The systemic iron-regulatory proteins hepcidin and ferroportin are reduced in the brain in Alzheimer’s disease
© Raha et al.; licensee BioMed Central Ltd. 2013
Received: 5 July 2013
Accepted: 25 August 2013
Published: 3 September 2013
The pathological features of the common neurodegenerative conditions, Alzheimer’s disease (AD), Parkinson’s disease and multiple sclerosis are all known to be associated with iron dysregulation in regions of the brain where the specific pathology is most highly expressed. Iron accumulates in cortical plaques and neurofibrillary tangles in AD where it participates in redox cycling and causes oxidative damage to neurons. To understand these abnormalities in the distribution of iron the expression of proteins that maintain systemic iron balance was investigated in human AD brains and in the APP-transgenic (APP-tg) mouse.
Protein levels of hepcidin, the iron-homeostatic peptide, and ferroportin, the iron exporter, were significantly reduced in hippocampal lysates from AD brains. By histochemistry, hepcidin and ferroportin were widely distributed in the normal human brain and co-localised in neurons and astrocytes suggesting a role in regulating iron release. In AD brains, hepcidin expression was reduced and restricted to the neuropil, blood vessels and damaged neurons. In the APP-tg mouse immunoreactivity for ferritin light-chain, the iron storage isoform, was initially distributed throughout the brain and as the disease progressed accumulated in the core of amyloid plaques. In human and mouse tissues, extensive AD pathology with amyloid plaques and severe vascular damage with loss of pericytes and endothelial disruption was seen. In AD brains, hepcidin and ferroportin were associated with haem-positive granular deposits in the region of damaged blood vessels.
Our results suggest that the reduction in ferroportin levels are likely associated with cerebral ischaemia, inflammation, the loss of neurons due to the well-characterised protein misfolding, senile plaque formation and possibly the ageing process itself. The reasons for the reduction in hepcidin levels are less clear but future investigation could examine circulating levels of the peptide in AD and a possible reduction in the passage of hepcidin across damaged vascular endothelium. Imbalance in the levels and distribution of ferritin light-chain further indicate a failure to utilize and release iron by damaged and degenerating neurons.
KeywordsAlzheimer’s disease Amyloid Brain iron homeostasis Ferritin Neurodegeneration Vascular endothelium damage
Alzheimer’s disease (AD) is characterized by cerebrovascular and neuronal dysfunction leading to a progressive decline in cognitive functions and the development of dementia [1–3]. Pathological hallmarks of AD include neurofibrillary tangles consisting of hyper-phosphorylated microtubule-associated protein tau [4, 5] and extracellular amyloid plaques derived from amyloid precursor protein (APP), a widely expressed trans-membrane metalloprotein essential for neuronal growth, survival, post-injury and repair . The main component of plaques is amyloid β (Aβ) peptide, (38–43 amino acids) generated by sequential cleavage of APP by β- and γ-secretase [7–10]. Recently, it has been shown that oligomeric Aβ species (the smallest of which are dimers) isolated from AD brains are the most synaptotoxic forms found in amyloid plaques . Another key protein involved in AD is apolipoprotein E (ApoE), a major genetic risk factor with 60-80% of affected individuals having at least one ApoE4 allele [12–15]. The majority of plasma ApoE is produced by hepatocytes, creating a hepatic pool that is important for lipid metabolism, while the second most common site of synthesis is the brain [16, 17]. ApoE is an Aβ chaperone, promoting its transport across the blood brain barrier (BBB), a process that is known to be impaired in AD [18–20].
Iron homeostasis in the mammalian brain is important, yet poorly understood. Excess iron in the form of ferritin has been described in many neurodegenerative disorders including AD and furthermore there is an apparent link between an age-associated increase in iron stores in the brain and the increasing incidence of AD with advancing age [21–25]. Aβ and ferritin have been shown to co-localise in the vascular amyloid deposits of plaques in post-mortem AD brains [26, 27] while iron accumulation in AD has been found to be a ready source of redox generated free radicals that promote neuronal cell death [28, 29].
