弗里德赖希氏共济失调病人神经元丢失的病理机制研究

弗里德赖希共济失调(FRDA）是由FXN表达量减少引起中枢神经系统中小脑和脊髓神经元的丢失造成的神经退行性疾病。这种特定部位的神经元是如何丢失的问题还未见报道。前期研究中我们发现，FXN表达量减少后，引起细胞内铁代谢异常，胞质内铁缺乏，依赖于铁硫簇的酶活性下降，从而导致线粒体功能减退；另有文献报道，FXN减少引起细胞氧化应激增加，总GSH含量下降，细胞内膜结构发生聚集。以上特点与最新报道的一种依赖于铁的细胞死亡方式ferroptosis的特征非常吻合。我们推测FXN减少导致神经元的丢失是ferroptosis的结果。本项目拟采用多学科的技术手段，研究FXN表达量减少后神经元死亡的分子通路及可能的干预途径。计划利用细胞模型、动物模型及临床病人的资料和药物干预， 阐明FXN的缺失导致FRDA疾病的分子病理机制，为临床上治疗FRDA和其他神经退行性疾病的药物筛选提供有的放矢的靶点。

Friedreich ataxia (FRDA) is a progressive disease caused by expansion of a GAA trinucleotide repeat in the gene that encodes frataxin (FXN). Patients develop ataxia, sensory loss and cardiomyopathy, and symptoms are limited mainly to the central nervous system, e.g. cerebellum and spinal cord, and heart. FXN is important in iron-sulfur cluster assembly where it likely functions as a chaperone that provides iron in a bioavailable form in the early steps of iron-sulfur cluster biosynthesis. Our previous study showed that patients manifest mitochondrial iron-sulfur cluster assembly deficiency, with decreased activity of iron-sulfur proteins such as mitochondrial aconitase and succinate dehydrogenase. Because of the mitochondrial iron overload as disease progresses, it has been considered that iron chelation therapy should ameliorate disease by protecting mitochondria from the adverse effects of iron overload and have shown positive results. However, we found that cells of FRDA patients have evidence of cytosolic iron depletion, as indicated by increased levels of iron regulatory protein 2 (IRP2) and/or increased IRE-binding activity of IRP1. A link to oxidative stress was anticipated and confirmed by the treatment of FRDA patients with idebenone, a free-radical scavenger. In fact, idebenone remains the best drug for the treatment of FRDA. And FXN-deficient cells showed change of glutathione-dependent redox status in a yeast and cell culture model of FRDA. Therefore, the misregulation of intracellular iron imposed by FXN deficiency could not only elicit a Fenton chemistry-based mechanism of ROS toxicity leading to prominent abnormalities of the neuronal cytoskeleton, but also change of glutathione-dependent redox status causing abnormally increased protein glutathionylation, both of which result in neuronal cell death. However, the detail signaling pathway of the mechanism is unknown. Very recently reported an iron-dependent form of nonapoptotic cell death, ferroptosis, presents a similar characters of FXN-deficient cells. A specific inhibitor, Fer-1, of ferroptosis was identified. We demonstrate that overexpression of human frataxin or alpha-lipoic acid treatment of cells derived from FRDA patients increased cellular antioxidant defense via elevation of reduced thiols/glutathione. We hypothesize that ferroptosis is the pathway for the death of FXN-deficient neuronal cells and interventions of Fer-1 and alpha-lipoic acids will reverse or slow down the cell death, further clinically should reduce the severity of at least some aspects of FRDA disease. We will test the validity of this hypothesis by using the FXN-deficient cell and mouse models and combining IRP2 knockout mice to reveal the molecular pathological mechanism of neuronal cell death. Identification and characterization of the pathway may aid in the development of new therapeutic avenues for the treatment of Friedreich ataxia and other neurodegenerative disease.