Quantification of different iron forms in the aceruloplasminemia brain to explore iron-related neurodegeneration

IntroductionAceruloplasminemia is an ultra-rare neurodegenerative disorder associated with massive brain iron accumulation. It is unknown which molecular forms of iron accumulate in the brain of patients with aceruloplasminemia. As the disease is associated with at least a fivefold increase in brain iron concentration compared to the healthy brain, it offers a unique model to study the role of iron in neurodegeneration and the molecular basis of iron-sensitive MRI contrast.MethodsThe iron-sensitive MRI metrics inhomogeneous transverse relaxation rate (R2*) and magnetic susceptibility obtained at 7T were combined with Electron Paramagnetic Resonance (EPR) and Superconducting Quantum Interference Device (SQUID) magnetometry to specify and quantify the different iron forms per gram wet-weight in a post-mortem aceruloplasminemia brain, with focus on the basal ganglia, thalamus, red nucleus, dentate nucleus, superior-and middle temporal gyrus and white matter. MRI, EPR and SQUID results that had been previously obtained from the temporal cortex of healthy controls were included for comparison.ResultsThe brain iron pool in aceruloplasminemia consisted of EPR-detectable Fe3+ ions, magnetic Fe3+ embedded in the core of ferritin and hemosiderin (ferrihydrite-iron), and magnetic Fe3+ embedded in oxidized magnetite/maghemite minerals (maghemite-iron). Of all the studied iron pools, above 90% was made of ferrihydrite-iron, of which concentrations up to 1065 µg/g were detected in the red nucleus. Although deep gray matter structures in the aceruloplasminemia brain were three times richer in ferrihydrite-iron than the temporal cortex, ferrihydrite-iron in the temporal cortex of the patient with aceruloplasminemia was already six times more abundant compared to the healthy situation (162 µg/g vs. 27 µg/g). The concentration of Fe3+ ions and maghemite-iron were 1.7 times higher in the temporal cortex in aceruloplasminemia than in the control subjects. Of the two quantitative MRI metrics, R2* was the most illustrative of the pattern of iron accumulation and returned relaxation rates up to 0.49 ms-1, which were primarily driven by the abundance of ferrihydrite-iron. Maghemite-iron did not follow the spatial distribution of ferrihydrite-iron and did not significantly contribute to MRI contrast in most of the studied regions.ConclusionsEven in extremely iron-loaded cases, iron-related neurodegeneration remains primarily associated with an increase in ferrihydrite-iron, with ferrihydrite-iron being the major determinant of iron-sensitive MRI contrast.