An increased understanding of how iron homeostasis is maintained at the whole body and cellular levels has followed from the identification of a number of iron related proteins [30–34]. Hepcidin is a regulatory hormone playing a key role in whole body iron homeostasis . This liver-derived peptide regulates systemic iron homeostasis by controlling iron flux into the plasma from the duodenum as well as iron recycling macrophages through binding to its receptor, the iron exporter ferroportin [35–39]. Low serum hepcidin levels cause iron overload, as in haemochromatosis, while increased serum hepcidin expression plays an important role in the anaemia of inflammation by restricting intestinal iron absorption and macrophage iron release . Hepcidin expression is modulated by systemic stimuli such as iron stores, hypoxia, oxidative stress and inflammation [31, 41]. Ferroportin (FPN) is a transmembrane protein, (also known as SLC40A1, IREG1, MTP1) that exports iron from cells to plasma [42–44]. It is found on the surface of macrophages, Kupffer cells, hepatocytes, intestinal enterocytes and placental cells [32, 45]. It is also localized in the brain in most cell types including neuronal perikarya, axons, dendrites and synaptic vesicles [46–49]. Recently, Duce and colleagues have reported that APP may bind to ferroportin to facilitate neuronal iron export and that disturbances in these processes may be implicated in AD brain pathology .
The aim of our study was to explore a possible role for these recently described proteins in the abnormalities of iron metabolism previously described in the brain in AD. Hepcidin and ferroportin proteins levels were assessed by Western blotting in AD brains as compared to age-matched controls. Cellular expression was investigated by immunohistochemistry of brain sections and comparisons were made with the distribution of AD markers Aβ and ApoE.
In conjunction with exploring the progressive abnormalities in iron homeostasis in AD, we also investigated age-associated changes in the expression of iron-handling proteins in the well-characterised APP transgenic (APP-tg) mouse (APP/PS1-tg2576) model [51–54]. We describe extensive blood vessel damage in AD brain and a reduction in hepcidin and ferroportin levels. In the APP-tg mouse model although the overall levels of ferritin in the brain were not increased there may have been a re-distribution of iron as an increase in ferritin immunoreactivity was found in the core of plaques.
Reagents and antibodies
Synthetic hepcidin and ferroportin peptides were purchased from Abcam, Cambridge. 4′,6-Diamidino-2′ phenylindole dihydrochloride (DAPI), 3,3′-diaminobenzidine (DAB) and methanol were from Sigma. Precast 4-12% NuPAGE BisTris minigel and prestained protein molecular weight markers were from Life technologies. Polyvinylidene difluoride (PVDF) membrane, Western blotting detection reagents (ECL Plus chemiluminescence reagents and Hyperfilm) were from GE Healthcare (UK). Vectastain ABC kit was from Vector laboratories (USA). Protease and phosphatase inhibitors were from Roche laboratories (Germany). BCA™ protein assay kit was from Thermo Scientific (USA).
