Innate immunity in Rett pathology and repair

描述（由申请人提供）：Rett综合征是一种毁灭性的神经系统疾病，几乎只影响女孩。Rett主要是由编码甲基cpg结合蛋白（MECP）2的x连锁基因突变引起的。直到最近，这种疾病的病因被认为是纯粹的神经元。然而，神经胶质在Rett综合征（和其他中枢神经系统疾病）病理中的作用现在已经被认识到。野生型Mecp2在Mecp2缺失宿主星形胶质细胞中的表达已被证明可显著改善疾病病理。小胶质细胞最近也被认为在Rett病理生理中发挥作用；据报道，mecp2缺失的小胶质细胞通过产生高水平的谷氨酸而对神经元有毒。沿着这些思路，现在人们普遍认为中枢神经系统的非神经元细胞对大脑功能至关重要。我们的初步数据作为这一建议的基础，证明了骨髓细胞在Rett病理阻滞中发挥的独特作用。将野生型骨髓移植到光照条件下的Mecp2-null宿主中，导致小胶质表型的骨髓源性髓样细胞植入脑实质，并阻止疾病发展。然而，当头部照射被铅屏蔽，并阻止小胶质细胞植入时，疾病并未停止。同样，在Mecp2缺失的背景下，Lysmcre在髓细胞中靶向表达Mecp2，可显著减轻疾病症状。在受辐射的小鼠中，新移植的脑单核吞噬细胞产生高水平的胰岛素样生长因子1 (IGF-1)，与常驻突变小胶质细胞不同，这可能是一种潜在的分子机制。然而，有趣的是，使用膜联蛋白V抑制吞噬活性，阻断凋亡靶点上的磷酸二丝氨酸残基，阻止它们被组织驻留的吞噬细胞识别和吞噬，从而消除了疾病停滞。根据我们的初步结果，我们假设mecp2缺失的小胶质细胞不能提供神经营养支持，也不能完成碎片清除的任务（包括突触修剪、清除死细胞等），从而导致Rett综合征的病理生理。我们的数据暗示小胶质细胞在Rett病理生理中起主要作用，并提示骨髓移植可能为这种破坏性疾病提供可行的治疗方法。然而，在分子水平上进一步了解小胶质细胞在疾病病理和修复中的作用对于将这些研究有效地转化为临床至关重要。本研究旨在通过以下方法研究小胶质细胞介导的Rett病理阻滞的潜在机制：(1)利用遗传和药理学方法建立小胶质细胞在Rett病理和修复中的作用；(2)利用骨髓移植和基因杂交研究外周免疫源性IGF-1和小胶质源性IGF-1在Rett病治疗中的相对作用；(3)利用巨噬细胞吞噬活性不足的小鼠作为骨髓供体，验证小胶质细胞吞噬清除能力不足可能在一定程度上导致Rett的病理生理学。

DESCRIPTION (provided by applicant): Rett syndrome is a devastating neurological disorder, almost exclusively affecting girls. Rett is predominantly caused by mutations in an X-linked gene encoding methyl-CpG-binding protein (MECP)2. Until recently, the etiology of the disease was assumed to be purely neuronal. However, the role of glia in the pathology of Rett syndrome (and of other CNS diseases) has now been recognized. Expression of wild type Mecp2 in astrocytes of Mecp2-null hosts has been shown to dramatically ameliorate disease pathology. Microglia were also recently suggested to play a role in Rett pathophysiology; Mecp2-null microglia were reported to be toxic to neurons through production of high levels of glutamate. Along these lines, it is generally well accepted by now that non-neuronal cells of the CNS are critically important for brain function. Our preliminary data that serves as a basis for this proposal, demonstrates the unique role myeloid cells play in arrest of Rett pathology. Transplantation of wild type bone marrow into irradiation-conditioned Mecp2-null hosts resulted in engraftment of brain parenchyma by bone marrow-derived myeloid cells of microglial phenotype, and arrest of disease development. However, when cranial irradiation was blocked by lead shield, and microglial engraftment was prevented, disease was not arrested. Similarly, targeted expression of Mecp2 in myeloid cells, driven by Lysmcre on an Mecp2-null background, dramatically attenuated disease symptoms. In irradiated mice, newly- engrafted brain mononuclear phagocytes produced high levels of insulin-like growth factor 1 (IGF-1), unlike resident mutant microglia, possibly one underlying molecular mechanism of rescue. Interestingly, however, inhibition of phagocytic activity using annexin V that blocks phosphatydilserine residues on apoptotic targets and prevents their recognition and engulfment by tissue-resident phagocytes, abolished disease arrest. Based on our preliminary results, we hypothesize that Mecp2-null microglia are incapable of providing neurotrophic support and are insufficient to the task of debris clearance (including synaptic pruning, elimination of dead cells etc.), thus contributing to the ongoing pathophysiology seen in Rett syndrome. Our data implicate microglia as major players in Rett pathophysiology, and suggest that bone marrow transplantation might offer a feasible therapeutic approach for this devastating disorder. However, a further understanding of the role played by microglia in disease pathology and repair on a molecular level is crucial for efficient translation of these studies to clinic. This proposal is aimed to study the underlying mechanism of microglia-mediated arrest of Rett pathology by (1) establishing the role of microglia in Rett pathology and repair using genetic and pharmacological approaches; (2) addressing the relative contribution of peripheral immune-derived IGF-1 and microglia-derived IGF-1 in arrest of Rett disease, using bone marrow transplantation and genetic crosses; and (3) testing the hypothesis that the deficiency in phagocytic clearance by microglia may underlie, in part, the pathophysiology of Rett using mice deficient in macrophage phagocytic activity as bone marrow donors.