involved in iron transport from human blood-brain barrier endothelial cells and are modified by endothelial cell iron status.

Iron is essential for normal brain development and function. Hence, understanding the mechanisms of iron efflux at the blood-brain barrier and their regulation are critical for the establishment of brain iron homeostasis. Here, we have investigated the role of exosomes in mediating the transfer of H-ferritin (FTH1)- or transferrin (Tf)-bound iron across the blood-brain barrier endothelial cells (BBBECs). Our study used ECs derived from human-induced pluripotent stem cells that are grown in bicameral chambers. When cells were exposed to 55Fe-Tf or 55Fe-FTH1, the 55Fe activity in the exosome fraction in the basal chamber was significantly higher compared to the supernatant fraction. Furthermore, we determined that the release of endogenous Tf, FTH1, and exosome number is regulated by the iron concentration of the endothelial cells. Moreover, the release of exogenously added Tf or FTH1 to the basal side via exosomes was significantly higher when ECs were iron loaded compared to when they were iron deficient. The release of exosomes containing iron bound to Tf or FTH1 was independent of hepcidin regulation, indicating this mechanism by-passes a major iron regulatory pathway. A potent inhibitor of exosome formation, GW4869, reduced exosomes released from the ECs and also decreased the Tf- and FTH1-bound iron within the exosomes. Collectively, these results indicate that iron transport across the blood-brain barrier is mediated via the exosome pathway and is modified by the iron status of the ECs, providing evidence for a novel alternate mechanism of iron transport into the brain.

Apo- and holo- transferrin differentially interact with ferroportin and hephaestin to regulate iron release at the blood-brain barrier.

Background : Apo- (iron free) and holo- (iron bound) transferrin (Tf) participate in precise regulation of brain iron uptake at endothelial cells of the blood-brain barrier. Apo-Tf indicates an iron deficient environment and stimulates iron release, while holo-Tf indicates an iron sufficient environment and suppresses additional iron release. Free iron is exported through ferroportin, with hephaestin as an aid to the process. Until now, the molecular mechanism of apo- and holo-Tf's influence on iron release was largely unknown. Methods : Here we use a variety of cell culture techniques, including co-immunoprecipitation and proximity ligation assay, in iPSC-derived endothelial cells and HEK 293 cells to investigate the mechanism of apo- and holo-Tf's influence over iron release. We placed our findings in physiological context by further deciphering how hepcidin played a role in this mechanism as well. Results : We demonstrate that holo-Tf induces the internalization of ferroportin through the established ferroportin degradation pathway. Furthermore, holo-Tf directly binds to ferroportin, whereas apo-Tf directly binds to hephaestin. Only pathological levels of hepcidin disrupt the interaction between holo-Tf and ferroportin, and no amount of hepcidin disrupts the interaction between apo-Tf and hephaestin. The disruption of the holo-Tf and ferroportin interaction by hepcidin is due to hepcidin's ability to rapidly internalize ferroportin compared to holo-Tf. Conclusions : These novel findings provide a molecular mechanism for apo- and holo-Tf regulation of iron release from endothelial cells. They further demonstrate how hepcidin impacts these protein-protein interactions, and offer a model for how holo-Tf and hepcidin corporate to suppress iron release. We have established a more thorough understanding of the mechanisms behind iron release regulation with great clinical impact for a variety of neurological conditions in which iron release is dysregulated.

Apo- and holo- transferrin differentially interact with ferroportin and hephaestin to regulate iron release at the blood-brain barrier.

Background: Apo- (iron free) and holo- (iron bound) transferrin (Tf) participate in precise regulation of brain iron uptake at endothelial cells of the blood-brain barrier. Apo-Tf indicates an iron deficient environment and stimulates iron release, while holo-Tf indicates an iron sufficient environment and suppresses additional iron release. Free iron is exported through ferroportin, with hephaestin as an aid to the process. Until now, the molecular mechanism of apo- and holo-Tf's influence on iron release was largely unknown. Methods: Here we use a variety of cell culture techniques, including co-immunoprecipitation and proximity ligation assay, in iPSC-derived endothelial cells and HEK 293 cells to investigate the mechanism of apo- and holo-Tf's influence over iron release. We placed our findings in physiological context by further deciphering how hepcidin played a role in this mechanism as well. Results: We demonstrate that holo-Tf induces the internalization of ferroportin through the established ferroportin degradation pathway. Furthermore, holo-Tf directly binds to ferroportin, whereas apo-Tf directly binds to hephaestin. Only pathological levels of hepcidin disrupt the interaction between holo-Tf and ferroportin, and no amount of hepcidin disrupts the interaction between apo-Tf and hephaestin. The disruption of the holo-Tf and ferroportin interaction by hepcidin is due to hepcidin's ability to rapidly internalize ferroportin compared to holo-Tf. Conclusions: These novel findings provide a molecular mechanism for apo- and holo-Tf regulation of iron release from endothelial cells. They further demonstrate how hepcidin impacts these protein-protein interactions, and offer a model for how holo-Tf and hepcidin corporate to suppress iron release. We have established a more thorough understanding of the mechanisms behind iron release regulation with great clinical impact for a variety of neurological conditions in which iron release is dysregulated.

