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

Aceruloplasminemia 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.
The 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.
The 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.
Even 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

铜蓝蛋白缺乏症（aceruloplasminemia）是一种与大脑大量铁蓄积相关的超罕见神经退行性疾病。目前尚不明确铜蓝蛋白缺乏症患者脑内蓄积的铁以何种分子形式存在。相较于健康大脑，该病患者的脑铁浓度至少升高五倍，因此其为研究铁在神经退行性变中的作用以及铁敏感磁共振成像（MRI）对比的分子基础提供了独特的模型。
本研究将7特斯拉（7T）场强下获取的铁敏感磁共振成像指标——非均匀横向弛豫率（R2*）与磁敏感度，与电子顺磁共振（EPR）、超导量子干涉器件（SQUID）磁强计技术相结合，对一例死后铜蓝蛋白缺乏症患者脑组织每克湿重中的不同铁形式进行定性与定量分析，分析重点覆盖基底神经节、丘脑、红核、齿状核、颞上回、颞中回及白质。为便于对照，本研究同时纳入了此前从健康对照者颞皮层获取的磁共振成像、电子顺磁共振及超导量子干涉器件磁强计检测结果。
铜蓝蛋白缺乏症患者的脑铁池由可经电子顺磁共振检测的三价铁离子、嵌于铁蛋白及含铁血黄素核心中的磁性三价铁（水铁矿铁，ferrihydrite-iron），以及嵌于氧化磁铁矿/磁赤铁矿矿物中的磁性三价铁（磁赤铁矿铁，maghemite-iron）组成。在所有被分析的铁池中，超过90%为水铁矿铁，其中红核内检测到的水铁矿铁浓度最高可达1065 µg/g。尽管铜蓝蛋白缺乏症患者脑内的深部灰质结构中水铁矿铁含量是颞皮层的三倍，但该患者颞皮层内的水铁矿铁含量仍较健康状态高出六倍（162 µg/g 对比 27 µg/g）。铜蓝蛋白缺乏症患者颞皮层内的三价铁离子与磁赤铁矿铁浓度较健康对照者高出1.7倍。在两项定量磁共振成像指标中，横向弛豫率（R2*）最能反映铁蓄积模式，其弛豫率最高可达0.49 ms⁻¹，这一结果主要由水铁矿铁的丰度所驱动。磁赤铁矿铁的空间分布与水铁矿铁并不一致，且在大多数被分析脑区中并未对磁共振成像对比产生显著贡献。
即便在铁蓄积程度极高的病例中，铁相关神经退行性变仍主要与水铁矿铁的升高相关，而水铁矿铁正是铁敏感磁共振成像对比的主要决定因素。