Sources of antibodies
List of the primary antibodies used in this study
Anti-β amyloid 1-16 amino acid (6E10)
1: 1000 for IHC
Covance cat number (SIG 39320)
1:2000 for WB
1: 200 for IHC
Abcam (Ab30760) (0.9 mg/ml)
1:100 for WB
1: 1000 for IHC
Abcam (ab93438) (1 mg/ml)
Anti- Ferroportin (SLC40A1)
1:200 for IHC
Abcam (ab85370) (1 mg/ml)
1:500 for IHC
Abcam (ab83508) (0.5 mg/ml)
1:500 for IHC
Abcam (ab83178) (0.4 mg/ml)
1:1000 for IHC
Sigma (G3893 Clone G-A-5)
1:200 for IHC
Abcam (ab48050) (1 mg/ml)
1:1000 for IHC
Millipore (clone 2G10, neuronal | 05-559)
1:10000 for WB
Sigma (clone AC-74, A5316 )
1:1000 for WB
Abcam (ab1907) (1 mg/ml)
1:500 for IHC
1:100 for WB
Abcam (ab85311) (0.1 mg/ml)
1:100 for IHC
Anti-Ferritin heavy chain
1:200 for IHC
Abcam (65080) (1 mg/ml)
Anti-Ferritin light chain
1:250 for IHC
Abcam (69090) (0.8 mg/ml)
1: 500 for IHC
Abcam (10060) (1 mg/ml)
1:1000 for WB
Anti-Myelin basic protein
1:500 for IHC
Abcam (ab62631) (1 mg/ml)
1:100 for IHC
Abcam (ab9498) (0.15 mg/ml)
1: 1000 for IHC
Abcam (ab32457) (1 mg/ml)
1: 1000 for IHC
Abcam (ab32570) (1 mg/ml)
Human brain tissues
Alzheimer’s disease cases and age matched control brain samples
Cause of death
AD, gradual decline
Cancer, Heart failure
Chronic obstructive airway disease
Bowel resection with complication
Chronic obstructive airway disease
APP-tg mice (Tg2576) over-expressing two human mutations (K670N and V717F) and one PS1 mutation (M146V), driven by Thy1 promoter were purchased from Jackson Laboratory, USA.
All animals were housed under standard conditions (12 h light-dark cycle, 20˚C ambient temperature) with free access to food and water. All procedures were performed under licence in accordance to the UK Animals (Scientific Procedures) Act 1986.
Neuropathological characterisation of these animals has previously been described in detail [51, 52]. By 4-6 months of age, extensive amyloid plaque deposition is seen in hippocampus and cortical regions. Six mice from each age group (2, 4, 6, and 10 months-old) were used for immunohistochemistry and as described subsequently, six for protein quantitation by Western blotting. C57/bl mice were (n=6, age-matched to APP-tg mice) were used as control samples.
In accordance with institutional guidelines for the humane treatment of animals, mice were terminally anaesthetised with carbon dioxide and culled. Unfixed tissues were carefully dissected from various brain regions from control and APP-tg mouse brains and snap frozen in dry ice until analysed by Western blotting. For histochemical analyses, animals were anesthetised with pento-barbitone and flash-perfused transcardially with 0.9% saline followed with 4% (v/v) paraformaldehyde (PFA) in 0.1 M phosphate buffer (pH 7.4). Brains were sectioned by microtome as described previously . Free-floating sections were prepared (25 μm coronal sections in 0.1 M PBS) through the entire olfactory bulb, hippocampus, cortex, mid brain and cerebellum. Sections were then stained by immunohistochemistry as described below.
SDS-PAGE and Western blotting
Levels of hepcidin, ferroportin, ApoE and aβ-42 (6E10) proteins were examined by Western blotting. Protein lysates were prepared from hippocampus and SVZ of control, and AD brain (n = 6) as described previously . 20 μg protein samples were separated using a 12-20% gradient SDS-PAGE gel and transferred to polyvinylidene difluoride (PVDF, pore sizes 0.45 μm or 0.2 μm for hepcidin) membrane (Bio-Rad). The membrane was incubated with the appropriate primary antibody in blocking buffer (5% non-fat milk in 1 × tris buffer saline (TBS) for 24 h at 4˚C. The membrane was then washed three times with 1 × TBS plus 0.1% Tween 20 (1× TBST) and incubated for 1 hour at room temperature with HRP-conjugated secondary antibodies (anti mouse IgG, 1:3000, DAKO) or anti-rabbit IgG (1:3000; DAKO) antibodies). Finally, membranes were incubated with ECL Plus chemiluminescence reagents and were exposed to an X-ray film (Pierce). Levels of proteins were estimated by densitometry analysis using the Gel Analyzer module in the Image J program (NIH). Anti-actin immunoblot was used to normalise protein loading. Similarly protein lysates were prepared from cortex, hippocampus and SVZ of control and APP-Tg mouse brains (n = 6) and followed the same methodology.
