Clioquinol

The essential metal iron has long been implicated in the neuronal damage associated with Parkinson's disease. Recent findings show that iron chelation may prevent the reductions in dopamine and motor disturbances associated with this disease, and suggest the need to examine the role of dietary iron and the use of metal chelators in neurodegenerative disorders.

Key words: iron, Parkinson's disease, Alzheimer's disease, dopamine, oxidation, chelation

Whereas the accumulation of iron has been linked to a variety of neurodegenerative disorders including Alzheimer's disease,1 Hallervorden-Spatz syndrome,2 and Huntington's disease,3 there is a particularly strong association between iron and Parkinson's disease. Parkinson's disease (PD), the most common neurodegenerative disorder after Alzheimer's disease, affects approximately 1% of the population of the United States over 60 years of age. It is a progressive disorder characterized by tremor at rest, bradykinesia (slowness of movement), rigidity, postural dysfunction, gait abnormalities (difficultly walking), and loss of balance. These symptoms result from the progressive loss of dopaminergic neurons in the substantia nigra and reduced dopamine production. Dopaminergic neurons in PD patients may also contain Lewy bodies, filamentous deposits in the cytosol comprised largely of the protein [alpha]-synuclein, which have been associated not only with PD but also with several other forms of dementia and AD when found in other regions of the brain.

A possible role for iron in the etiology of PD was hypothesized after several reports were published showing that iron levels were increased in postmortem substantia nigra samples from PD patients.4,5 Recent advances in high-field magnetic resonance imaging (MRI) techniques have allowed investigators to confirm that iron is indeed deposited in the substantia nigra of patients diagnosed with PD.6 Furthermore, use of transcranial ultrasound has suggested that the accumulation of iron may precede the development of Parkinsonian symptoms.7

The mechanisms of iron-mediated damage to nigrostriatal neurons are related, at least in part, to the fact that iron is a significant generator of reactive iron species (ROS). Hydrogen peroxide, a byproduct of the oxidative deamination of dopamine, is converted to hydroxyl radicals in the presence of ferrous iron (Fe^sup 2+^). The resulting oxidative stress can damage cellular lipids, including membrane phospholipids, proteins, and nucleic acids.8 In addition, dopamine and its metabolite salsolinol are both converted into highly neurotoxic compounds by oxidation, rendering dopaminergic neurons particularly vulnerable to ROS-mediated damage. Iron has also been shown to facilitate the fibrillation of human [alpha]-synuclein.9

While the relationship between PD and abnormalities in a variety of iron-related proteins (e.g., lactoferrin and its receptor, melanotransferrin; ceruloplasmin; the divalent cation transporter, DCT1; and neuromelanin)10,11 has been studied, a recent report suggests that sequestration of iron by the iron-binding protein ferritin can prevent death of nigro-striatal neurons in an experimental model of PD.12 Ferritin, the primary iron storage protein, comprises 24 heavy (H) and light (L) subunits. These two subunits, which arise from separate genes on different chromosomes, have distinct functions. The H subunit, which has a molecular weight of 21 kDa, is associated with cellular iron utilization, whereas the L subunit (19 kDa) has been linked to cellular iron storage. Organs with ferritin molecules having a high L:H ratio (such as liver and spleen) participate in iron storage. Cells with a high demand for iron utilization synthesize ferritin composed largely of H subunits. For example, whereas microglia in the central nervous system (CNS) that participate in buffering metal concentrations contain ferritin that comprises mainly L subunits, neurons have a high demand for iron and express H ferritin.13 Oligodendrocytes appear to be the only CNS cell type that has an abundance of both H and L subunits.14 In addition to its role in iron binding and utilization, the H subunit has also been shown to have ferroxidase activity that reduces the potential for iron-mediated damage. Reductions in iron availability result in the association of iron-sensing proteins with an iron-response element (IRE) in the 5' non-coding region of ferritin mRNA. Binding of these proteins, known as IRE-binding proteins (IRE-BP), to the IRE blocks ferritin translation. Increases in cellular iron result in the removal of IRE-BP from the 5'-IRE, permitting ferritin translation and synthesis.

