Modulation of brain iron by local hepcidin in prion disorders

朊病毒病是一种传染性的、致命的神经退行性疾病，与朊病毒的积累有关。 PrP-scrapie (PrPSc)，一种富含 β-折叠的正常朊病毒蛋白 (PrPC) 亚型，存在于人类大脑和视网膜中 和某些动物物种。散发性克雅氏病 (sCJD) 是最常见的人类朊病毒 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，后者导致 铁催化活性氧（ROS）的神经毒性。然而，铁积累的原因是 仍然难以捉摸。我实验室的最新数据表明细胞因子介导的铁调素上调 由星形胶质细胞合成是一个重要原因。铁调素通过下调铁转运蛋白 (Fpn) 来调节铁， 唯一已知的铁输出蛋白质。正常情况下，铁调素在铁饱和时上调 转铁蛋白 (Tf-Fe) 较低。然而，细胞因子的上调会取代来自 Tf-Fe 的信号。很可能是这样 星形胶质细胞细胞因子介导的铁调素上调是表达铁的神经元积累的原因 Fpn 在其质膜上，以及 ROS 的毒性。为了支持这一假设，sCJD 大脑匀浆 显示铁调素 mRNA 和蛋白质的上调、Fpn 的下调以及铁蛋白的增加。同样，大脑 PrPSc 感染小鼠的匀浆显示铁调素 mRNA 上调，PrPSc 的视网膜切片显示铁调素 mRNA 上调 受感染的仓鼠在疾病期间铁蛋白上调之前或同时表现出小胶质细胞的激活 进展。基于这些观察结果，我们假设 sCJD 和 PrPSc 感染者体内的铁积累 大脑和视网膜是细胞因子介导的局部铁调素上调的结果。提出了两个具体目标 来检验这个假设。在目标 1 中，将检查额外的 sCJD 大脑和视网膜组织中铁调素的增加情况 mRNA 和铁的积累，并与神经元和视网膜神经节细胞 (RGC) 死亡相关 免疫染色切片。此外，PrPSc感染小鼠的大脑和视网膜将在患病期间进行检查 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。这将为视网膜成像预铺平道路 sCJD 的临床诊断测试。在目标2中，铁调素在大脑和视网膜铁积累中的作用将进一步得到证实。 使用接种 PrPSc 的铁调素敲除小鼠 (hepc-/-) 和同窝小鼠 (hepc / ) 对照小鼠进行研究。一个 与 hepc/对照组相比，hepc-/- 小鼠的铁积累显着减少，尽管 随着疾病进展，细胞因子和 PrPSc 负荷表明局部铁调素是铁积累的原因。 此外，hepc-/- 小鼠神经元和 RGC 死亡的显着减少表明铁在 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 积累的铁作为治疗选择。 hepc-/- 小鼠中铁水平的变化不会反驳我们的假设，并且 建议铁作为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.