Increased iron and free radical generation in preclinical Alzheimer disease and mild cognitive impairment

Abstract

It is now established that oxidative stress is one of the earliest, if not the earliest, change that occurs in the pathogenesis of Alzheimer's disease (AD). Consistent with this, mild cognitive impairment (MCI), the clinical precursor of AD, is also characterized by elevations in oxidative stress. Since such stress does not operate in vacuo, in this study we sought to determine whether redox-active iron, a potent source of free radicals, was elevated in MCI and preclinical AD as compared to cognitively-intact age-matched control patients. Increased iron was found at the highest levels both in the cortex and cerebellum from the pre-clinical AD/MCI cases. Interestingly, glial accumulations of redox-active iron in the cerebellum were also evident in preclinical AD patients and tended to increase as patients became progressively cognitively impaired. Our findings suggests that an imbalance in iron homeostasis is a precursor to the neurodegenerative processes leading to AD and that iron imbalance is not necessarily unique to affected regions. In fact, an understanding of iron deposition in other regions of the brain may provide insights into neuroprotective strategies. Iron deposition at the preclinical stage of AD may be useful as a diagnostic tool, using iron imaging methods, as well as a potential therapeutic target, through metal ion chelators.

Figure 1

Redox metals accumulate within the cortex in preclinical/MCI cases. Cognitively normal patients (n=5) have very low levels (A), while preclinical cases (n=4) (B), and cases with either CDR scores of 0.5 (n-6) (C) or 1.0 (n=5) have substantially higher levels of cellular accumulation of redox metals. Quantification of all the cases in the study of the metal deposition of 5 fields of representatively stained cortical regions, show that metal deposition increases with disease progression and that the control cases have significantly lower levels (* p<0.05, D). Scale bar = 50 μm.

Figure 2

Redox metals accumulate within the cerebellum in preclinical/MCI cases. In preclinical cases (B), CDR=0.5 cases (C), and CDR=1.0 cases (D), redox metals accumulate within spherical structures within the Purkinje cell layer of the cerebellum at much higher levels than normal controls (A). Quantification reveals that only the MCI cases with CDR=0.5 show significantly greater levels of redox metal accumulation than normal controls (*p<0.05; E).

Figure 3

In a representative MCI case, many of the iron-positive structures (A) are associated with glial cells recognized by fluorescence microscopy using GFAP as the antibody (C). B is a merged image showing the iron-stained bodies associated with GFAP-positive glia (arrows). The same structures on adjacent serial sections are labeled using both the detection method with potassium ferrocyanide (D) as well as incubation with only DAB/ H₂O₂ to detect sites of metal-catalyzed redox activity (E). Redox metal positive structures within the cerebellum of an MCI case (F) are sensitive to chelation treatment with EDTA (G) and deferroxamine (H).