The effect of chronic exposure to metformin in a new type-2 diabetic NONcNZO10/LtJ mouse model of stroke.

BACKGROUND: Diabetes is an independent risk factor of stroke and previous studies have confirmed that diabetic patients and animals experience poorer clinical outcomes following stroke. In this study, we aim to determine the effect of chronic exposure of the first-line antidiabetic agent, metformin, to restore euglycemia and to impact brain cell death following stroke in a new type-2 diabetes, NONcNZO10/LtJ (RCS-10) mouse model of stroke. METHODS: Male RCS-10 mice received a moderate (11%) fat diet post-weaning, at 4 weeks of age, and became diabetic by 12-14 weeks, thus resembling human maturity-onset diabetes. The mice received either metformin or vehicle for 4 weeks before undergoing a hypoxic/ischemic (HI) insult. Blood samples were collected pre-, post-treatment, and post HI for glucose and lipid measurements, and brains were analyzed for infarct size, glial activation, neuronal cell death, and metformin-mediated adenosine monophosphate-activated protein kinase (AMPK) signaling at 48 h post HI. RESULTS: Pretreatment with metformin maintained euglycemia for 4 weeks but did not change body weight or lipid profile. Metformin treatment significantly enhanced the microglial Bfl-1 mRNA expression and showed a non-significant increase in GFAP mRNA, however, GFAP protein levels were reduced. Metformin treatment slightly increased neuronal NeuN and MAP-2 protein levels and significantly reduced overall mortality post HI but did not elicit any significant change in infarct size. CONCLUSION: The study suggests that the prolonged effect of metformin-induced euglycemia promoted the microglial activation, reduced neuronal cell death, and improved the overall survival following stroke, without any change in infarct size.

Regulation of brain iron uptake by apo- and holo-transferrin is dependent on sex and delivery protein.

BACKGROUND: The brain requires iron for a number of processes, including energy production. Inadequate or excessive amounts of iron can be detrimental and lead to a number of neurological disorders. As such, regulation of brain iron uptake is required for proper functioning. Understanding both the movement of iron into the brain and how this process is regulated is crucial to both address dysfunctions with brain iron uptake in disease and successfully use the transferrin receptor uptake system for drug delivery. METHODS: Using in vivo steady state infusions of apo- and holo-transferrin into the lateral ventricle, we demonstrate the regulatory effects of brain apo- and holo-transferrin ratios on the delivery of radioactive 55Fe bound to transferrin or H-ferritin in male and female mice. In discovering sex differences in the response to apo- and holo-transferrin infusions, ovariectomies were performed on female mice to interrogate the influence of circulating estrogen on regulation of iron uptake. RESULTS: Our model reveals that apo- and holo-transferrin significantly regulate iron uptake into the microvasculature and subsequent release into the brain parenchyma and their ability to regulate iron uptake is significantly influenced by both sex and type of iron delivery protein. Furthermore, we show that cells of the microvasculature act as reservoirs of iron and release the iron in response to cues from the interstitial fluid of the brain. CONCLUSIONS: These findings extend our previous work to demonstrate that the regulation of brain iron uptake is influenced by both the mode in which iron is delivered and sex. These findings further emphasize the role of the microvasculature in regulating brain iron uptake and the importance of cues regarding iron status in the extracellular fluid.

The role of neutrophils in mediating stroke injury in the diabetic db/db mouse brain following hypoxia-ischemia.

Diabetic mice exhibit increased mortality and morbidity following stroke. Recent studies from our laboratory have indicated that increased morbidity in diabetic db/db mice relative to their non-diabetic db/+ littermates is associated with increased levels of MMP-9 protease activity, increased blood-brain barrier (BBB) permeability, and greater neutrophil infiltration following hypoxic/ischemic (H/I) insult. Neutrophils are a major source of proteases and reactive oxygen species and studies have reported neutrophil depletion/inhibition is protective in certain models of experimental stroke. The objective of the current study is to determine the role of neutrophils in the increased morbidity seen in db/db mice following acute ischemic stroke. In this study, we found a significant increase in circulating neutrophils in the db/db mice at 4 h post H/I, which bound to endothelial cells in the ipsilateral hemisphere and infiltrated into brain tissue by 24 h of recovery. Depletion of circulating neutrophils resulted in reduced neutrophil concentrations in blood and in the ipsilateral hemispheres of the brain of both db/+ and db/db mice and decreased the levels of MMP-9 within the infarcted area. This resulted in smaller infarct size in the db/db mice compared to non-treated controls but did not affect stroke outcome in db/+ mice. While there was a significant correlation between neutrophil number and the levels of MMP-9 in the ipsilateral hemisphere of control and diabetic mice, surprisingly, neutrophil depletion had no effect on BBB permeability in either group. Thus, the current study suggests that neutrophil depletion reduces MMP-9 protease levels and improves stroke outcome in db/db mice but not in their db/+ counterparts.

