Parkinson’s disease (PD) is the second most prevalent age-related neurodegenerative disease affecting 1–2% of the population over 65. It has received the most attention in explaining iron’s contribution to the pathogenesis of neurodegeneration. Patients present with motor dysfunction broadly arising from a loss of dopaminergic neurons within the pars compacta region of SN, while the SN reticulate is relatively unaffected. Elevated levels of total iron and a shift in the equilibrium of iron to the oxidized state within a region that already has a high level of iron in the brain is considered to contribute to the oxidative stress-induced neurotoxicity (Dexter et al., 1991; Halliwell, 1992; Wypijewska et al., 2010). However, more recently it has been observed that the combination of iron with dopamine is a greater risk factor than each element on their own, and that the SN pars compacta has a greater “iron-dopamine index” than other regions of the brain (Hare et al., 2014). Altered redox-active labile iron in PD is compounded by a loss of the buffering capacity of iron storage proteins; neuromelanin (Faucheux et al., 2003) and Ft (Connor et al., 1995) as well as iron-catalyzed aggregation of α-synuclein to form Lewy bodies in surviving neurons (Lotharius and Brundin, 2002). Contributing factors that can explain PD-increased iron accumulation are the elevated expression of the iron import transporter DMT1 (Salazar et al., 2008) and reduced expression of the iron export pore protein Fpn (Song et al., 2010) as well as CP ferroxidase activity (Olivieri et al., 2011; Ayton et al., 2012). In vivo imaging of iron by transcranial sonography (TCS) and T₂^* -weighted magnetic resonance imaging (MRI) has strengthened the iron hypothesis of PD by illustrating a strong correlation for SN iron levels with disease severity and duration (Menke et al., 2009; Ulla et al., 2013). Typically, intraneuronal iron would be controlled by IRP1/2 response to the cytosolic labile iron pool. However, iron accumulation with an iron-regulated protein profile that correlates with decreased iron, infers a breakdown in the neuron’s iron regulatory system with PD. Support for this theory come from an inability for IRP to correctly respond in models of PD. Upregulation of IRP1/2 induces a downregulation in Fpn, thus exacerbating iron accumulation in a 6-hydroxydopamine model of PD (Song et al., 2010), but is unable to control Ft mRNA translation despite the elevated labile iron pool in PD (Hirsch, 2006).

Accounting for 50–80% of all dementia cases, AD is the most common neurodegenerative disease of individuals over 65. The neuropathological hallmarks of AD are an accumulation of extracellular amyloid plaques comprising mainly of amyloid-β (Aβ), and the presence of intraneuronal neurofibrillary tangles that are comprised of hyperphosphorylated tau. Aβ is proteolytically derived from APP, a ubiquitously expressed type 1 transmembrane protein also predominantly expressed on the neuronal surface. A small number of AD cases who tend to have an earlier onset of disease are caused by autosomal dominance in familial mutations. These mutations are present within regions of APP or its γ-secretase cleavage proteins; presenilin 1 and 2, and promote the amyloidogenic processing of APP to increase Aβ generation. The trisomy mutation associated with Down’s syndrome also increases Aβ accumulation leading to early AD pathology and is considered to be caused by the increased copy number of APP that lies within chromosome 21.

While strong evidence suggests that Aβ is the principal cause of neurotoxicity and may be a significant contributor to synaptic dysfunction in AD (Roberts et al., 2012), iron accumulation in affected brain regions, as reported in both post mortem and MRI studies (Falangola et al., 2005; Jack et al., 2005; Antharam et al., 2012), may also be a factor in the increased oxidative stress observed in AD (Castellani et al., 2007). Hippocampal iron accumulation localized in neurofibrillary tangle-containing neurons and the neuritic processes surrounding senile plaques in AD (Quintana et al., 2006) correlates well with cognitive decline (Ding et al., 2009). Within the same regions, proteins regulated by iron are also altered whereby Ft is elevated (Grundke-Iqbal et al., 1990; Morris et al., 1994; Bouras et al., 1997; LeVine, 1997) and both CP expression and activity are lowered (Connor et al., 1993; Torsdottir et al., 2010). Despite these changes, transferrin expression levels are a little less clear with some evidence of a decrease (Connor et al., 1992) while later reports showing a localized increase within the frontal lobe (Loeffler et al., 1995). As previously suggested, the transport of iron by transferrin within the brain is minor and only required by select cells. This may account for the uncertainty in protein observations despite weak association with AD risk of the C2 variant to the Tf gene (Robson et al., 2004; Bertram et al., 2007). A known partner of Tf called HFE protein is also expressed in glia and neurons around neurofibrillary tangles and senile plaques of AD (Connor and Lee, 2006) and over the previous decade numerous genetic association studies have illustrated HFE gene mutations increase the risk of AD (Nandar and Connor, 2011). In particular the mutations H63D and C282Y (Sampietro et al., 2001; Pulliam et al., 2003; Blazquez et al., 2007) cause peripheral iron accumulation in AD and possibly has a link with the APOE gene. The HFE protein carrying the H63D mutation has also been shown to upregulate the phosphorylation of tau (Hall et al., 2010).

