Iron: Oxidative Stress and Neurodegeneration
`
`Dev Neurosci 2002;24:188–196
`DOI: 10.1159/000065701
`
`Received: May 2, 2002
`Accepted: June 6, 2002
`
`Brain Iron Metabolism and
`Neurodegenerative Disorders
`
`Jack C. Sipe Pauline Lee Ernest Beutler
`
`Department of Molecular and Experimental Medicine, Scripps Research Institute (MEM-215), La Jolla, Calif., USA
`
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`Key Words
`Iron W Metabolism W Gene W Mutation W Single nucleotide
`polymorphism W Neurodegeneration
`
`Abstract
`Iron, an essential element for central nervous system
`(CNS) function, has frequently been found to accumulate
`in brain regions that undergo degeneration in neurologi-
`cal diseases such as Alzheimer disease, Parkinson dis-
`ease, Friedreich ataxia and other disorders. However, the
`precise role of iron in the cause of many neurodegenera-
`tive diseases is unclear. To assist in understanding the
`potential importance of iron in CNS disease, this review
`summarizes the present knowledge in the areas of CNS
`iron metabolism, homeostasis and disregulation of iron
`balance caused by mutations in genes encoding proteins
`involved in iron transport, storage and metabolism. This
`review encompasses neurodegenerative disorders asso-
`ciated with both iron overload and deficiency to highlight
`areas where iron misregulation is likely to be important
`in the pathophysiology of several human brain dis-
`eases.
`
`Copyright © 2002 S. Karger AG, Basel
`
`Introduction
`
`Elemental iron (Fe) is essential for nearly all living
`organisms since it participates in a variety critical meta-
`bolic processes including oxygen transport, electron trans-
`port, DNA synthesis, and redox/non-redox reactions and
`other cell functions [1, 2]. Cellular iron concentrations
`must be tightly controlled due to low iron solubility and
`the ability of Fe to form toxic reactive oxygen interme-
`diates (free radicals) in most biological systems [2].
`In the central nervous system (CNS), iron is necessary
`for many critical metabolic functions. One role of iron in
`the CNS is for the biosynthesis of the neurotransmitters
`dopamine [3] and serotonin [4]. Iron is also important in
`the formation of myelin by oligodendrocytes [5]. In mam-
`mals, brain iron accumulation increases after birth with
`selective uptake in regions undergoing myelination [6]. In
`humans, there is accumulation of iron during adolescence
`in motor areas such as the motor cortex and substantia
`nigra [7, 8]. Iron stores gradually increase until about age
`40 and then remain steady [9]. Many of these iron-depen-
`dent events within the CNS are a function of age, as is the
`case with myelination, or are region specific, as is the case
`with dopamine synthesis in substantia nigra neurons [10].
`In the CNS, iron is also an essential component of
`enzymes of oxidative metabolism, such as those catalyzed
`by the cytochromes and iron-sulfur enzymes. Brain ener-
`
`ABC
`Fax + 41 61 306 12 34
`E-Mail karger@karger.ch
`www.karger.com
`
`© 2002 S. Karger AG, Basel
`
`Accessible online at:
`www.karger.com/journals/dne
`
`Jack C. Sipe, MD
`Department of Molecular and Experimental Medicine
`Scripps Research Institute, 10550 N. Torrey Pines Road
`La Jolla, CA 92037 (USA)
`Tel. +1 818 784 9681, Fax +1 858 784 2083, E-Mail jcsipe@scripps.edu
`
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`gy requirements are high, with particular demand for
`ATP as the principal energy source for stable cellular ionic
`gradients, synaptic transmission and distant axoplasmic
`transport [10]. Thus, it is not surprising that Fe-related
`mitochondrial dysfunction has been associated with neu-
`rodegeneration [11]. During CNS oxidative metabolism,
`free radicals are normally produced and metabolized [12],
`but an excess of free Fe results in rapid formation of toxic
`reactive oxygen intermediates that may be one mecha-
`nism of CNS cell death in neurodegeneration [10].
