A large, collaborative group led by Julie Andersen at the Buck Institute for Research in Aging, Novato, California, demonstrates that lowering the bioavailability of iron-both systemically and in dopaminergic neurons-protects mice against chemically induced Parkinson's disease (PD). Published in the March 27 Neuron, this work supports previous studies implicating the metal in the etiology of PD.

First author Deepinder Kaur and colleagues used two approaches to reduce the amount of iron available to neurons in the substantia nigra (SN), the part of the brain most affected by PD. First, the authors made transgenic mice which express large amounts of the iron storage protein ferritin in dopaminergic neurons of the SN; ferritin is a magnet for iron, sequestering upwards of 4,500 molecules of the atom per protein. When Kaur and colleagues exposed these mice to the chemical MPTP, which induces Parkinson's-like neurodegeneration, the animals were protected. In contrast to wild-type mice, which lose about 30 percent of SN dopaminergic neurons in response to MPTP, there were no significant losses of neurons in the transgenic mice. Second, Kaur et al. used the iron chelator clioquinol to achieve a systemic reduction in reactive iron. Clioquinol reduced the amount of free iron in the substantia nigra by about 30 percent, and also prevented loss of dopaminergic neurons in response to MPTP. Furthermore, in both clioquinol-treated and transgenic animals, MPTP-associated loss of motor activity was attenuated by 30 and 60 percent, respectively.

Iron is thought to mediate oxidative damage by catalyzing the conversion of superoxide-a reactive form of oxygen formed during normal respiration-to the hydroxyl radical. The latter is even more reactive, making and breaking bonds in proteins that can render them inactive. By measuring such protein oxidation, Kaur and colleagues show that clioquinol prevents MPTP-induced oxidative stress.

The chelator has also shown promise for treatment for other neurodegenerative diseases. Ashley Bush at Massachusetts General Hospital in Charlestown and colleagues recently showed that it can reduce amyloid burden in mouse models of Alzheimer's disease (see ARF related news story), probably by virtue of its ability also to chelate copper and zinc atoms, which contribute to formation of amyloid-β plaques (see ARF related news story). A phase 2 trial has been completed in Australia to assess its usefulness for human AD (Masters, 2002). However, as Greg Cole of the University of California in Los Angeles points out in an accompanying preview article, results from the trial are still being analyzed. The drug is not without its drawbacks, having been removed from the market after linkage to thousands of cases of subacute myeloneuropathy in Japan. Nevertheless, Kaur et al. find no toxic effect in mice, suggesting that, at least for the doses required to ameliorate symptoms of Parkinson's disease, clioquinol may prove safe and effective.

Meanwhile, Shawn Burdette and Stephen Lippard from MIT draw attention to molecular advances and future directions of what they term "metalloneurochemistry," the study of metal ions in the brain and nervous system. There are major advances to be made at the intersection of bioinorganic chemistry and neurobiology, they remind us. Zinc, for example, is bound by a variety of proteins, including metallothioneins and membrane-bound transporters, which may play important roles in neurodegenerative diseases. For example, Lee et al. recently reported that ablating a version of the zinc transporter reduces the formation of amyloid plaques in transgenic mice expressing human AβPP (see ARF related news story); in Alzheimer's patients, levels of the protein metallothionein III, which has multiple zinc binding sites, are reduced.

Burdette and Lippard also discuss the role of the other metals and advances in development of fluorescent metal sensors. They call on bioinorganic chemists to enter this area of research to make new inroads into neurodegeneration. Their review appears in the March 24 PNAS early online edition.—Tom Fagan

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  1. The very interesting review article by Burdette and Lippard addresses recent advances in understanding the molecular mechanisms of metal ion function in the nervous system, including the roles of potassium, calcium, and particularly zinc in various neuronal processes. The authors propose a new term, "metalloneurochemistry" to encompass this exciting area of biological research.