Huntington’s disease (HD) is a progressive neurodegenerative disorder characterized by motor, psychiatric and cognitive disturbances that progress to dementia (The Huntington’s disease Collaborative Research Group, 1993). Prevalence in Europe, North America, and Australia is ~5.70 per 100,000 (Pringsheim et al., 2012). HD is caused by a dominant CAG expansion in the exon 1 encoded region of the huntingtin gene resulting in the expression of polyglutamine-expanded mutant huntingtin protein (The Huntington’s disease Collaborative Research Group, 1993). Similar to most neurodegenerative diseases and iron accumulative disorders, numerous mechanisms have been implicated in the pathogenesis of HD including oxidative stress (Browne and Beal, 1994), energetic dysfunction (Panov et al., 2002; Cui et al., 2006), transcriptional dysregulation (Nucifora et al., 2001; Dunah et al., 2002), and defective axonal transport (Trushina et al., 2004). Iron dysregulation occurs in human HD (Dexter et al., 1991; Rosas et al., 2012) and brain field map MRI values of gene-positive individuals have suggested that alterations of brain iron homeostasis occur before the onset of clinical signs (Rosas et al., 2012). Genetic mouse models of HD have also accurately recapitulated the elevated levels of brain iron (Fox et al., 2007; Chen et al., 2013). These findings were interpreted to indicate a compensatory response to iron stress occurring in HD striatum.

Prion diseases are a group of disorders whereby extreme cellular destruction leads to vacuolization and spongiosis of large areas of the brain. Within humans the most common form of prion disease is Creutzfeldt–Jacob disease (CJD) of which ~80% is sporadic cases. Despite being comparatively rare considering other forms of neurodegenerative disease, its high risk of infectivity both within and between species has prompted intense research into the disorder. Prion disease is now known to, at least in part, be caused by the conversion of the prion protein from its regular form (PrP^C) into more of a β-sheeted isoform termed PrP-scrapie (PrP^Sc; Prusiner, 1998; Aguzzi and Falsig, 2012). Little doubt remains that PrP^Sc is able to initiate infection when inoculated into a recipient animals brain (Wang et al., 2010) and that this is done by PrP^Sc autocatalyzing its conversion from PrP^C. However, the mechanism by which PrP^Sc induces neurotoxicity remains to be fully elucidated as PrP^Sc levels poorly correlate with disease progression (Caughey and Baron, 2006).

Accumulation of redox-active iron, partly co-aggregated with Ft and in association with PrP^Sc plaques, has been reported in CJD brains (Petersen et al., 2005; Singh et al., 2009a, 2012). However, it appears that the increase in total iron may be biologically unavailable as the IRP response in the disease indicates iron deficiency. An increase in both Tf and its receptor as well as transcriptional changes with Ft and IRP1/2 are reported with Tf increase correlating with PrP^Sc levels (Kim et al., 2007; Singh et al., 2009a). In reflection to the prion diseased brain, cerebrospinal fluid (CSF) has decreased levels of Tf and increased total ferroxidase activity (Singh et al., 2011; Haldar et al., 2013). When used in combination these CSF markers of disease have an accuracy of 88.9% in detecting CJD over other forms of neurodegenerative disease (Haldar et al., 2013).

As the name infers APP is the precursor of Aβ; the prevalent peptide found in senile plaques from a range of amyloidogenic diseases including AD. Proteolytic processing of APP from the neuron is predominantly through cell surface α-secretase cleavage followed by cleavage with the γ-secretase complex. This non-amyloidogenic processing of APP excludes Aβ production due to the α-secretase cleavage site residing within the Aβ peptide sequence. The alternative processing of APP through the amyloidogenic pathway requires β-secretase instead of α-secretase to produce Aβ as one of its products. The amyloidogenic processing of APP requires the protein to be endocytosed to allow optimal pH conditions for β-secretase cleavage. As yet it is not functionally clear as to why two intricate proteolytic pathways are required to cleave APP, however, the altered cellular location and function of cleaved products could be a likely reason.

