Burdette SC, Lippard SJ.
Meeting of the minds: metalloneurochemistry.
Proc Natl Acad Sci U S A. 2003 Apr 1;100(7):3605-10.
Please login to recommend the paper.
To make a comment you must login or register.
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.
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.
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.
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.
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.