To show that iron chelation in dopaminergic neurons by ferritin H could be neuroprotective, a team of researchers headed by Julie Anderson at the Buck Institute for Age Research generated transgenic mice that expressed the human ferritin H gene. The transgene, which lacked the 5'-IRE, was driven by the rat tyrosine hydroxylase promoter.12 This construct permitted the overexpression of ferritin H in dopaminergic neurons of the substania nigra. MRI, used to detect ferritin-bound iron in the substantia nigra, showed that there was an over 40% increase in signal intensity in transgenic animals compared with wild-type controls, suggesting increased association of iron with the ferritin in the transgenic mice. Furthermore, transgenic animals had more than 20% less available iron. Together these data suggest that not only was ferritin H increased in transgenic animals, but that it was chelating iron in dopaminergic neurons.

Mice were then treated with 1-methyl-4-phenyl-1,2,3,6-tetrapyridine (MPTP) to induce Parkinsonian-like symptoms via selective degeneration of the dopaminergic neurons of the substantia nigra. This experimental model has been shown to be particularly useful in the study of PD because it induces motor symptoms of the disorder that are very similar to those observed in humans. Furthermore, it appears that these symptoms arise as a result of many of the same mechanisms responsible for the human form of this disorder including decreased glutathione (GSH), increased oxidative stress, reduced activity of complex I of the mitochondrial electron transport chain, and significantly decreased dopamine production. Overexpression of ferritin H prevented the decrease in GSH and the increase in oxidative stress seen in MPTP-treated mice compared with wild-type controls. Ferritin also protected dopaminergic neurons from MPTP-mediated death and significantly attenutated the loss of dopamine in the substania nigra. The deleterious effects of MPTP on motor behavior were also prevented by ferritin overexpression.12 This genetic model of iron chelation therefore appears to prevent MPTP-induction of experimental Parkinson's disease.

The next challenge to was to determine if the pharmacologic chelation of iron could also be used to prevent the neuronal damage and symptoms associated with MPTP administration. To answer this question, transgenic and wild-type mice were fed the antibiotic 5-chloro-7-iodo-8-hydroxyquinoline, also known as clioquinol (CQ), which was previously shown to chelate both ferrous and ferric iron.15 Eight weeks of CQ administration reduced iron levels in the substania nigra by 30% and prevented the increases in oxidative stress associated with MPTP treatment. While pre-treatment with CQ did not prevent the loss of dopamine production in MPTP-treated animals, the loss was halved. Furthermore, CQ protected MPTP-treated mice from the motor dysfunction observed in MPTP animals that did not receive the chelator.12

The finding that the chelation of iron by ferritin H and CQ attenuated the effects of MPTP in this experimental model suggests a possible role for iron chelators in the clinical treatment of PD. There are several advantages to the use of CQ over other iron chelators. First, CQ is lipophilic and thus crosses the blood-brain barrier. It reduces brain iron but appears to do so (at least at the reported doses) without compromising normal brain function. Second, CQ reduces brain iron without significantly reducing systemic iron.15 It may therefore be possible to use CQ as a treatment or adjunct to treatment while avoiding side effects such as anemia. Finally, CQ not only has the potential to be effective in the treatment for PD, but may also be useful in the treatment of other neurodegenerative disorders in which iron accumulation has been observed. CQ reduced plaque formation in the transgenic mouse model of Alzheimer's disease16 and appeared to delay the course of disease in a two-year, double-blind, phase-II clinical trail.17 However, caution should be used in interpreting these results both because the clinical trial was a small study and because CQ chelates copper and zinc in addition to iron. The trace metals copper and zinc have both been shown to play a role in the aggregation of [beta]-amyloid.18

Although some side effects have been reported, it may be possible to overcome them. For example, oral CQ reduces both serum and brain levels of vitamin B12.15 This problem was circumvented in human trails by supplying a B12 supplement.17 There was also a report that linked CQ to subacute myelo-optic neuropathy. The clinical trail using CQ to treat Alzheimer's disease used significantly lower doses of CQ and seemed to avoid this dangerous side effect; however, this study was only two years in length. Because PD is a progressive disorder, it is likely that CQ, if used as a therapeutic agent, would need to be administered for life. Future work will need to address the safety of long-term administration of CQ, including its effect on cellular copper and zinc levels, CNS function, and other unintended consequences of long-term chelation therapy.