    One particular metal that I feel receives short shrift in the review article, however, is iron, both the part it plays in normal neuronal function and how imbalances in iron homeostasis can lead to neurological dysfunction. The authors mention it only in passing, and only in relation to its regulation of levels of the iron-storage protein ferritin and the iron-uptake molecule transferrin. Iron is an essential element for several normal metabolic functions in the brain, including as a co-factor for tyrosine hydroxylase, the rate-limiting enzyme in synthesis of the neurotransmitter dopamine (for review, see Yantiri et al., 2001; Kaur and Andersen, 2002). Deficiencies in iron during neurodevelopment are known to lead to neurobehavioral dysfunction (Connor et al; 1995). Conversely, increases in brain iron have been associated with several neurodegenerative conditions, including Parkinson's, Alzheimer's, and Huntington's disease, progressive supranuclear palsy, aceruloplasminemia, and Hallervorden-Spatz, to name but a few (e.g., Dexter et al., 1991, Connor et al., 1992, Smith et al., 1997, Gitlin et al., 1998, Janetzky et al., 1997).

    Building on past work in the field, recent evidence from our laboratory suggests that genetic and/or pharmacological binding of excess iron in the brain in a manner that is well-tolerated by mammalian subjects, including humans, may be a possible preventative in Parkinson's disease (Kaur et al., 2003). This challenges the view that iron accumulation is a late-stage, irreversible event in the disorder, and gives hope that its regulation can be used for the preventative treatment of several neurological conditions.

    See also:

    Janetzky B, Reichmann H, Youdim MB, and Reiderer P. 1997. In Mitochondria and Free Radicals in Neurodegenerative Diseases, F. Beal, N. Howell, and I. Bodis-Wollner, eds. (Wiley-Liss Inc), pp 407-421.

    References:

    . Glutamyl cysteine synthetase catalytic and regulatory subunits localize to dopaminergic nigral neurons as well as to astrocytes. J Neurosci Res. 2001 Apr 15;64(2):203-6. PubMed.

    . Ironing out Parkinson's disease: is therapeutic treatment with iron chelators a real possibility?. Aging Cell. 2002 Oct;1(1):17-21. PubMed.

    . Alterations in the levels of iron, ferritin and other trace metals in Parkinson's disease and other neurodegenerative diseases affecting the basal ganglia. Brain. 1991 Aug;114 ( Pt 4):1953-75. PubMed.

    . Regional distribution of iron and iron-regulatory proteins in the brain in aging and Alzheimer's disease. J Neurosci Res. 1992 Feb;31(2):327-35. PubMed.

    . A quantitative analysis of isoferritins in select regions of aged, parkinsonian, and Alzheimer's diseased brains. J Neurochem. 1995 Aug;65(2):717-24. PubMed.

    . Iron accumulation in Alzheimer disease is a source of redox-generated free radicals. Proc Natl Acad Sci U S A. 1997 Sep 2;94(18):9866-8. PubMed.

    . Aceruloplasminemia. Pediatr Res. 1998 Sep;44(3):271-6. PubMed.

    . Genetic or pharmacological iron chelation prevents MPTP-induced neurotoxicity in vivo: a novel therapy for Parkinson's disease. Neuron. 2003 Mar 27;37(6):899-909. PubMed.

  2. Chelation, Metals and Parkinson’s Disease
    This paper by Julie Andersen and colleagues highlights the importance of iron in the pathogenesis of Parkinson’s disease (PD). Reduction of reactive iron in the brains of mice either by genetic (overexpression of ferritin) or by pharmacological (metal ion chelation) intervention protected against the Parkinson-inducing effects of MPTP administration. The beneficial actions of clioquinol on the substantia nigra were attributed to a decrease in bioavailable iron and a decrease in oxidative stress. These results suggest the chelation of reactive iron to be a promising therapy for this disease. Further studies will no doubt determine if iron chelation can slow or stabilize disease progression, or its prophylactic use can offset the disease. Iron chelation may, however, only be palliative, since such therapies are unlikely to attack the underlying cause of the disease, i.e., what causes the accumulation of iron in the first place.