PFA fixed tissues were first quenched with 5% hydrogen peroxide and 20% methanol in 0.01M PBS for 30 min at room temperature followed by three rinses for 10 min in 0.01M phosphate buffer saline (PBS). Non-specific binding sites were blocked using blocking buffer (0.1 M PBS, 0.3% Triton-X100, and 10% normal goat serum for polyclonal antibodies or 10% normal horse serum for monoclonal antibodies) for 1 h at room temperature. Tissue sections were incubated overnight with the primary antibody diluted in blocking buffer (Table 1). Binding of the primary antibody was detected using a biotinylated secondary antibody followed by an avidin-biotin complex conjugated to peroxidase (Elite standard kit SK6100, Vector Laboratories) and DAB substrate (ABC substrate SK-4200, Vector Laboratories).
Bright field images were taken and quantified using Lucia imaging software and a Leica FW 4000 upright microscope equipped with SPOT digital camera. Fluorescence images were obtained using a Leica DM6000 wide field fluorescence microscope equipped with a Leica FX350 camera and 20× and 40× objectives. Images were taken through several z-sections and deconvolved using Leica software. A Leica TCS SP2 confocal laser-scanning microscope equipped with 40× and 63× objectives was used to acquire high-resolution images.
Image and statistics analysis
All experiments were performed in triplicate. Western blot and immunofluorescence images were quantified using ImageJ software (NIH). For Western blots, the gel analyser module was used. Selected bands were quantified based on their relative intensities, adjusted for background with fold-change in intensity subjected to statistical analysis as described below. Immunofluorescence was quantified using methods previously described . Values in the figures are expressed as mean ± SEM. To determine the statistical significance, values were analysed by Student’s t-test when comparing difference between case (AD or APP-Tg brain) and control. A probability value of p<0.05 was considered to be statistically significant.
Decreased hepcidin and ferroportin protein levels in AD brain
Analysis by Western blotting showed that Aβ42 levels were increased in diseased brain as compared to controls (P<0.005) while hepcidin and ferroportin proteins were both significantly decreased (P<0.005 and P<0.01 respectively, Figure 1a-b).
Hepcidin expression in neuritic processes and amyloid plaques
Hepcidin and ferroportin protein expression in control mouse brain
Hepcidin and ferroportin expression in APP-tg mice
Early on in the disease process (2-6 months), hepcidin was observed throughout the cortex, including the hippocampus, dentate gyrus, white matter tracts of the olfactory bulb and corpus callosum of APP-tg mice. With disease progression (10 months) hepcidin levels were reduced in the dentate gyrus (Figure 5f) as evaluated by densitometric analysis (IMAGE J, Figure 2k, p<0.0001). In sections from 10 months-old animals stained additionally for Aβ42, hepcidin was seen around the periphery of the maturing plaques (Figure 5d-f). In the early stages (2 months) extensive ferroportin staining was seen in the hippocampus (Figure 5g) and limited co-localisation with the neuronal marker βIIIT was observed in some cells (Figure 5h-i). Ferroportin levels were reduced as the disease progressed, with expression limited to axons (Figure 5l) while hepcidin expression was restricted to glial cells (astrocytes and oligodendrocytes) (Figure 5j-k).
Ferritin accumulation in the APP-tg mouse brain during disease progression
Finally, protein steady state levels were measured and compared by Western blotting using brain tissue (cortex including hippocampus and dentate gyrus) from 6 month-old APP-tg mice and age-matched controls (n = 6). Aβ42 levels were higher in APP-tg mice compared to control (p<0.0002) while hepcidin levels were decreased (p<0.005). However significant changes were not seen in levels of ferroportin, ApoE and FTL (Figure 7n-o).
Vascular endothelial damage in the APP-tg mouse
By 10 months APP-tg mice showed widespread endothelium damage, loss of cellular integrity and, in some sections, loss of endothelium in the wall of ventricles (Figure 8e-i). Sections were also stained with CD31 (endothelium marker) and a pericyte marker (PDGFβR1) to confirm the appropriate staining pattern in controls (Figure 8f-k) and extensive endothelial disruption in the APP-tg mouse model (Figure 8e-k). Both hepcidin and ApoE were found in APP-tg mice close to the blood vessels (Figure 8l).