`The purpose of this review is to summarize recent
`advances in the biochemistry, cellular biology and genet-
`ics of iron metabolism as they relate to the critical role of
`iron in normal nervous system function and to proven or
`potential molecular mechanisms of disease in selected
`neurodegenerative disorders. Here, we will focus on the
`molecular control of iron homeostasis in the CNS in both
`health and in neurodegenerative diseases. For more com-
`prehensive reviews of total body iron homeostasis, the
`reader is referred to several recent review articles [1, 2,
`13–15].
`
`Iron Metabolism in the CNS
`
`Iron Uptake and Transport
`Iron uptake in the brain is limited by the blood-brain
`barrier and blood-cerebrospinal fluid (CSF) barrier [16,
`17] maintained by tight junctions between microvascular
`endothelial cells and choroid plexus epithelium. Thus,
`CNS-regulated expression of transport proteins is re-
`quired for uptake of iron [10]. Iron uptake into the brain
`is mediated in part by transferrin receptor expression in
`the endothelial cells and choroid plexus cells [18] or lacto-
`ferrin receptor expression on neurons and microvessels
`[19, 20]. The Fe-transferrin complex or Fe-lactoferrin
`undergoes endocytosis, followed by release of Fe within
`the endothelial cell [10]. Movement of iron from the cyto-
`sol to the extracellular spaces of the CNS involves brain-
`specific proteins. In the CNS, a brain-specific ceruloplas-
`min is expressed as a membrane anchored protein [21]
`and in situ hybridization of CNS tissue reveals abundant
`ceruloplasmin gene expression in specific populations of
`perivascular glial cells associated closely with dopaminer-
`gic melanized neurons in the substantia nigra [22]. Thus,
`ceruloplasmin may play a role in iron export from endo-
`thelial cells into the extracellular spaces or from iron-
`loaded cells such as dopaminergic neurons in the substan-
`tia nigra. The subsequent transport of Fe into other areas
`of the brain is likely mediated by transferrin and non-
`
`transferrin-bound mechanisms [23]. Serum transferrin is
`not found within the CNS unless there is a breakdown of
`the blood-brain barrier [24]. Thus, the CNS must synthe-
`size its own transferrin, a synthesis that is regulated by a
`nervous system-specific promoter [3, 25]. Cellular iron
`uptake may be stimulated by ceruloplasmin ferroxidase
`activity by means of a recently reported trivalent cation-
`specific transport mechanism [26].
`The divalent metal transporter 1 (DMT1, Nramp2)
`has been implicated in transport of iron from endosomes
`and is localized in neurons and oligodendrocytes, which
`show a significantly decreased iron content in the Bel-
`grade rat with a mutation in DMT1/Nramp2 [27]. Alter-
`natively, evidence suggests that Fe may be transported by
`glial cell processes, from astrocyte foot processes that sur-
`round the microvasculature, through the astrocyte cell
`extensions to remote regions [28]. Redistribution studies
`of cerebral iron demonstrate regional movement of iron
`over several weeks in patterns that often suggest the use of
`axonal transport mechanisms to areas of axon projections
`[29]. Ferritin binding, presumably due to receptors, has
`been identified in the human brain [30]. Ferritin-binding
`sites are distributed in iron-rich areas of the brain in con-
`trast to the transferrin receptor distribution which are
`located primarily in the grey matter [30]. As a result, ferri-
`tin receptor binding has been postulated to play a role in
`iron uptake since the localization of ferritin-binding sites
`is better correlated with brain iron accumulation.
`
`Iron Storage
`Ferritin, a cytosolic protein, binds and stores intracel-
`lular iron in most cells of the CNS [12] and prevents it
`from forming reactive oxygen intermediates [1]. In the
`CNS, the synthesis of ferritin takes place in the neuronal
`cell body and holoferritin is transported by axoplasmic
`flow within axons where ferritin may be degraded distally
`within lysosomes [31–33]. Very early in mammalian CNS
`development, iron first appears in microglia [6] and then,
`as brain development continues, ferritin and stored iron
`appear in oligodendroglia in both temporal and regional
`correlation with myelination of axons [5]. The expression
`of ferritin is induced by the presence of excess iron [1].
`Intact human ceruloplasmin appears to be required for
`the incorporation and loading of iron into ferritin [34].