Recent work has shown that the chelation of iron may protect dopaminergic neurons from the oxidative stress associated with cellular damage, neuronal death, and PD. Protection from MPTP-induced damage was only seen following two months of pretreatment with CQ, however; it remains to be determined whether CQ would be effective in delaying the progression of the disease once the clinical symptoms appear. Future clinical trials are needed to determine if CQ is safe and efficacious in the treatment of PD when used chronically in humans diagnosed with the disorder. Furthermore, the role of dietary iron and iron supplements will need to be addressed so that nutritional recommendations can be made to physicians, PD patients, and the general public.

1. Connor JR, Snyder BS, Beard JL, Fine RE, Mufson EJ. Regional distribution of iron and iron-regulatory proteins in the brain in aging and Alzheimer's disease. J Neurosci Res. 1992;31:327-335.

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3. Bartzokis G, Cummings J, Perlman S, Hance DB, Mintz J. Increased basal ganglia iron levels in Huntington disease. Arch Neurol. 1999;56:569-574.

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5. Sofic E, Riederer P, Heinsen H, et al. Increased iron (III) and total iron content in post mortem substantia nigra of parkinsonian brain. J Neural Transm. 1988; 74:199-205.

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7. Berg D, Becker G, Zeiler B, et al. Vulnerability of the nigrostriatal system as detected by transcranial ultrasound. Neurology. 1999;53:1026-1031.

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9. Berg D, Gerlach M, Youdim MBH, et al. Brain iron pathways and their relevance to Parkinson's disease. J Neurochem. 2001;79:225-236.

10. Biederer P, Sofic WD, Rausch B, et al. Transition metal, ferritin, glutathione, and ascorbic acid in parkinsonian brain. J Neurochem. 1989;52:515-520.

11. Uversky VN, Li J, Fink AL. Metal-triggered structural transformations, aggregation, and fibrillation of human [alpha]-synuclein. A possible molecular link between Parkinson's disease and heavy metal exposure. J Biol Chem. 2001;47:44284-44296.

12. Kaur D, Yanitiri F, Subramanian R, et al. Genetic or pharmacological iron chelation prevent MPTP-induced neurotoxicity in vivo: a novel therapy for Parkinson's disease. Neuron. 2003;37:899-909.

13. Cheepsunthorn P, Palmer C, Connor JR. Cellular distribution of ferritin subunits in postnatal rat brain. J Comp Neurol. 1998;400:73-86.

14. Connor JR, Menzies SL Cellular management of iron in the brain. J Neurol Sci. 1995;134:33-44. 15. Yassin MS, Ekblom J, Xilinas M, Gottfries CG, Oreland L Changes in uptake of vitamin B(12) and trace metals in brain of mice treated with clioquinol. J Neurol Sci. 2000;173:40-44.

16. Cherny RA, Atwood CS, Xilinas ME, et al. Treatment with a copper-zinc chelator markedly and rapidly inhibits beta-amyloid accumulation in Alzheimer's disease transgenic mice. Neuron. 2001;30: 665-676.

17. Masters C. Alzheimer's disease: modulation of the APP/beta-amyloid pathways toward rational therapeutic intervention. 8th International Conference on Alzheimer's Disease and Related Disorders; 2002.

18. Atwood CS, Scarpa RC, Huang X, et al. Characterization of copper interactions with alzheimer amyloid beta peptides: identification of an attomolaraffinity copper binding site on amyloid beta1-42. J Neurochem. 2000;75:1219-1233.

This review was prepared by Cathy W. Levenson, Ph.D., Florida State University, Program in Neuroscience and Department of Nutrition, Food and Exercise Sciences, Tallahassee, FL 32306-4340, USA.

Copyright International Life Sciences Institute and Nutrition Foundation Sep 2003
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