. In situ characterization and mapping of iron compounds in Alzheimer's disease tissue. J Alzheimers Dis. 2005 Aug;7(4):267-72. PubMed.

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  1. One could argue that the dismetabolism of iron, whether throughout the whole body or only in the brain, is a recurring theme in neurodegenerative disease. Evidence of such has been available for several decades and has taken many forms, including techniques allied to quantitative determinations of biogenic iron associated with pathogenic structures such as proteinaceous plaques in AD and MS and Lewy bodies in PD.

    Iron is no stranger to biochemical evolution, and its deposition in the brain may well be an early adaptation to excess free iron, for example, leading to the essential function of biogenic magnetite in magnetotactic bacteria (see Scheffel et al., 2006). We do not know why iron is deposited in nervous tissue in neurodegenerative diseases, nor do we know very much about the forms of iron that are deposited. The latter has only recently been the subject of intense investigation. Jon Dobson (a colleague of mine at Keele), Mark Davidson, and co-workers are leading exponents of this field.

    Their application of high resolution synchrotron techniques (Collingwood et al., 2005), as well as highly sensitive interference magnetometry, SQUID (Hautot et al., 2003), has moved our understanding of the structures of brain iron deposits into the twenty-first century. Only now are we beginning to differentiate these deposits based on their primary biogenic structures. The latter is important for a number of reasons. For example, we may be able to use early deposition of iron in a particular form, for example, magnetite, as a signature for an ongoing disease process. The ability to "see" iron spectroscopically with noninvasive in-vivo imaging techniques could, potentially at least, revolutionize diagnosis of chronic neurodegenerative conditions such as AD and MS.

    Equally important is that knowing the form of deposited iron in a particular condition could give an important insight into disease etiology. I am convinced that the form of iron in, for example, senile plaques in AD, is dictated by the "protein" environment that "templates" its deposition. For example, the protein amyloid-β is only involved in the deposition; it is not part of the biogenic product (though some could conceivably be occluded within the matrix of the final product). The biogenic product is extremely insoluble in comparison to any putative protein/peptide-iron complexes.

    Thus, the disease process dictates the form of deposited iron. If this were the end of the story, then probably the brain would be sufficiently robust to cope with the excess iron using evolutionarily conserved mechanisms. However, the additional presence of aluminum in the brain upsets the apple-cart (see Khan et al., 2006). Aluminum, most likely as an aluminum superoxide semi-reduced radical ion (AlO2•2+), is able to reduce Fe3+ and thereby delay its deposition as biogenic iron. It is perhaps by this route that iron remains active as an oxidant in the vicinity of senile plaques, etc., and Fe2+ promotes the oxidative damage that is characteristic of neurodegenerative diseases.

    References:

    . An acidic protein aligns magnetosomes along a filamentous structure in magnetotactic bacteria. Nature. 2006 Mar 2;440(7080):110-4. PubMed.

    . In situ characterization and mapping of iron compounds in Alzheimer's disease tissue. J Alzheimers Dis. 2005 Aug;7(4):267-72. PubMed.

    . Preliminary evaluation of nanoscale biogenic magnetite in Alzheimer's disease brain tissue. Proc Biol Sci. 2003 Aug 7;270 Suppl 1:S62-4. PubMed.

    . Redox cycling of iron by Abeta42. Free Radic Biol Med. 2006 Feb 15;40(4):557-69. PubMed.

  2. Imagine Imaging Iron in Alzheimer Disease
    The early detection and diagnosis of Alzheimer disease (AD) is an extremely active area since it is likely to provide better therapeutic opportunities for patients both in the very earliest stages of disease as well as those at risk of developing disease. To date, the majority of studies have focused on structural changes (MRI) or metabolic analysis (PET) that likely represent downstream consequences of neuronal atrophy rather than initiators of disease. More recently, a great deal of attention has been given to the imaging of amyloid-β deposits using the Pittsburgh compound (PIB). However, while amyloid-β deposits are pathognomonic for AD, their high prevalence in normal aged individuals makes diagnosis problematic in the absence of clinical symptoms. On the other hand, oxidative stress, which is known to predate amyloid-β deposits (Odetti et al., 1998; Nunomura et al., 2001; Pratico et al., 2001; Pratico et al., 2002), may represent a superior diagnostic target. Since imbalances in iron homeostasis appear to be intimately related to oxidative stress (Smith et al., 1997; Sayre et al., 2000), the studies by Collingwood and Dobson (Collingwood et al., 2005; Collingwood and Dobson, 2006) are likely to be of paramount importance for future imaging studies to capture individuals at greatest risk of progressing to occult disease.