Despite a reduced affinity compared to other transitional metals, iron binds Aβ and induces Aβ aggregation (Huang et al., 2004; Ha et al., 2007; Bousejra-ElGarah et al., 2011). This iron interaction is via His6, His13, and His14 of Aβ and is thought to be facilitated in a more reduced environment such as the brain due to the prevalence for the Fe²⁺ form of iron to bind Aβ (Bousejra-ElGarah et al., 2011). ROS generated through iron aggregated Aβ is toxic to neurons (Liu et al., 2011) and may partly contribute to the neurotoxicity present within the iron enriched environment around senile plaques (Meadowcroft et al., 2009; Gallagher et al., 2012). Evidence also supports the same histidine residues in Aβ binding to the iron center as well as porphyrin ring of heme (Atamna et al., 2009; Yuan and Gao, 2013). While the interaction with heme reduces Aβ aggregation (Zhao et al., 2013) it is unclear whether oligomeric Aβ, now considered to be the neurotoxic species within AD, are preferentially formed instead (Thiabaud et al., 2013). Heme binding to Aβ also restricts the bioavailability of regulatory heme and the complex formed has been shown to have peroxidase activity (Atamna and Boyle, 2006).

Translational regulation of APP through an IRE within the 5′UTR implies an interaction with iron status whereby increased cytosolic free iron levels translationally upregulate APP expression (Rogers et al., 2002). APP has recently been identified as a facilitator of neuronal iron efflux through an interaction with Fpn (Duce et al., 2010). While some controversy surrounds the exact mechanism of how APP is involved in the release of iron from the cell (Ebrahimi et al., 2012; Honarmand Ebrahimi et al., 2013), APP within a neuronal environment still appears to be essential to efflux iron (Duce et al., 2010; Wan et al., 2012). Depletion on APP in both cultured neurons and mouse models leads to intracellular iron retention that can be rescued upon the addition of APP to the extracellular environment (Duce et al., 2010) or by overexpression of cellular APP (Wan et al., 2012). Of significance, children suffering from Down’s syndrome that have an increased expression of APP have a reported high risk of iron deficiency and anemia (Dixon et al., 2010; Tenenbaum et al., 2011), however, further investigation is required to confirm whether this is due to an increase in APP facilitated iron efflux. As with Fpn, it appears that the surface presence of APP is essential for its role in iron efflux. When APP trafficking to the cell surface is impaired (Lei et al., 2012) or altered by processing through the amyloidogenic pathway, as with the AD-associated familial mutation in APP, iron accumulation arises (Wan et al., 2011).

Hyperphosphorylated tau has mostly been recognized as the principal component of neurofibrillary tangles, a pathological hallmark in a number of neurodegenerative disorders including AD. Various repeat motifs on tau are known to bind iron in a pH- and stoichiometric-dependent manner that results in the promotion of phosphorylation and aggregation of the protein (Ma et al., 2006; Malm et al., 2007; Zhou et al., 2007). While the affinity for iron within a physiological environment has yet to be established for tau, binding of Fe²⁺ appears to preferentially induce phosphorylation of tau (Lovell et al., 2004; Chan and Shea, 2006) despite Fe³⁺ being the favored state in causing aggregation of tau once it has been hyperphosphorylated (Yamamoto et al., 2002; Amit et al., 2008).

Recently, it has been identified that tau may be required in the iron-modulatory role of APP (Lei et al., 2012). Mice deficient in tau have neuronal iron accumulation that can be reduced in primary cultures by the extracellular addition of APP or moderate chelators such as clioquinol (Lei et al., 2012). Tau has been implicated in axonal trafficking of proteins including APP (Islam and Levy, 1997) and it is proposed that impaired trafficking of APP to the cell surface in tau^-/- neurons restricts APP’s ability to facilitate iron efflux through Fpn leading to intracellular iron accumulation (Lei et al., 2012).