`The concentration of brain ferritin iron may be elevated
`in some neurodegenerative disorders, for example, Alz-
`heimer disease [1] and neuroferrinopathy [35]. Neurome-
`lanin [36] is another major iron storage mechanism in cer-
`tain neuronal systems such as the substantia nigra. Intra-
`neuronal membrane-bound neuromelanin has a strong
`
`Iron and Neurodegeneration
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`Dev Neurosci 2002;24:188–196
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`chelating ability for iron and may be another mechanism
`of intracellular iron sequestration [37].
`
`Intracellular Iron Utilization
`Iron may be used in the CNS for a wide variety of cellu-
`lar metabolic processes, for example, as a cofactor for the
`enzymes, tyrosine hydroxylase and tryptophan hydroxy-
`lase, in the synthesis of dopamine and serotonin in neu-
`rons or incorporation into iron-sulfur cluster-containing
`proteins [10]. Iron is also essential for the biosynthesis of
`CNS lipids and cholesterol, the important building blocks
`of myelin and as a co-factor for metabolic enzymes in olig-
`odendroglia [5]. The mitochondrion is a particularly criti-
`cal site of intracellular iron utilization since this organelle
`is where iron is incorporated into protoporphyrin IX to
`form heme and where vital non-heme proteins, cyto-
`chromes a, b and c, and cytochrome oxidase required for
`mitochondrial function are also assembled [10]. Mito-
`chondrial iron sequestration has been demonstrated in
`dopamine-challenged astrocytes, providing a possible al-
`ternative pathogenetic mechanism for iron accumulation,
`mitochondrial damage and oxidative injury in neurode-
`generation [38]. Recently, another protein believed to be
`involved in iron metabolism, frataxin, has been localized
`to the mitochondrion in several tissues, including the
`CNS [39–41]. The crystal structure of frataxin has been
`reported [42] and these structural findings predict mecha-
`nisms of protein-protein and protein-iron interactions
`similar to ferritin suggesting that frataxin may serve an
`iron storage function in mitochondria [43].
`
`Iron Recycling and Export
`Most of the iron taken up by the human body is exten-
`sively recycled and reused and the CNS is no exception,
`although little is known about how or if iron exits the
`CNS. Macrophages play a key role in the body by phago-
`cytosis of senescent erythrocytes but red cells are not nor-
`mally present inside the CNS blood-brain barrier except
`in brain injuries [44]. Transferrin-bound Fe in the inter-
`stitial fluid could exit the CNS through the arachnoid villi
`that normally participate in the exit of CSF into the
`venous system [3]. Alternatively, vascular endothelial
`cells could export transferrin-bound iron back into the cir-
`culation [3, 14]. In addition, heme oxygenase (HO) is
`present in the brain in two forms (HO-1 and HO-2) that
`appear to have functions other than heme recycling [45].
`Recent studies of HO distribution demonstrate high lev-
`els of HO-2 in the brain [45] with the highest levels local-
`ized in the substantia nigra [46]. HO-1 appears to be
`involved in mitochondrial iron trapping via the mito-
`
`chondrial transition pore which mediates the transfer of
`non-transferrin iron into mitochondria [38]. HO is upreg-
`ulated in stroke, stress and proinflammatory cytokine
`release which might contribute to brain iron accumula-
`tion in neurodegeneration [47, 48] but one experimental
`brain ischemic injury model suggested that increased HO
`activity does not contribute to iron accumulation [49].
`Consistent with this hypothesis, Ferris et al. [50] showed
`that cells from mice with a targeted deletion of the HO-1
`gene demonstrated reduced iron efflux in HO-1-deficient
`fibroblasts, suggesting a possible mechanism for intracel-
`lular iron accumulation and cell death.
`
`Mechanisms of Iron Homeostasis
`Some fundamentals of CNS iron homeostasis remain
`unknown but clues to these basic mechanisms may be
`uncovered by animal models of iron metabolism dysregu-
`lation produced by targeted genetic deletions. For exam-
`ple, iron regulatory protein-2 (IRP2) gene disruption in
`mice results in iron accumulation in distinctive brain
`regions and is associated with iron overload in degenerat-
`ing neurons and in oligodendrocytes with ubiquitin posi-
`tive inclusions [31]. This leads to a neurodegenerative dis-
`ease in IRP2–/– mice [31] suggesting possible contribu-
`tions to the pathogenesis of comparable human neurologi-
`cal disorders. The function of the hemochromatosis gene,
`HFE, in the brain is unclear but HFE protein has been
`localized to vascular endothelial cells in the cortex and
`cerebellum [51]. However, hereditary hemochromatosis,
`a disorder typically associated with systemic iron over-
`load, has not been associated with excessive brain iron
`levels or neurodegenerative disease. Consistent with this
`notion, Moos et al. [109] examined whether high circula-
`tory levels of iron could cause brain iron accumulation
`and clearly showed that there is exclusion of excess plas-
`ma iron from transfer into the brain.