Hepcidin co-staining with haem-rich deposit and red blood cells in AD brain
The pathological features of the common neurodegenerative conditions, Alzheimer’s disease, Parkinson’s disease and multiple sclerosis are all known to be associated with iron dysregulation in regions of the brain where the specific pathology is most highly expressed [21, 25]. The diversity of the pathological processes involved make it unlikely that there is a primary abnormality of brain iron metabolism common to these diseases, although this is the case with a group of rare genetic disorders characterized by neurodegeneration with brain iron accumulation (NBIA), . Even if the abnormalities in iron metabolism in common neurodegenerative disorders are secondary phenomena the finding that iron-related oxidative damage in Alzheimer’s disease is an early event in the disease process [29, 62] suggests that the control of iron levels in the brain remains a worthwhile therapeutic target .
In the present study the expression of proteins that play a central role in maintaining systemic iron homeostasis, hepcidin and its receptor, ferroportin, was investigated in human AD brains and in the APP transgenic mouse model to further characterize abnormalities of iron metabolism. Hepcidin and ferroportin protein were found to be widely distributed in normal human and mouse brain but levels were decreased significantly in AD brains and in the later stages of the mouse model. The expression of ferroportin protein has been reported to be decreased by ischaemia [64, 65] and inflammation in the rat cortex [66, 67] and in primary cultures of rat brain cells [68, 69]. The down-regulation of ferroportin by inflammatory stimuli in cells derived from the brain mirrors the findings in multiple cell types in systemic iron metabolism [70–72]. The primary pathology of AD, that of protein misfolding [5, 10], is accompanied by other pathological processes, notably vascular damage with associated ischaemic changes [73, 74] and inflammation [75, 76] and we believe that the reduction in ferroportin expression found in the present study is likely to be a secondary phenomenon caused by these factors that clearly contribute to AD pathogenesis [77, 78]. Ferroportin is also down-regulated when bound by hepcidin at the cell surface, an event that leads to the internalization and degradation of the iron carrier . This was observed in rat brain when the expression of ferroportin was reduced following the intra-ventricular administration of hepcidin [79, 80] or when hepcidin was added directly to primary cultures of neurons, astrocytes and microglia . We found that hepcidin levels were reduced in human and mouse brains exhibiting severe AD pathology but early in the course of the disease, as shown in the mouse model, hepcidin levels did not differ significantly from controls and the interaction with ferroportin as seen in cortical neurons by immunohistochemical staining could contribute to the decline in levels of the iron carrier. Inflammation in AD  could be a further reason for the increase in hepcidin levels as in the systemic environment hepcidin synthesis by hepatocytes is transcriptionally regulated by IL-6 through the STAT-3 signalling pathway . Interestingly in the dentate gyrus of the APP mouse and in AD brains we found that hepcidin was distributed around the periphery of amyloid plaques and in surviving neurons, in a similar distribution to that of IL-6 around plaques and in large cortical neurons reported previously in AD .
The finding that hepcidin and ferroportin were co-localised in cortical neurons in control brains is consistent with a role in for these protein in regulating neuronal iron release . However, neither the constitutive loss of hepcidin through gene mutations in either human  or mouse models of haemochromatosis [84, 85] or the targeted loss of ferroportin in the brain  appear to cause cerebral or cerebellar dysfunction and a role for hepcidin and ferroportin in the brain is currently undefined. Our finding that hepcidin protein was widely distributed in normal human and mouse brain is consistent with previous reports [64, 86] and raises the question of the origin of this protein given that hepcidin mRNA was not consistently detected by in situ hybridization in normal mouse brain . Hepcidin is a gene-encoded antimicrobial peptide structurally related to members of the defensin and protegrin families , most of which are cationic, a property that facilitates adsorption and insertion into anionic bacterial cell walls . Cationic peptides also cross mammalian cell membranes  and the blood brain barrier [89–91] and it is possible that hepcidin may cross the vascular endothelium to enter the brain interstitium.
