The systemic iron-regulatory proteins hepcidin and ferroportin are reduced in the brain in Alzheimer’s disease

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 [6]. 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 [11]. 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 [30]. 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 [40]. 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 [50].

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.

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), [61]. 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 [63].

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 [35]. 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 [69]. 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 [75] 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 [81]. 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 [82].

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 [35]. However, neither the constitutive loss of hepcidin through gene mutations in either human [83] or mouse models of haemochromatosis [84, 85] or the targeted loss of ferroportin in the brain [45] 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 [86]. Hepcidin is a gene-encoded antimicrobial peptide structurally related to members of the defensin and protegrin families [87], most of which are cationic, a property that facilitates adsorption and insertion into anionic bacterial cell walls [87]. Cationic peptides also cross mammalian cell membranes [88] 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 [94] and iron levels were also found to be increased in neurofibrillary tangles and plaques using laser microprobe mass analysis [95] and particle-induced X-ray emission [96]. It is not clear whether this represents increased deposition of iron and other transition metals [97] 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 [34]. 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 [103]. 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 [34], 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 [103] 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 [73] 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 [74] and endothelial dysfunction [77]. Furthermore, these abnormalities in vascular structure and function were recapitulated in the arcAβ mouse model of AD [106] 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.