`
`Neurodegenerative Disorders Associated with
`Increased Total Iron Content
`
`Increased total brain iron content is a feature of some
`neurodegenerative disorders and is thought to be involved
`in the pathophysiology of many of these diseases. In sever-
`al neurodegenerative diseases, an increased concentration
`of brain parenchymal iron has been localized to specific
`brain regions or neuronal systems [3, 43, 52]. Similarly, in
`certain neurodegenerative basal ganglia disorders includ-
`ing Parkinson disease, Huntington disease, Hallervorden-
`Spatz syndrome (neurodegeneration with brain iron accu-
`
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`mulation), Friedreich ataxia, neuroferrinopathy (domi-
`nantly inherited adult-onset basal ganglia disease) and
`hereditary aceruloplasminemia, there is accumulation of
`iron in the basal ganglia [1, 43, 52, 53]. Exactly how
`increased regional or cellular brain iron is involved in dis-
`ease pathophysiology is uncertain but there are several
`possible mechanisms. The potential mechanism most of-
`ten cited is the ability of iron to promote oxidative dam-
`age [3, 43, 54]. It has been proposed that some brain
`regions or specific neuronal groups, i.e., dopaminergic
`neurons, with a high rate of free oxygen radical produc-
`tion would be vulnerable to excessive iron-mediated free
`radicals via the Fenton reaction [55]. Alternatively, the
`presence of high iron levels in Lewy bodies and senile
`plaques in Parkinson disease [56] suggests a possible rela-
`tionship. Iron and oxidative stress might result in the
`damage and aggregation of proteins. In fact, in vitro iron
`has been shown to promote aggregation of ·-synuclein,
`reminiscent of Lewy bodies or amyloid [57]. Hydroxyl
`radicals and other oxygen-free radicals are produced dur-
`ing normal brain metabolism [12, 55] and significantly
`higher plasma levels of hydroxyl radicals have been dem-
`onstrated in Parkinson disease [58], together with in-
`creased lipid peroxidation products in the CSF and brain
`[59, 60], suggesting a role in neurodegeneration. Subse-
`quently, the ubiquitin-proteasome system, essential for
`non-lysosomal catabolism, could fail to adequately clear
`damaged or aggregated proteins in CNS cells [61].
`
`Alzheimer Disease
`Excessive iron accumulation and oxidative damage
`have been associated with Alzheimer disease and may
`contribute to the pathogenesis or disease progression [10].
`In this disease, iron has been associated with senile
`plaques [62], deposition of ß-amyloid peptide fragments
`and abnormal processing of amyloid precursor protein [7,
`63–65]. Analysis of Alzheimer disease brains indicates
`iron accumulation within specific brain regions of se-
`lective vulnerability to neurodegeneration such as the hip-
`pocampus and cerebral cortex [66, 67]. Iron accumulation
`in Alzheimer disease has been suggested as an important
`source of redox-generated free radicals and a contributor
`to oxidative damage in this disease [68]. Neurofibrillary
`tangles and senile plaques contain redox-active metals
`like iron and are major sites for catalytic reactivity [69].
`HO protein immunoreactivity is greatly enhanced in neu-
`rons and astrocytes of the hippocampus and cortex in Alz-
`heimer subjects [70]. Other iron-binding proteins have
`been associated with Alzheimer neurodegeneration in-
`cluding transferrin, ferritin and IRP2. Specifically, brain
`
`tissue from Alzheimer subjects has shown transferrin
`localized to glial cells and melanotransferrin in a subset of
`reactive glia associated with senile plaques [71]. Ferritin-
`containing microglia, ferritin cores and nanoscale iron
`oxides have been demonstrated in senile plaques of Alz-
`heimer brains [63, 72, 73], thus suggesting a possible role
`for ferritin in the disruption of Alzheimer brain iron
`homeostasis. Clearly IRP2 also has an important role in
`neurodegeneration evidenced by IRP2 colocalization in
`senile neurites [74] and by CNS iron overload and degen-
`eration of neurons with associated ubiquitin-positive in-
`clusions in mice with deletion of the IRP2 gene [31].
