Cortical Iron Reflects Severity of Alzheimer's Disease

Abstract

Abnormal iron distribution in the isocortex is increasingly recognized as an in vivo marker for Alzheimer's disease (AD). However, the contribution of iron accumulation to the AD pathology is still poorly understood. In this study, we investigated: 1) frontal cortical iron distribution in AD and normal aging and 2) the relation between iron distribution and degree of AD pathology. We used formalin fixed paraffin embedded frontal cortex from 10 AD patients, 10 elder, 10 middle aged, and 10 young controls and visualized iron with a modified Perl's histochemical procedure. AD and elderly subjects were not different with respect to age and sex distribution. Iron distribution in the frontal cortex was not affected by normal aging but was clearly different between AD and controls. AD showed accumulation of iron in plaques, activated microglia, and, in the most severe cases, in the mid-cortical layers along myelinated fibers. The degree of altered iron accumulations was correlated to the amount of amyloid-β plaques and tau pathology in the same block, as well as to Braak stage (p < 0.001). AD and normal aging show different iron and myelin distribution in frontal cortex. These changes appear to occur after the development of the AD pathological hallmarks. These findings may help the interpretation of high resolution in vivo MRI and suggest the potential of using changes in iron-based MRI contrast to indirectly determine the degree of AD pathology in the frontal cortex.

Keywords: Alzheimer’s disease; iron; magnetic resonance imaging; myelin.

Fig.1

A) Each row represents consecutive sections of frontal cortex stained for iron, myelin (PLP immunohistochemistry), and Aβ immunohistochemistry. Controls show a similar iron distribution, irrespective of age, with increased amounts in the myelin rich areas: white matter and the inner and outer line of Baillarger (arrows). AD cortex (bottom row) shows a diffuse band shaped iron and PLP increase in cortical layer IV/V, extending to the inner part of layer III (arrow, and indicated by lines). B) Higher magnification of A, Meguro iron stain. Controls show evenly distributed iron concentration in myelinated fibers and a good distinction between the lines of Baillarger. AD cortex shows diffuse iron increase in the middle cortical layers (arrows), as well as iron positive plaques (asterisk), also concentrated in the middle cortical layers.

Fig.2

A) Iron accumulation in plaques (left) and microglia (middle) (both Meguro stain). On the right double labelling of iron positive cells by ferritin (brown) and MHCII (red). B) Band shaped increase of iron and PLP labelling in AD (right column) compared to an old control (left column). The right column illustrates that the normal distinction of the lines of Baillarger (arrow) is blurred by a band shaped iron/PLP increase in cortical layer IV/V extending to layer III (arrow). This is due to darker and thicker iron/PLP labelling of more crowded myelinated fibers (bottom panel).

Fig.3

Semi-quantitative correlation between iron accumulation and Braak stage (A), Aβ plaques (B), and tau pathology (C). Iron accumulation: 0: no iron accumulation, 1: low, 2: intermediate and 3: high iron accumulation in plaques and microglia, 4: high iron accumulation in plaques and microglia and a band shaped iron/PLP increase in the middle cortical layers. For grading definitions, see Methods. All three correlations were statistically significant (p < 0.001).

Fig.4

Iron accumulation in AD, related to tau pathology and Aβ plaques. A) Both patients (first and second row) show similar amounts of Aβ plaques but only the tissue with the most severe tau pathology shows a diffuse band shaped increase of iron/PLP labelling in the middle cortical layers (arrows). B) 20 μm sections of the same blocks. Cortex with the most severe tau pathology shows more iron accumulation in plaques and microglia.

Fig.5

Schematic representation of the observed combinations of AD pathology and iron accumulation in frontal cortex. A) Aging without plaques or tau pathology. B) Controls with little diffuse plaques without iron accumulation. C, D) Controls with increasing iron accumulation in plaques and microglia depending on the amount of plaques. E) AD with high Aβ plaque load and diffuse limited increase of tau pathology with increased iron accumulation. F-H) Similar high plaque load with increasing tau pathology and iron accumulation.