    References:

    . Mapping and characterization of iron compounds in Alzheimer's tissue. J Alzheimers Dis. 2006 Nov;10(2-3):215-22. PubMed.

    . In situ characterization and mapping of iron compounds in Alzheimer's disease tissue. J Alzheimers Dis. 2005 Aug;7(4):267-72. PubMed.

    . Oxidative damage is the earliest event in Alzheimer disease. J Neuropathol Exp Neurol. 2001 Aug;60(8):759-67. PubMed.

    . Early glycoxidation damage in brains from Down's syndrome. Biochem Biophys Res Commun. 1998 Feb 24;243(3):849-51. PubMed.

    . Increase of brain oxidative stress in mild cognitive impairment: a possible predictor of Alzheimer disease. Arch Neurol. 2002 Jun;59(6):972-6. PubMed.

    . Increased lipid peroxidation precedes amyloid plaque formation in an animal model of Alzheimer amyloidosis. J Neurosci. 2001 Jun 15;21(12):4183-7. PubMed.

    . In situ oxidative catalysis by neurofibrillary tangles and senile plaques in Alzheimer's disease: a central role for bound transition metals. J Neurochem. 2000 Jan;74(1):270-9. 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. It is well known that the brain contains an intriguing and distinctive pattern of iron deposition (1) and there is a rapidly growing interest in possible roles for iron metabolism in the pathogenesis and possible therapy of neurodegenerative diseases including Alzheimer disease (2). It is generally thought that the involvement of iron in these diseases involves the exacerbation of oxidative stress (3). However, it has been difficult to establish clear-cut mechanisms linking iron storage to disease progression. Many discussions of brain iron have focused on the compound ferrihydrite as the mineral core of the iron-protein complex ferritin. However, these tend to be based not on direct studies of brain iron, but on extrapolations from other tissues (both normal and iron-overloaded), particularly liver and spleen (4-7). The authors of the papers I discuss here (8,9) provide evidence for magnetite as one of the components of brain iron in Alzheimer disease (AD), and they point out that "the oxidative chemistry cannot be understood unless we first understand which species are present." Studies such as theirs are welcome, as they contribute to an understanding of how iron metabolism in the central nervous system may exhibit features not present in the rest of the body.

    In the first paper under review (8), the authors provide important new information on the relation of brain iron to AD derived from x-ray analyses; in the second paper (9) these findings are compared with those from a number of previous studies. The first paper reports the modification of a powerful synchrotron radiation source at the Argonne National Laboratory to permit the focusing of a nominally 400 µm x-ray beam to the order of 3 μm while maintaining the majority of the beam intensity. Using 50 μm brain slices (superior frontal gyrus) from an AD autopsy, the researchers scanned the samples for iron by x-ray fluorescence, first at low resolution (100 μm pixels) to study iron signals from a relatively large area (6 mm x 4 mm), and then at high resolution (~5 μm) to study regions where this signal was found to be exceptionally high. Steps were taken during tissue fixation to avoid changes in the chemical and charge state of the iron and to prevent contamination with exogenous magnetic particles. A K-edge XANES (x-ray absorption near edge spectroscopy) profile was obtained and compared with previously obtained spectra from a number of iron-containing compounds (hemoglobin, ferritin, magnetite, etc.). This analysis suggested that the regions of high iron signal contained predominantly magnetite or a combination of magnetite and ferritin. Slices from brain adjacent to the sections studied by x-ray analysis were subjected to conventional histopathology analysis to demonstrate the presence of AD.

