AbstractWith an increasingly ageing population, the prevalence of neurodegenerative diseases (NDDs) is set to rise. Diagnosis is still currently limited to clinical assessment and treatment options remain ineffective. Iron is thought to play a role in the progression of NDDs due to increased levels being discovered in the brain regions most affected by the specific disorders and exacerbating the loss of neurons, likely through oxidative stress mechanisms. Therefore, this research investigates magnetic resonance imaging (MRI) relaxometry methods to non-invasively and accurately, detect and quantify iron during neurodegeneration and to understand the role of iron-induced toxicity for neuronal death. This work is the first to link iron assessment using MRI in an in vitro model with ex vivo post-mortem tissue and in vivo assessment of iron, as well as providing further validation of inconclusive and conflicting studies evaluating differences in iron content in neurodegenerative disease pathology.
The accuracy of iron content measured using MRI relaxometry was assessed using a variety of model systems; ranging from the use of agarose standards, post-mortem human brain samples, and an in vivo animal model of iron-toxicity. Agarose standards containing different concentrations of iron demonstrated that iron linearly correlated to R1, R2 and R2*. This was the case for both ferrous and ferric iron forms, however ferric iron had a greater effect than ferrous iron. Human post-mortem brain tissue from the medial temporal gyrus (MTG) was then taken from control and Alzheimer’s disease (AD) subjects and assessed by MRI relaxometry. The MTG is one of the brain regions affected in AD and contains both white (WM) and grey matter (GM). Iron content was determined using synchrotron radiation X-ray fluorescence (SR-XRF) elemental mapping, a quantitative measure of iron content rather than using Perl’s stain, a qualitative method of iron localisation. R2* correlation to iron content was strongest in both control and AD tissue, and was more sensitive within GM than in WM. Subsequent myelin assessment of brain samples using luxol fast blue demonstrated the effect of myelin content on iron-relaxometry correlations. Myelin was detectably lower in AD WM than control WM tissue, and contributed to an improved correlation of iron content to R2* measurements in AD samples. Finally, validation of iron content correlation to MRI relaxometry measurement was performed in vivo using a novel animal model by direct injection of ferric citrate into the mouse hippocampus. R2* and R2’ provided the best method for detection of injected iron ex vivo, however no changes were observed in vivo after 10 days recovery due to conflicting signals from iron content and oedema. Greater levels of neuronal cell death and neuroinflammation were associated with the presence of iron in the brain, confirming the contribution of iron to toxicity.
Overall, this work improves our understanding of the relationship of iron to relaxometry measurement and the impact of myelin on such measurements. The work also confirms that the presence of high amounts of iron in the brain can lead to both neuronal cell death and neuroinflammation.
|Date of Award||2016|
|Supervisor||Po-Wah So (Supervisor) & David Lythgoe (Supervisor)|