`Increased expression of melanotransferrin (p97) has
`been evaluated as a biochemical marker for Alzheimer
`disease [75]. Two laboratories have reported that p97 is
`not elevated in other neurodegenerative diseases and may
`be related to overexpression by senile plaque-associated
`reactive microglia [75, 76].
`
`Parkinson Disease
`Impaired iron homeostasis has been demonstrated in
`Parkinson disease and other basal ganglia disorders [77].
`The origin of the increased iron content in Parkinson dis-
`ease is presently unknown. Specifically in Parkinson dis-
`ease, there is increased iron loading of intracellular ferri-
`tin [55] and the iron deposits are found within degenerat-
`ing neurons of the substantia nigra undergoing apoptosis
`and in Lewy bodies [12, 78]. Because Fe is sequestered in
`substantia nigra Lewy bodies [56], increased iron trans-
`port into the substantia nigra and basal ganglia or redistri-
`bution of ferritin or neuromelanin stores is suspected [55].
`It is noteworthy that iron associated with neuromelanin
`significantly accumulates within the substantia nigra in
`Parkinson disease [79] and it has been suggested that this
`mechanism may play a role in the selective neurodegener-
`ation seen in Parkinson disease [12, 80]. HO immunore-
`activity is greatly enhanced in Lewy bodies found in the
`substantia nigra of brains affected with Parkinson disease
`[70]. MPTP, a specific neuronal toxin in the substantia
`nigra that causes parkinsonism, selectively induces HO
`expression in striatal astrocytes [81], thus suggesting a
`possible role of HO in the pathogenesis of Parkinson dis-
`ease. Lactoferrin is structurally similar to transferrin and
`binds iron reversibly but the exact function of lactoferrin
`and lactoferrin receptor in the CNS remain unknown.
`Expression of lactoferrin receptors on neurons and micro-
`vessels has been found to be increased in regions of neu-
`ronal degeneration in the mesencephalon of postmortem
`Parkinson brain tissue and suggests a possible link to iron
`overload in affected brain regions [82].
`
`Iron and Neurodegeneration
`
`Dev Neurosci 2002;24:188–196
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`Table 1. Ceruloplasmin gene
`polymorphisms in Parkinson disease
`
`Location
`
`SNP
`
`Amino
`acid
`
`Control subjects
`n+/n total subjects
`
`Parkinson subjects
`n+/n total subjects
`
`5)-UTR nt-4385)
`UTR nt-16955)
`UTR nt-1936
`
`Ex 9 nt-1652
`
`Ex 11 nt-1950
`
`Ex 17 nt-2991
`
`T → C
`C → T
`A → G
`C → T
`A → C
`T → C
`
`2/2 T/C, C/C
`1/5 C/T
`2/5 A/G
`
`1/5 T/C, 4/5 C/C
`2/8 C/T
`1/8 G/G
`
`T551I
`
`G650G
`
`H997H
`
`0/10
`
`1/10
`
`0/10
`
`3/60
`
`5/60
`
`1/60
`
`Ceruloplasmin gene SNPs and frequencies found in the 5)-untranslated region (5)-UTR)
`and coding exons of white Parkinson disease patients and matched controls. n+ = Number of
`subjects positive for the mutation over the total number (n) of subjects that were genotyped in
`each group. None of the Parkinson subject groups vs. control comparisons were found to be
`statistically significant.