    At least 15 different forms of iron oxide have been characterized (10), and several of these have biological implications. For the most part, based on studies of normal and iron-overloaded liver and spleen, it has usually been assumed that the predominant form of brain iron is some version of ferrihydrite present as mineralized cores associated with the protein ferritin or the related compound, hemosiderin. Hemosiderin is usually considered to be an insoluble ferritin degradation product, and it is difficult to characterize precisely (11). The details of its structure may vary from tissue to tissue. Although hemosiderin is often considered to be an important form of brain iron storage, it is seldom directly demonstrated in brain. Within the last few weeks, a new book devoted to iron and neurodegenerative diseases has appeared which questions whether this compound is even present in brain (12). Recent studies have applied new technologies based on Mossbauer spectroscopy, SQUID magnetometry, and electron microscopy to brain hemosiderin and related compounds (13-18). In addition to the current studies, these have raised a number of alternatives to the chemistry of brain iron storage as being predominantly a matter of ferritin, hemosiderin, and ferrihydrite. These alternatives include iron in neuromelanin (19) and bound to lipofuscin (20). The x-ray techniques used in the Collingwood et al. studies do not provide information concerning the possible association of the magnetite detected with proteins or other macromolecules.

    image

    New Iron Detection Technique
    A growing number of AD researchers would love to find out just what role iron plays in neurodegenerative diseases including Alzheimer's. However, they've been hampered to date by a lack of techniques for defining which forms of iron occur in human and AD brain, and exactly where. [Figure courtesy of Joanna Collingwood]

    The question of the extent to which iron storage in the brain involves ferrihydrite, magnetite, neuromelanin, and other compounds is important to the use of magnetic resonance imaging to follow disease-related changes in this storage (21,22). In our own work, we have used T2-mapping of high-field MRI to produce evidence of increased iron deposition in the hippocampus and other brain regions of AD patients compared to age-matched controls (23).

    Iron affects the MR image by producing microscopic variations in magnetic field strength, which lead to a shortening of the transverse relaxation time (known as T2 or T2*) of water molecules in the vicinity of the iron deposits. On T2-weighted images, this results in a distinctive loss of signal (hypointensity) in the vicinity of iron deposits. The extent of this effect depends on the amount of iron present, but it is also very dependent on the magnetic properties of the compound in which the iron is contained. For example, magnetite is ferromagnetic, and this implies that, on an atom-by-atom basis, iron in this compound is expected to be more effective in producing endogenous contrast in MRI than a similar amount of iron in ferrihydrite. Theoretically, at least, conversion of iron from one chemical compound to another could be as effective in modifying MRI results as an increase in the total iron present (24). This underscores the importance of the Collingwood et al. studies in investigating the form of iron actually present in the diseased brain. A number of studies have found evidence for increased iron deposition in a wide range of neurodegenerative diseases.

    The smallest practical voxel elements in brain MRI of living subjects are on the order of 1 mm,3 although much smaller voxels can be imaged in postmortem tissue. These limitations on the resolution of MRI suggest that very small regions of magnetite accumulation, such as seen in the present studies (diameter ~5 μm), may not be detected in MRI unless their density is such that several of them occur within a single voxel. New MRI techniques are being developed to supplement traditional T2- and T2*-weighting and these may extend the ability of MRI to detect and quantify small regions of iron storage (25,26).

    Collingwood et al. have provided new technology and information on the chemical and physical properties of brain iron in AD. Such information, I hope, will eventually lead to a deeper understanding of the role of iron in this important disease and illuminate its potential in the areas of diagnosis and treatment.

    The work of our group is sponsored in part by the Department of the Army, U.S. Army Medical Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21701-5014 under contract number W81XWH-05-0331. It does not represent the official government position or policy and no official endorsement should be inferred.

    See also:

    St Pierre TG, Webb J, Mann S. Ferritin and hemosiderin: structural and magnetic properties of the iron core. In: Mann S, Webb J, Williams RJP, eds. Biomineralization: chemical and biochemical perspectives. Weinheim; New York: VCH, 1989:295-344.

    Artymiuk PJ, Bauminger ER, Harrison PM, et al. Ferritin: a model system for iron biomineralization. In: Frankel RB, Blakemore RP, eds. Iron biominerals. New York: Plenum Press, 1991.

