NOVEL MODEL FOR LOADING BRAIN IRON IN MICE: IMPLICATIONS FOR STUDYING AGING AND AMYLOID PATHOLOGY
Open Access
- Author:
- Peters, Douglas Gordon
- Graduate Program:
- Neuroscience
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 06, 2017
- Committee Members:
- Mark David Meadowcroft, Dissertation Advisor/Co-Advisor
James Robert Connor, Committee Chair/Co-Chair
Qing X Yang, Committee Member
Paul Eslinger, Committee Member
Ralph Lauren Keil, Outside Member - Keywords:
- Alzheimer's disease
Iron
Aging
MRI
Histology
Neurodegeneration - Abstract:
- Brain iron accumulation occurs normally in the aging brain to facilitate myelination of white matter (WM); neurotransmitter synthesis; and ATP production. However, mishandling of iron may lead to early senescence and apoptosis because of impaired waste clearance, decreased cellular energy supply and increased oxidative stress. During the aging process, iron accumulates in regionally specific brain areas: basal ganglia, olfactory bulb, hippocampus, and cerebellum that are clinically associated with the manifestations of brain dysfunction in Alzheimer’s disease (AD) and neurodegeneration with brain iron accumulation (NBIA). Both conditions are linked to iron imbalance. Recent work illustrates that amyloid precursor protein (APP), a protein that is cleaved to form amyloid beta (Aβ), is important for iron clearance. Dysfunction of APP may impair normal iron balance, and iron imbalance may increase Aβ plaque genesis associated with AD. This dissertation has explored the effect of dietary iron overload and deficiency in a wildtype control (C57BL6) and humanized APP knock-in mouse models (NL-F and NL-G-F), by using a small lipophilic iron molecule, TMHF. We found TMHF in both the blood and brain of mice after dietary exposure. Chronic TMHF exposure significantly increased brain iron in all animals by 15-35% within the hippocampus, cortex, basal ganglia, and cerebellum. Iron deficiency was only associated with decreased brain iron in the APP mutant mice. In vivo MRI analysis and immunohistological findings support the notion that MRI relaxation (R2) is driven by iron deposition. TMHF sustained the MRI WM volume fraction that decreased over time in the normal brain iron condition. WM loss with aging may propagate free iron increases, contributing to AD Aβ deposition. APP synthesis is a common marker for WM damage and is found to be elevated in AD. This work found Aβ40 clustering and dense Aβ42 deposition near WM tracts. Although Aβ42 saturation is not altered by brain iron load, Aβ42 plaques was denser. In contrast, Aβ40 plaque load was shown to decrease over time with increasing brain iron accumulation. Increased Aβ42/40 ratio drives the proliferation of more hydrophobic oligomers that are believed to be more neurotoxic. The altered Aβ42/40 ratio found in the setting of iron accumulation may differentiate a primitive/diffuse plaque from a senile cored plaque. This altered ratio may also explain the accelerated process of iron accumulation and subsequent amyloid plaque deposition noted in AD, when compared to the normal aging process. ELISA analysis illustrates that TMHF iron linearly increases the iron storage protein, L-ferritin, to facilitate normal handling of elevated iron. Despite the normative response to sequester toxic free iron, TMHF also elevated the protein markers for oxidative stress, inflammation and gliosis associated with AD pathology. TMHF fed animals also developed impaired spatial learning as shown by Barnes maze testing. With respect to iron dyshomeostasis, iron overload with TMHF increased Aβ plaque iron and iron-rich microglia. These iron inclusions may increase neurodegenerative processes during normal aging and disease. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and Perl’s iron stain, we found regional heterogeneity of these iron deposits. The content of iron-rich plaques had a higher Aβ42/40 ratio and presented with more densely packed senile core-like features. IBA1+ microglia were also found to have shorter branches, rounder bodies, and intense iron staining in the TMHF group. An increased phagocytic profile during amyloid deposition with increased brain iron may perturb microglial housekeeping functions in the brain. Taken together, we found that iron overload with TMHF increases brain iron in a physiologically normal and regionally specific manner. Brain iron elevation increases WM volume, but it may also increase inflammation, oxidative stress, and neurotoxicity of Aβ plaques, all of which are linked to memory impairment in normal aging and AD.