    The use of clioquinol to decrease the toxic effects of redox metal ions will also be of great interest to Alzheimer’s disease, since both redox metal ions and oxidative stress have been implicated in the pathogenesis of the disease (Smith et al., 1997). Although the importance of each of the metal ions (iron, copper, and zinc) implicated in the disease process remains to be clarified, the administration of clioquinol to AβPP transgenic mice (Cherny et al., 2001) led to a reduction in Aβ load, reflecting on the importance of copper and zinc in the formation of pathological lesions in AβPP-transgenic mice (these metals having the highest affinity for Aβ; see Atwood et al., 2000). The study by Andersen and colleagues further suggests that clioquinol will be efficient in chelating iron from the AD brain. Taken together, these studies indicate that the use of clioquinol in AD patients could potentially neutralize copper, zinc, and iron, and prevent the oxidative stress caused by these metals. Thus, a single drug could alleviate toxicity by covering many bases in one hit.

    References:

    . Characterization of copper interactions with alzheimer amyloid beta peptides: identification of an attomolar-affinity copper binding site on amyloid beta1-42. J Neurochem. 2000 Sep;75(3):1219-33. PubMed.

    . Treatment with a copper-zinc chelator markedly and rapidly inhibits beta-amyloid accumulation in Alzheimer's disease transgenic mice. Neuron. 2001 Jun;30(3):665-76. PubMed.

    . Genetic or pharmacological iron chelation prevents MPTP-induced neurotoxicity in vivo: a novel therapy for Parkinson's disease. Neuron. 2003 Mar 27;37(6):899-909. PubMed.

    . Iron accumulation in Alzheimer disease is a source of redox-generated free radicals. Proc Natl Acad Sci U S A. 1997 Sep 2;94(18):9866-8. PubMed.

  3. Chelation as a Therapeutic for Neurodegenerative Diseases
    In this study by Kaur and colleagues, transgenic mice generated by the embryonic injection of a human H ferritin gene construct driven by the rat tyrosine hydroxylase promoter were shown to be less susceptible to MPTP-induced neurotoxicity. The protective effects of H ferritin were demonstrated by comparing the percentage change in reactive oxygen species levels, glutathione (GSH) levels, substantia nigra (SN) cell counts, and levels of striatal dopamine and its metabolites in transgenic vs. wild-type mice after acute MPTP administration. The authors speculate that the protective effect of H ferritin is due to this molecule’s ferroxidase activity, through which it is able to convert harmful ferrous iron to the unreactive ferric form and subsequently retard the iron-catalyzed oxidative damage found in Parkinson’s disease. To support this hypothesis, the investigators preformed a parallel study in which wild-type mice were orally pretreated with a pharmacological metal chelating agent, clioquinol (CQ), and then subjected to the same acute MPTP administration. Similar quantitative analysis preformed on these mice confirmed that the administration of a pharmacological chelator demonstrates protective effects similar to the H ferritin transgenic mice after acute MPTP administration.

    The data obtained from the parallel experiments presented in this paper strongly support the possibly protective role of iron chelators in Parkinson’s disease. However, the data from the H ferritin transgenic mice and the CQ-pretreated mice do not precisely correspond with one another. According to Figure 3, the transgenic mice appear to be completely unaffected by the acute administration of MPTP in all of the quantitative analysis performed. In Figure 5, the CQ-pretreated mice also appear to be unaffected by MPTP administration in regard to their 4HNE-protein conjugate levels, carbonyl content and total GSH levels. However, decreases in the levels of striatal dopamine and SN neuronal numbers after MPTP administration are not completely attenuated in the CQ-pretreated mice as is reported in the H ferritin transgenic mice. Although the authors discuss the basic nature of CQ as a general metal chelator, there is no discussion regarding what possibilities could account for discrepancies between the level of MPTP-protection demonstrated by the transgenic mice and that of the CQ-pretreated mice.