    Powell AK. Ferritin. Its mineralization. In: Sigel A, Sigel H, eds. Iron transport and storage in microorganisms, plants, and animals. New York: Marcel Dekker, 1998:515-561.

    Crichton RR. Inorganic biochemistry of iron metabolism: from molecular mechanisms to clinical consequences. 2nd ed. Chichester; New York: Wiley, 2001.

    Crichton RR, Ward RJ. Metal-based neurodegeneration: from molecular mechanisms to therapeutic strategies. Chichester; Hoboken, NJ: J. Wiley & Sons, 2006, p. 5.

    References:

    . A brief history of brain iron research. J Neurol Sci. 2003 Mar 15;207(1-2):95-7. PubMed.

    . Iron, brain ageing and neurodegenerative disorders. Nat Rev Neurosci. 2004 Nov;5(11):863-73. 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.

    . In situ characterization and mapping of iron compounds in Alzheimer's disease tissue. J Alzheimers Dis. 2005 Aug;7(4):267-72. PubMed.

    . Mapping and characterization of iron compounds in Alzheimer's tissue. J Alzheimers Dis. 2006 Nov;10(2-3):215-22. PubMed.

    . Hemosiderin: nature, formation, and significance. Int Rev Exp Pathol. 1980;22:193-225. PubMed.

    . Magnetic properties of human liver and brain ferritin. Eur Biophys J. 1999;28(3):263-7. PubMed.

    . The structure of ferritin cores determined by electron nanodiffraction. J Struct Biol. 2000 Sep;131(3):210-6. PubMed.

    . Electron nanodiffraction and high-resolution electron microscopy studies of the structure and composition of physiological and pathological ferritin. J Struct Biol. 2004 Aug;147(2):166-78. PubMed.

    . Looking for biogenic magnetite in brain ferritin using NMR relaxometry. NMR Biomed. 2005 Nov;18(7):469-72. PubMed.

    . Electron tomography of degenerating neurons in mice with abnormal regulation of iron metabolism. J Struct Biol. 2005 May;150(2):144-53. PubMed.

    . Study of the localization of iron, ferritin, and hemosiderin in Alzheimer's disease hippocampus by analytical microscopy at the subcellular level. J Struct Biol. 2006 Jan;153(1):42-54. PubMed.

    . Neuromelanin of the substantia nigra: a neuronal black hole with protective and toxic characteristics. Trends Neurosci. 2003 Nov;26(11):578-80. PubMed.

    . Abnormal iron deposition associated with lipid peroxidation in transgenic mice expressing interleukin-6 in the brain. J Neuropathol Exp Neurol. 1998 Mar;57(3):268-82. PubMed.

    . High-field magnetic resonance imaging of brain iron: birth of a biomarker?. NMR Biomed. 2004 Nov;17(7):433-45. PubMed.

    . Imaging iron stores in the brain using magnetic resonance imaging. Magn Reson Imaging. 2005 Jan;23(1):1-25. PubMed.

    . Looking for biogenic magnetite in brain ferritin using NMR relaxometry. NMR Biomed. 2005 Nov;18(7):469-72. PubMed.

    . Relaxometry and magnetometry of ferritin. Magn Reson Med. 1998 Aug;40(2):227-35. PubMed.

    . Susceptibility weighted imaging (SWI). Magn Reson Med. 2004 Sep;52(3):612-8. PubMed.

  4. A number of studies demonstrate that iron and aluminum are co-deposited in the brains of Alzheimer patients (1) and that the metals interact in enhancement of oxidation. Walton has developed a method of staining aluminum in hippocampal neurons in humans with and without AD (2). Higher levels of the metal were associated with sufficient density of neurofibrillary tangles to kill brain cells by enucleation. One looks forward to a future multi-metal study that compares the location of iron and aluminum in the brain, and compares their interaction.

    References:

    . A laser microprobe mass analysis of brain aluminum and iron in dementia pugilistica: comparison with Alzheimer's disease. Eur Neurol. 1997;38(1):53-8. PubMed.

    . Aluminum in hippocampal neurons from humans with Alzheimer's disease. Neurotoxicology. 2006 May;27(3):385-94. PubMed.

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