Evidence for communication of peripheral iron status to cerebrospinal fluid: clinical implications for therapeutic strategy.

BACKGROUND: Iron is crucial for proper functioning of all organs including the brain. Deficiencies and excess of iron are common and contribute to substantial morbidity and mortality. Whereas iron's involvement in erythropoiesis drives clinical practice, the guidelines informing interventional strategies for iron repletion in neurological disorders are poorly defined. The objective of this study was to determine if peripheral iron status is communicated to the brain. METHODS: We used a bi-chamber cell culture model of the blood-brain-barrier to determine transcytosis of iron delivered by transferrin as a metric of iron transport. In the apical chamber (representative of the blood) we placed transferrin complexed with iron59 and in the basal chamber (representative of the brain) we placed human cerebrospinal fluid. Cerebrospinal fluid (CSF) samples (N = 24) were collected via lumbar puncture. The integrity of the tight junctions were monitored throughout the experiments using RITC-Dextran. RESULTS: We demonstrate that iron transport correlates positively with plasma hemoglobin concentrations but not serum ferritin levels. CONCLUSIONS: The clinical ramifications of these findings are several- fold. They suggest that erythropoietic demands for iron take precedence over brain requirements, and that the metric traditionally considered to be the most specific test reflecting total body iron stores and relied upon to inform treatment decisions-i.e., serum ferritin-may not be the preferred peripheral indicator when attempting to promote brain iron uptake. The future direction of this line of investigation is to identify the factor(s) in the CSF that influence iron transport at the level of the BBB.

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.

Endothelial cells are critical regulators of iron transport in a model of the human blood-brain barrier.

Iron delivery to the brain is essential for multiple neurological processes such as myelination, neurotransmitter synthesis, and energy production. Loss of brain iron homeostasis is a significant factor in multiple neurological disorders. Understanding the mechanism by which the transport of iron across the blood-brain barrier (BBB) is regulated is crucial to address the impact of iron deficiency on brain development and excessive accumulation of iron in neurodegenerative diseases. Using induced pluripotent stem cell (iPSC)-derived brain endothelial cells (huECs) as a human BBB model, we demonstrate the ability of transferrin, hepcidin, and DMT1 to impact iron transport and release. Our model reveals a new function for H-ferritin to transport iron across the BBB by binding to the T-cell immunoglobulin and mucin receptor 1. We show that huECs secrete both transferrin and H-ferritin, which can serve as iron sources for the brain. Based on our data, brain iron status can exert control of iron transport across the endothelial cells that constitute the BBB. These data address a number of pertinent questions such as how brain iron uptake is regulated at the regional level, the source of iron delivery to the brain, and the clinical strategies for attempting to treat brain iron deficiency.

Pharmaceutical iron formulations do not cross a model of the human blood-brain barrier.

Whether iron formulations used therapeutically for a variety of conditions involving iron deficiency can deliver iron to the brain is a significant clinical question given the impact that iron loading has on the brain in neurodegenerative diseases. In this study, we examine the ability of 5 pharmaceutical iron formulations that are given intravenously for treatment of iron deficiency to cross an in vitro model of the blood-brain barrier. The model uses human brain endothelial cells derived from induced pluripotent stem cells. We report that, compared to the natural iron delivery proteins, transferrin and H-ferritin, the pharmaceutical iron formulations neither cross the blood-brain barrier model nor significantly load the endothelial cells with iron. Furthermore, we report that mimicking brain iron sufficiency or deficiency by exposing the endothelial cells to apo- or holo-transferrin does not alter the amount of iron compound transported by or loaded into the cells. Coupled with previous studies, we propose that pharmaceutical iron formulations must first be processed in macrophages to make iron bioavailable. The results of this study have significant clinical and mechanistic implications for the use of therapeutic iron formulations.

The role of HFE genotype in macrophage phenotype.

