The role of FGF signaling in suppressing astrogliosis

描述(申请人提供)：神经胶质增生症是对大脑损伤的一种反应，是许多神经疾病的显著特征，包括癫痫、神经退行性变、肿瘤生长、中风和创伤。对损伤的反应涉及中枢神经系统内的细胞类型，可能包括渗透的免疫细胞。反应的程度是不同的，并受许多秘密因素的调节(Sofroniew，2009；Kang和Hibert，2011)。然而，对于这些因素中的一些，它们在胶质细胞增生症中的需求和作用还没有明确的定义或在体内得到证实。成纤维细胞生长因子(FGFs)尤其如此。以前的免疫组织化学分析表明，星形胶质细胞对损伤的反应是核FGF2和至少一个受体上调(Eddleston和Mucke，1993；Ridet等人，1997)。然而，这些免疫标记的意义尚不清楚。此外，在正常情况下和损伤后的体内成熟星形胶质细胞中，成纤维细胞生长因子信号的作用仍未被阐明。这项建议的目的是为FGFs在正常情况下星形胶质细胞增生症和损伤反应中的作用提供确凿的证据。为了研究成纤维细胞生长因子信号在成人大脑中的功能，我们在FGFR3缺失的背景下有条件地删除了FGFR1和FGFR2的FGFR2等位基因。我们使用了NestinCreer小鼠品系，在该品系中，在他莫昔芬治疗后，Creer出人意料地在整个海马区和皮质实质(除了预期的神经源性区域之外)的星形胶质细胞中活跃。我们的初步数据表明，在成纤维细胞生长因子受体的三个突变体中，星形细胞GFAP、Vimentin和Nestin的表达显著增加，这表明星形胶质细胞增生。在携带NestinCreer的三苯氧胺处理的杂合子对照或只缺少一个或两个受体的突变体中，没有观察到中间丝的这种增加。在单突变体和双突变体中没有这种表型，这表明受体之间的功能代偿，这可能解释了为什么以前没有报道这种表型。在三个突变体中，胶质细胞标志物的增加并没有伴随着可检测到的组织损伤、细胞死亡或神经源性区域的明显变化。此外，我们的初步数据表明，突变体中具有反应性的星形胶质细胞是那些失去了成纤维细胞生长因子信号的星形胶质细胞，而邻近的非反应性星形胶质细胞保持了信号传递。这导致了我们的假设，即成纤维细胞生长因子受体在没有损伤的情况下自主地作用于细胞，以抑制星形胶质细胞增生。 与公共健康相关：在大脑中，星形胶质细胞是一种对损伤做出反应的细胞类型。尽管对损伤的反应很重要，但控制它的分子机制仍然知之甚少。反应的程度取决于损害的类型和严重程度。例如，在某些情况下，星形胶质细胞形成疤痕组织，而在某些情况下，它们不会。虽然疤痕的形成对于最大限度地减少损害的扩散是必不可少的，但它也不利于再生。因此，对损伤的反应程度被认为受到分泌因子、细胞内信号因子和结构蛋白(中间丝)的严格调控。尽管缺乏确凿的证据，但有一类因素被认为可以促进星形胶质细胞的反应，那就是FGFs。然而，在这项提案中，我们的目标是提供第一个与之相反的确凿证据：FGFs实际上抑制了星形胶质细胞的反应。这一点意义重大，因为几乎没有其他明显的例子表明，在未受干扰的大脑中和受伤后，保持反应受控的因素即使有也很少。我们的初步数据表明，即使在没有损伤的情况下，成人也需要成纤维细胞生长因子信号来积极抑制星形胶质细胞的反应。因此，这提供了一种潜在的手段来调节反应的程度或持续时间以达到治疗目的。

DESCRIPTION (provided by applicant): Gliosis is a response to brain damage and is a prominent feature of many neurological disorders including epilepsy, neurodegeneration, tumor growth, stroke, and trauma. The response to damage involves cell types from within the CNS and can include infiltrating immune cells. The extent of the response varies and is modulated by a number of secreted factors (Sofroniew, 2009; Kang and Hibert, 2011). For some of these factors, however, their requirement and role in gliosis has not been clearly defined or demonstrated in vivo. This is particularly true for Fibroblast Growth Factors (FGFs). Previous immunohistochemical analyses suggested that nuclear FGF2 and at least one receptor are up-regulated in astrocytes in response to damage (Eddleston and Mucke, 1993; Ridet et al., 1997). However, the significance of these immunostains is unknown. Moreover the role of FGF signaling in mature astrocytes in vivo under normal conditions as well as after injury remains unaddressed. The goal of this proposal is to provide definitive evidence for the role of FGFs in astrogliosis under normal conditions and in response to damage. To address the function of FGF signaling in the adult cerebrum, we conditionally deleted floxed alleles of Fgfr1 and Fgfr2 in an Fgfr3 null background. We used a NestinCreER mouse line in which CreER, upon tamoxifen treatment, is unexpectedly active specifically in astrocytes throughout the hippocampal and cortical parenchyma (in addition to the expected neurogenic regions). Our preliminary data indicate that in FGF receptor triple mutants there is a tremendous increase in astrocytic GFAP, Vimentin, and Nestin expression, indicative of astrogliosis. This increase in intermediate filaments is not observed in tamoxifen-treated heterozygous littermate controls that carry NestinCreER or in mutants that lack only one or two receptors. The absence of the phenotype in single and double mutants suggests functional compensation between receptors and may explain why this phenotype has not previously been reported. The increase in gliotic markers in the triple mutant is not accompanied by detectable signs of tissue damage, cell death, or obvious changes in the neurogenic regions. Moreover, our preliminary data suggests that the astrocytes that are reactive in the mutants are the ones that have lost FGF signaling whereas neighboring non-reactive ones have maintained signaling. This leads us to the hypothesis that FGF receptors act cell autonomously in the absence of injury to suppress astrogliosis. PUBLIC HEALTH RELEVANCE: In the brain, astrocytes are a cell type that responds to damage. Despite the importance of the response to damage, the molecular mechanisms that control it remain poorly understood. The degree of the response depends on the type and severity of the damage. For example, in some cases astrocytes form scar tissue and it some cases they do not. Although scar formation is essential to minimizing the spread of damage, it is also refractory to regeneration. Therefore the degree of the response to damage is believed to be tightly regulated by secreted factors, intracellular signaling factors, and structural proteins (intermediate filaments). One family of factors that has been postulated to promote the astrocyte response, despite the lack of definitive evidence, are FGFs. In this proposal, however, we aim to provide the first definitive evidence to the contrary: that FGFs in fact inhibit the astrocyte response. This is significant because there are few if any other clear examples of factors that maintain the response in check in the unperturbed brain and after injury. Our preliminary data suggest that FGF signaling is required in adults to actively suppress the astrocyte response, even in the absence of injury. Hence this provides a potential means of modulating the extent or duration of the response for therapeutic purposes.