Modulation of brain iron by local hepcidin in prion disorders

朊病毒疾病是感染性的，并且总是致命的神经退行性疾病，与朊病毒的积累有关。 PrP-scrapie（PrPSc），一种在人脑和视网膜中的正常朊蛋白（PrPC）的β折叠丰富的同种型 和某些动物物种。散发性克雅氏病（sCJD）是人类最常见的朊病毒 疾病和PrPSc感染的动物模型用于了解感染性和毒性的机制。 神经炎症和铁积累是这些疾病的一致特征，后者有助于 铁催化活性氧（ROS）的神经毒性。然而，铁积累的原因有 仍然难以捉摸我实验室的最新数据表明，马槟榔碱介导的铁调素上调 是由星形胶质细胞合成的铁调素通过下调膜铁转运蛋白（Fpn）来调节铁， 唯一已知的铁输出蛋白。在正常情况下，铁调素是上调时，铁饱和度 转铁蛋白（Tf-Fe）低。然而，细胞因子的上调取代了来自Tf-Fe的信号。很可能 星形胶质细胞介导的铁调素上调是铁神经元积累的原因， Fpn的细胞膜上，和ROS的毒性。为了支持这一假设，sCJD脑匀浆 显示铁调素mRNA和蛋白上调、Fpn下调和铁蛋白增加。同样，大脑 来自PrPSc感染的小鼠的匀浆显示铁调素mRNA的上调，来自PrPSc的视网膜切片显示铁调素mRNA的上调。 感染的仓鼠在疾病期间铁蛋白上调之前或同时显示小胶质细胞活化 进展基于这些观察结果，我们假设sCJD和PrPSc感染的 脑和视网膜的炎症是由精氨酸介导的局部铁调素上调引起的。提出了两个具体目标 来验证这个假设在目标1中，将检查额外的sCJD脑和视网膜组织中铁调素的增加 mRNA和铁的积累，并与神经元和视网膜神经节细胞（RGC）的死亡， 免疫染色切片。此外，将在疾病期间检查PrPSc感染小鼠的大脑和视网膜。 进展以探索细胞因子的增加是否先于铁调素和铁积累的上调，以及 视网膜变性是否先于神经变性这将为视网膜成像铺平道路， sCJD的临床诊断测试。在aim2中，hepcidin在脑和视网膜铁蓄积中的作用将进一步得到研究 使用用PrPSc接种的铁调素敲除（hepc-/-）和同窝（hepc +/+）对照小鼠进行探索。一 与hepc +/+对照组相比，hepc-/-小鼠中的铁积累显著降低，尽管 细胞因子和PrPSc负荷与疾病进展的关系表明局部铁调素是铁积累的原因。 此外，在hepc-/-小鼠中神经元和RGC死亡的显著减少将表明铁在以下方面的重要作用： 诱导神经毒性，并证明使用铁调素拮抗剂和Fpn稳定剂，以减少 铁作为治疗选择。在hepc-/-小鼠中铁水平的变化不会反驳我们的假设， 表明铁是疾病过程的附带现象。

Prion disorders are infectious and invariably fatal neurodegenerative conditions associated with accumulation of PrP-scrapie (PrPSc), a β-sheet rich isoform of the normal prion protein (PrPC), in the brain and retina of humans and certain animal species. Sporadic Creutzfeldt-Jakob-disease (sCJD) is the most common human prion disorder, and PrPSc-infected animal models are used to understand the mechanism of infectivity and toxicity. Neuroinflammation and iron accumulation are consistent features of these disorders, the latter contributing to neurotoxicity by iron-catalyzed reactive oxygen species (ROS). The cause of iron accumulation, however, has remained elusive. Recent data from my laboratory suggest cytokine-mediated upregulation of hepcidin synthesized by astrocytes as a significant cause. Hepcidin regulates iron by downregulating ferroportin (Fpn), the only known iron export protein. Under normal conditions, hepcidin is upregulated when iron saturation of transferrin (Tf-Fe) is low. Upregulation by cytokines, however, supersedes the signal from Tf-Fe. It is likely that cytokine-mediated upregulation of hepcidin by astrocytes is the cause of iron neuronal accumulation that express Fpn on their plasma membrane, and toxicity by ROS. In support of this hypothesis, sCJD brain homogenates show upregulation of hepcidin mRNA and protein, downregulation of Fpn, and increase in ferritin. Likewise, brain homogenates from PrPSc-infected mice show upregulation of hepcidin mRNA, and retinal sections from PrPSc- infected hamsters show activation of microglia before or concomitant with upregulation of ferritin during disease progression. Based on these observations, we hypothesize that iron accumulation in sCJD and PrPSc-infected brains and retina results from cytokine-mediated upregulation of local hepcidin. Two specific aims are proposed to test this hypothesis. In aim 1, additional sCJD brain and retinal tissue will be checked for increase in hepcidin mRNA and accumulation of iron, and correlated with neuronal and retinal ganglion cell (RGC) death in immunostained sections. In addition, the brain and retina of PrPSc-infected mice will be examined during disease progression to explore if increase in cytokines precedes upregulation of hepcidin and iron accumulation, and whether retinal degeneration precedes neurodegeneration. This will pave the way for retinal imaging a pre- clinical diagnostic test for sCJD. In aim 2, the role of hepcidin in brain and retinal iron accumulation will be further explored using hepcidin knock-out (hepc-/-) and littermate (hepc+/+) control mice inoculated with PrPSc. A significant decrease in iron accumulation in hepc-/- mice relative to hepc+/+ controls despite similar increase in cytokines and PrPSc load with disease progression will suggest local hepcidin as the cause of iron accumulation. Moreover, a marked reduction in neuronal and RGC death in hepc-/- mice will suggest a significant role of iron in inducing neurotoxicity, and justify the use of hepcidin antagonists and Fpn stabilizing agents to reduce accumulated iron as therapeutic options. No change in iron levels in hepc-/- mice will refute our hypothesis, and suggest iron as an epiphenomenon of the disease process.