BACKGROUND: Iron regulation is essential for cellular energy production. Loss of cellular iron homeostasis has critical implications for both normal function and disease progression. The H63D variant of the HFE gene is the most common gene variant in Caucasians. The resulting mutant protein alters cellular iron homeostasis and is associated with a number of neurological diseases and cancer. In the brain, microglial and infiltrating macrophages are critical to maintaining iron homeostasis and modulating inflammation associated with the pathogenic process in multiple diseases. This study addresses whether HFE genotype affects macrophage function and the implications of these findings for disease processes. METHODS: Bone marrow macrophages were isolated from wildtype and H67D HFE knock-in mice. The H67D gene variant in mice is the human equivalent of the H63D variant. Upon differentiation, the macrophages were used to analyze iron regulatory proteins, cellular iron release, migration, phagocytosis, and cytokine expression. RESULTS: The results of this study demonstrate that the H67D HFE genotype significantly impacts a number of critical macrophage functions. Specifically, fundamental activities such as proliferation in response to iron exposure, L-ferritin expression in response to iron loading, secretion of BMP6 and cytokines, and migration and phagocytic activity were all found to be impacted by genotype. Furthermore, we demonstrated that exposure to apo-Tf (iron-poor transferrin) can increase the release of iron from macrophages. In normal conditions, 70% of circulating transferrin is unsaturated. Therefore, the ability of apo-Tf to induce iron release could be a major regulatory mechanism for iron release from macrophages. CONCLUSIONS: These studies demonstrate that the HFE genotype impacts fundamental components of macrophage phenotype that could alter their role in degenerative and reparative processes in neurodegenerative disorders.

A role for sex and a common HFE gene variant in brain iron uptake.

HFE (high iron) is an essential protein for regulating iron transport into cells. Mutations of the HFE gene result in loss of this regulation causing accumulation of iron within the cell. The mutated protein has been found increasingly in numerous neurodegenerative disorders in which increased levels of iron in the brain are reported. Additionally, evidence that these mutations are associated with elevated brain iron challenges the paradigm that the brain is protected by the blood-brain barrier. While much has been studied regarding the role of HFE in cellular iron uptake, it has remained unclear what role the protein plays in the transport of iron into the brain. We investigated regulation of iron transport into the brain using a mouse model with a mutation in the HFE gene. We demonstrated that the rate of radiolabeled iron (59Fe) uptake was similar between the two genotypes despite higher brain iron concentrations in the mutant. However, there were significant differences in iron uptake between males and females regardless of genotype. These data indicate that brain iron status is consistently maintained and tightly regulated at the level of the blood-brain barrier.

Regulatory mechanisms for iron transport across the blood-brain barrier.

Many critical metabolic functions in the brain require adequate and timely delivery of iron. However, most studies when considering brain iron uptake have ignored the iron requirements of the endothelial cells that form the blood-brain barrier (BBB). Moreover, current models of BBB iron transport do not address regional regulation of brain iron uptake or how neurons, when adapting to metabolic demands, can acquire more iron. In this study, we demonstrate that both iron-poor transferrin (apo-Tf) and the iron chelator, deferoxamine, stimulate release of iron from iron-loaded endothelial cells in an in vitro BBB model. The role of the endosomal divalent metal transporter 1 (DMT1) in BBB iron acquisition and transport has been questioned. Here, we show that inhibition of DMT1 alters the transport of iron and Tf across the endothelial cells. These data support an endosome-mediated model of Tf-bound iron uptake into the brain and identifies mechanisms for local regional regulation of brain iron uptake. Moreover, our data provide an explanation for the disparity in the ratio of Tf to iron transport into the brain that has confounded the field.

Climate challenges, vulnerabilities, and food security.

This paper identifies rare climate challenges in the long-term history of seven areas, three in the subpolar North Atlantic Islands and four in the arid-to-semiarid deserts of the US Southwest. For each case, the vulnerability to food shortage before the climate challenge is quantified based on eight variables encompassing both environmental and social domains. These data are used to evaluate the relationship between the "weight" of vulnerability before a climate challenge and the nature of social change and food security following a challenge. The outcome of this work is directly applicable to debates about disaster management policy.

A novel model for brain iron uptake: introducing the concept of regulation.

Neurologic disorders such as Alzheimer's, Parkinson's disease, and Restless Legs Syndrome involve a loss of brain iron homeostasis. Moreover, iron deficiency is the most prevalent nutritional concern worldwide with many associated cognitive and neural ramifications. Therefore, understanding the mechanisms by which iron enters the brain and how those processes are regulated addresses significant global health issues. The existing paradigm assumes that the endothelial cells (ECs) forming the blood-brain barrier (BBB) serve as a simple conduit for transport of transferrin-bound iron. This concept is a significant oversimplification, at minimum failing to account for the iron needs of the ECs. Using an in vivo model of brain iron deficiency, the Belgrade rat, we show the distribution of transferrin receptors in brain microvasculature is altered in luminal, intracellular, and abluminal membranes dependent on brain iron status. We used a cell culture model of the BBB to show the presence of factors that influence iron release in non-human primate cerebrospinal fluid and conditioned media from astrocytes; specifically apo-transferrin and hepcidin were found to increase and decrease iron release, respectively. These data have been integrated into an interactive model where BBB ECs are central in the regulation of cerebral iron metabolism.