`
`Genes that encode iron-regulatory proteins expressed
`in the CNS have been investigated for a possible role in
`neurodegenerative diseases. Mice with a targeted disrup-
`tion in the IRP2 gene show misregulation of iron metabo-
`lism in the brain and a neurodegenerative disorder with
`significant brain iron overload [31], and the human brain
`distribution of IRP2 colocalizes with redox-active iron
`and the brain lesions of Alzheimer disease [74]. However,
`a new clinical study failed to show an association of any
`natural mutations in the IRP2 gene in patients with spo-
`radic Parkinson disease [83]. Because of a report of
`reduced ferroxidase activity in the CSF of Parkinson dis-
`ease patients [84], we studied the known human cerulo-
`plasmin-coding region single nucleotide polymorphisms
`(SNPs) and promoter region SNPs for association with
`Parkinson disease. With informed consent, we completed
`SNP genotyping on a cohort of up to 60 Parkinson sub-
`jects but the data presented in table 1 do not show any
`significant linkage of ceruloplasmin polymorphisms in
`Parkinson disease compared to matched controls or any
`evidence of SNPs in linkage disequilibrium in Parkinson
`patients [unpublished observations]. Investigations of
`polymorphisms in the 5) flanking region of transferrin
`indicated that the presence of transferrin haplotype 3 in
`white Parkinson patients was in slight excess over the con-
`trol white population suggesting a possible role of trans-
`ferrin SNPs in parkinsonism [85].
`
`Multiple System Atrophy
`Multiple system atrophy (MSA), is an adult-onset neu-
`rodegenerative disorder with parkinsonism, ataxia and
`
`autonomic failure. MSA is characterized by increased
`brain iron [86] and ·-synuclein-containing glial cytoplas-
`mic inclusions [87]. MSA provides a link to a group of
`disorders including Lewy body dementia, Parkinson, and
`Hallervorden-Spatz to which the term ‘synucleinopathy’
`has been applied [87]. A recent brain MRI study in
`patients with MSA demonstrated changes consistent with
`increased ferritin-bound iron and neuromelanin in the
`basal ganglia [88].
`
`Friedreich Ataxia
`Friedreich ataxia, a neurodegenerative disorder, has
`been associated with an intronic trinucleotide repeat ex-
`pansion in the frataxin gene [89] leading to reduced
`mRNA transcription and translation of frataxin protein
`[39]. Patients with Friedreich ataxia demonstrate mito-
`chondrial iron overload [90] and deficiency of iron-sulfur-
`containing enzymes in the CNS [91] In Friedreich ataxia,
`it has been suggested that the decreased levels of frataxin
`and increased iron within mitochondria lead to oxidative
`stress and associated damage of dorsal root ganglion and
`spinal sensory neurons [39, 43].
`
`Aceruloplasminemia
`This autosomal recessive disorder with degeneration of
`the retina and basal ganglia and associated brain iron
`overload results from several loss of function mutations in
`the ceruloplasmin gene [43] and has features of parkin-
`sonism with rigidity and gait disturbance but a lack of
`Parkinson-like tremors. This rare autosomal recessive dis-
`order is estimated to occur at a frequency of 1 per 2 mil-
`
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`lion in Japan [92]. This disorder is associated with several
`mutations in the ceruloplasmin gene. It produces marked
`accumulation of iron in the liver, pancreas, retina, and
`basal ganglia, the latter is thought to contribute to a neu-
`rodegenerative disorder resembling parkinsonism [92].
`CSF from affected patients has shown a threefold elevated
`iron concentration, increased superoxide dismutase activ-
`ity and lipid peroxidation products [59, 93] suggesting a
`direct link between iron overload and oxidative stress as
`one possible causative mechanism.
`
`Hallervorden Spatz
`(neurodegeneration
`syndrome
`Hallervorden-Spatz
`with brain iron accumulation-1, NBIA-1) is a neurodegen-
`erative disorder of childhood associated with iron depos-
`its in the basal ganglia and has been characterized as one
`of the synucleinopathies [94]. The gene responsible for
`NBIA-1 has recently been identified to be the pantothen-
`ate kinase gene (PANK2) that is expressed specifically in
`the brain [95]. Pantothenate kinase is a regulatory enzyme
`that catalyzes the phosphorylation of vitamin B5, panto-
`thenate, but is not directly related to iron metabolism in
`the brain. Thus, an indirect mechanism of secondary
`metabolite accumulation as the cause of basal ganglia iron
`overload in NBIA-1 has been postulated [95].