Variance in α-synuclein is sufficient to cause PD in humans and animal models suggesting a central role in PD pathogenesis (Hardy, 2010). This is apparent through several observations; the overexpression of wild-type α-synuclein through gene duplication is sufficient to cause parkinsonian symptoms (Singleton et al., 2003; Chartier-Harlin et al., 2004; Fuchs et al., 2007); the majority of familial cases of PD are associated with mutations in α- synuclein (Polymeropoulos et al., 1997; Kruger et al., 1998; Zarranz et al., 2004; Hardy, 2010); and aggregated α-synuclein is a core protein found in Lewy bodies. It has been proposed that this ubiquitously expressed protein is involved in synaptic vesicle formation however, it is currently not understood why the dopaminergic neurons of the SN that are targeted in PD are more susceptible to α-synuclein aggregation and toxicity. One theory as to the vulnerability of the SN is the high levels of iron within the region that can augment α-synuclein aggregation. Iron has been shown to bind to the C-terminal region of α-synuclein and under oxidizing conditions, such as that provided in dopamine’s presence, denatures the protein and promotes further aggregation (Cappai et al., 2005; Binolfi et al., 2006; Bharathi et al., 2007). Redox-active iron is detected in association with α-synuclein aggregates in Lewy bodies (Castellani et al., 2000), a phenotype that is likely to cyclically promote iron-mediated oxidation and exacerbate the aggregation of α-synuclein as well as the other proteins co-aggregated within the Lewy body (Giasson et al., 2000).

As with APP a strong indication of the importance of α-synuclein in the regulation of neuronal iron was through the identification of an iron-response element in its 5′UTR that is required to increase translation when intraneuronal iron is high (Friedlich et al., 2007; Febbraro et al., 2012). In support of an iron-associated role of α-synuclein, it has recently been identified to modulate cellular iron homeostasis through its ability to reduce Fe³⁺ into the biologically active Fe²⁺ form (Davies et al., 2011). In accordance with its ferrireductase activity, α-synuclein has a greater affinity for Fe³⁺ (Rouault and Tong, 2005) and within a normal physiological environment such as the SN may provide a consistent supply of Fe²⁺ for neuronal metabolic processes such as enzymatic synthesis of neurotransmitters.

As well as the significant role iron has in prion disease pathogenesis through PrP^Sc – Ft generated ROS that was described above, recent reports have also suggested that PrP^C has a normal physiological role in iron uptake. Cell surface presentation of PrP^C is required for this function and it appears the copper binding octapeptide repeat region of the protein may have ferrireductase activity (Singh et al., 2013). When the non-modified form of PrP^C is overexpressed in cultured cells, iron uptake, and storage is increased (Singh et al., 2009c, 2013) and similarly when expression is depleted by gene knockout in mouse models, tissue iron deficiency correlates with changes to proteins responsive to iron (Singh et al., 2009b). Intriguingly, a familial mutation in PrP^C (P102L) classically associated with the prion disorder called Gerstmann–Sträussler–Scheinker (GSS) disease is shown to increase ferrireductase activity and increase levels of intracellular labile iron (Singh et al., 2013). Accumulating evidence now suggests that PrP^C may have a role in the transferrin and NTBI import into the cell similar to DMT1 or Zip14. Of note, PrP^C bears a phylogenetic relationship to the ZIP family (Schmitt-Ulms et al., 2009) and has recently been implicated in neuronal zinc import when complexed to NMDA receptors (Watt et al., 2012). Similar to the recent identification that Zip14 is able to transport iron as well as zinc, it is worth noting that the NMDA- PrP^C complex involved in zinc import may also be implicated in neuronal iron import under certain conditions.

Mutant huntingtin protein aggregates to form inclusion bodies that represent a pathological hallmark of HD. As with most aggregated protein structures formed in neurodegenerative disease, these bodies bind iron and act as centers of oxidative stress with large amounts of oxidized protein present (Firdaus et al., 2006). Genetic mouse models of HD that transgenically overexpress mutant huntingtin accurately recapitulate the elevated levels of brain iron in the disease (Fox et al., 2007; Chen et al., 2013) and huntingtin knockdown in zebra fish models result in an iron deficiency phenotype (Hilditch-Maguire et al., 2000; Lumsden et al., 2007; Henshall et al., 2009). This suggests that huntingtin is not only regulated by iron but also involved in iron homeostasis. However, iron does not interact directly with N-terminal huntingtin fragments (Fox et al., 2007; Chen et al., 2013) indicating that the effect of huntingtin on iron may mediate downstream influences on iron homeostatic pathways.