    As the authors state, increases in reactive iron in the brain are not specific to Parkinson’s disease, but are also shown to be involved in Huntington’s disease and Alzheimer’s disease, as well as a number of other neurodegenerative disorders. The role of iron homeostatsis in several neurodegenerative diseases signifies the importance of this of research and suggests a variety of possible clinical applications for metal chelators in the prevention of neurodegenerative diseases. This type of clinical application has been confirmed in similar studies by Rottkamp et al., 2001 in which amyloid-β toxicity is shown to be significantly attenuated by treating Aβ with the iron chelator deferoxamine. Kaur et al., 2003 have laid the groundwork for the further investigation of CQ as an effective treatment for Parkinson’s disease and possibly other neurodegenerative diseases.

    References:

    . Redox-active iron mediates amyloid-beta toxicity. Free Radic Biol Med. 2001 Feb 15;30(4):447-50. PubMed.

    . Genetic or pharmacological iron chelation prevents MPTP-induced neurotoxicity in vivo: a novel therapy for Parkinson's disease. Neuron. 2003 Mar 27;37(6):899-909. PubMed.

  4. The article by Anderson and colleagues elegantly combines genetic and pharmacological approaches to argue for a primary role of iron in Parkinson's neurodegeneration in vivo. The authors suggest that chelators of iron abrogate cell death due to inhibition of Fenton chemistry and suppression of hydroxyl radical formation. However, the ability of hydroxyl radicals to interact with all biomolecules at diffusion-limited rates makes it unlikely that hydroxyl radicals could act as a specific mediators of cellular damage. In other words, since there are an infinite number of hydroxyl radical "sinks" in the cell, the notion that such radicals could actually damage a single cellular constituent suficiently to induce cell dysfunction seems unlikely. Moreover, a large number of putative mechanisms exist for cellular protection by iron chelators: 1) direct scavenging of radicals; 2) inhibiting the iron-dependent enzyme lipoxygenase; 3) slowing the formation of hypohalous acids by competing with peroxidases; 4) inhibiting cell cycle progression; and 5) activating the transcription factor involved in hypoxic adaptation, HIF-1. It would have been helpful if Anderson and colleagues had examined binding of the RNA-binding protein IRP to the Iron regulator element. This would provide some insight into whether iron is depleted to below physiological levels in the substantia nigra by H-ferritin or clioquinol. As low iron in the nigra has been associated with restless legs syndrome, this information is of more than academic interest.

    References:

    . Protection from oxidative stress-induced apoptosis in cortical neuronal cultures by iron chelators is associated with enhanced DNA binding of hypoxia-inducible factor-1 and ATF-1/CREB and increased expression of glycolytic enzymes, p21(waf1/cip1), and ery. J Neurosci. 1999 Nov 15;19(22):9821-30. PubMed.

    . Ironing-out mechanisms of neuronal injury under hypoxic-ischemic conditions and potential role of iron chelators as neuroprotective agents. Antioxid Redox Signal. 2000 Fall;2(3):421-36. PubMed.

References

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Further Reading

Papers

  1. . The effect of beta-amyloid on neurons and the influence of glucocorticoid and age on such effect. J Huazhong Univ Sci Technolog Med Sci. 2002;22(3):250-2. PubMed.

Primary Papers

  1. . Genetic or pharmacological iron chelation prevents MPTP-induced neurotoxicity in vivo: a novel therapy for Parkinson's disease. Neuron. 2003 Mar 27;37(6):899-909. PubMed.
  2. . Ironic fate: can a banned drug control metal heavies in neurodegenerative diseases?. Neuron. 2003 Mar 27;37(6):889-90. PubMed.
  3. . Meeting of the minds: metalloneurochemistry. Proc Natl Acad Sci U S A. 2003 Apr 1;100(7):3605-10. PubMed.