`
`Neuroferrinopathy
`This dominantly inherited disorder is characterized by
`motor system failure and ferritin-containing inclusions in
`the globus pallidus of the basal ganglia [52]. A mutation
`caused by an insertion of an adenine in the carboxy ter-
`minus of the L chain of ferritin appears to result in a ferri-
`tin molecule with a unique C-terminus. The aberrant fer-
`ritin accumulates in axonal swellings [35] and may be
`involved in the disease pathophysiology. The transport of
`ferritin may be important in the pathophysiology of mice
`lacking IRP2 with neurodegeneration where there is over-
`expression of ferritin H and L chains [52]. Recently, a
`mutation in the iron-responsive element of the Ferritin H
`subunit was associated with tissue iron overload, again
`demonstrating the role of ferritin in inherited iron accu-
`mulation [96].
`
`Other Neurodegenerative Disorders
`Huntington disease and progressive supranuclear palsy
`all show significantly elevated levels of ferritin-associated
`iron in the basal ganglia [72, 73, 78, 79], but the increases
`are present from the onset of the diseases and the relation-
`ship to the origin of these illnesses is uncertain. Multiple
`sclerosis (MS) has been associated with an accumulation
`
`of brain iron [97] although the significance of this finding
`may be only a nonspecific indicator of tissue destruction
`in MS.
`
`Iron Deficiency and CNS Dysfunction
`
`In contrast to CNS iron excess, there have been many
`studies over the years that have examined the issue of iron
`deficiency and CNS dysfunction. The effects of iron defi-
`ciency on the nervous system have been reviewed in the
`past [98, 99] and focused on developmental and intellec-
`tual retardation in experimental animals [100] and on
`developmental and intellectual retardation in humans as
`a consequence of iron deficiency [101, 102]. Neurological
`symptoms in iron-deficient patients have included head-
`aches, papilledema resolving with iron therapy [103], and
`parasthesias. The relationship of intellectual subnormali-
`ty to iron deficiency has been controversial due to the
`complexity of interacting factors [104]. In a study of the
`effects of iron therapy on developmental scores of iron-
`deficient infants, Oski and Honig [105] found improve-
`ment within 1 week of iron supplementation, suggesting
`that elemental Fe deficiency and not the anemia may
`have been responsible for the developmental retardation.
`This could be the result of a lack of functionally critical
`iron-containing proteins such as iron-sulfur proteins, me-
`talloflavoproteins and cytochromes [98]. Studies of iron
`deficiency in animals during development have demon-
`strated low brain iron concentrations and neurobehavio-
`ral abnormalities that persist despite iron treatment after
`weaning [106, 107] suggesting that sufficient brain iron
`content is critical to normal development and behavior
`[107]. Perinatal iron deficiency has also been found to
`decrease cytochrome c oxidase activity in the neonatal rat
`brain hippocampus, demonstrating one mechanism by
`which perinatal iron deficiency reduces neuronal meta-
`bolic activity, especially targeting the brain memory areas
`[108]. Recently, systemic iron deficiency, reduced ferritin
`in the CSF [110] and decreased basal ganglia levels of Fe
`[111] have been documented in some individuals with
`restless legs syndrome suggesting a possible causal link.
`
`Conclusions
`
`Both iron excess and deficiency in the nervous system
`may result in neurodegenerative disorders with motor sys-
`tem and neurobehavioral abnormalities that may have
`devastating or even progressively deteriorating effects
`
`Iron and Neurodegeneration
`
`Dev Neurosci 2002;24:188–196
`
`193
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`Luitpold Pharmaceuticals, Inc., Ex. 2051, P. 6
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`Pharmacosmos A/S v. Luitpold Ex. Pharmaceuticals, Inc., IPR2015-01490
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`upon normal brain function. The progressive disability
`seen in many of these disorders is at present largely incur-
`able, although the promise of advances in human genetics
`and molecular biology hold the best chance of identifying
`the cause and either prevention or effective treatment of
`sporadic and inherited neurodegenerative diseases.
`
`Acknowledgments
`
`This is manuscript number 14971-MEM from the Scripps Re-
`search Institute. Supported by the Skaggs Scholars in Clinical Science
`Program from the Scripps Research Institute and grants DK53505–
`02 and RR00833 from the NIH and by the Stein Endowment Fund.
`
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