Direct evidence for iron mishandling in AD brain comes from the histochemical demonstration of non-haem iron deposits in senile plaques [21, 92, 93] and Aβ plaques in APP mice  and iron levels were also found to be increased in neurofibrillary tangles and plaques using laser microprobe mass analysis  and particle-induced X-ray emission . It is not clear whether this represents increased deposition of iron and other transition metals  in the region of plaques or a more general increase as iron levels have not been found to be consistently increased in AD brains [98–100] compared to age-matched controls. Increased immunoreactivity for the iron storage protein ferritin, within and around plaques [101, 102] is further evidence of a local increase in iron as this protein is regulated primarily at the translational level through the binding of iron regulatory proteins to ferritin H- and L-chain mRNAs . Consistent with these findings in human AD brains, we found strong immunoreactivity for ferritin L-chain in maturing plaques in the later stages of the APP mouse while early on in the disease process this isoform was widely distributed throughout the brain in association with Aβ42 in the vicinity of blood vessels. There is recent evidence that ferritin L-chain may have a fundamental role in plaque pathology by binding to and stabilising PEN-2, a functional component of γ-secretase, the enzyme that cleaves APP to generate Aβ . Building on this observation it has been suggested that increased levels of iron and, hence, ferritin L-chain may lead to increased production of Aβ [34, 103]. The excess iron in plaques and associated increase in ferritin L-chain, the iron-storage isoform , is likely a secondary event resulting from a failure to utilise iron by dead and dying neurons. The suggestion that ferritin L-chain may lead to increased production of Aβ by increasing the activity of γ-secretase  would be consistent with a role for iron in promoting and maintaining plaque pathology. In agreement with earlier reports [25, 104] expression of ferritin H-chain was restricted to pyramidal neurons of the hippocampus.
Recent studies into the cause of vascular dysfunction in neurodegenerative diseases such as AD  suggest that the mechanisms include breakdown of the blood-brain barrier as a result of loss of pericytes [2, 73, 105], hypo-perfusion leading to hypoxia and brain ischaemia  and endothelial dysfunction . Furthermore, these abnormalities in vascular structure and function were recapitulated in the arcAβ mouse model of AD  while in the late stages of the APP-tg model used in the present study, loss of pericytes and extensive endothelial disruption was seen confirming the presence of severe vascular pathology. Loss of vascular integrity is also responsible for abnormal iron accumulation in addition to ferritin in AD brains in the form of haem-positive granular deposits. These have been demonstrated in aged brains in association with senile plaques and result from capillary bleeds or micro-haemorrhages [60, 107]. In AD brains where extensive neuronal damage was present, although levels of hepcidin and ferroportin were reduced, both proteins were found in association with haem-positive granular deposits in the region of damaged blood vessels.
Our results describe the progressive changes and dynamic interplay of iron homeostasis, vascular changes and neuronal degeneration in AD. In conclusion, as vascular damage with associated ischaemic change is widespread in AD brains it is likely that the reasons for a reduction in ferroportin levels are multifactorial but include cerebral ischaemia, inflammation, the loss of neurons due to the well characterised protein misfolding, senile plaque formation and possibly the ageing process itself. Progressive accumulation of ferritin light chain in the core of amyloid plaques of the APP tg mouse implies that there is an imbalance in iron utilization and release.
Amyloid precursor protein
APP transgenic mouse
β Tubulin type III
Bovine serum albumin
Ferritin light chain
Ferritin heavy chain
Polyacrylamide gel electrophoresis
Phosphate buffered saline
Sodium dodecyl sulphate
Superior frontal gyrus
We thank for Professor James Fawcett and Dr Simon Stott for critically reading the manuscript and endless encouragement. We also acknowledge Abcam (Cambridge, UK) for providing iron-related antibodies. This study was supported by the John Van Geest foundation, Scholl Foundation, and the Medical Research Council, UK.
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