Role of FGF-Signaling in Myelinogenesis

描述(申请人提供)：髓鞘生物学中的一个关键问题是少突胶质细胞(OL)和雪旺细胞如何被指示形成髓鞘轴突，以及什么分子机制控制髓鞘生长以实现有效的神经传导。虽然在定义髓鞘形成所必需的转录因子方面已经取得了很大进展，但控制OL发育、髓鞘生长和维持的信号转导途径仍然知之甚少。在三叉神经节，轴突NeuRegin-1-type III已成为雪旺细胞发育和髓鞘形成的“主调节器”。然而，它在中枢神经系统轴突髓鞘形成中的作用一直受到质疑。我们最近的研究表明，FGFR1/2(成纤维细胞生长因子受体-1和-2)信号在控制中枢神经系统髓鞘生长方面发挥着重要作用。我们发现，在缺乏FGFR1/2(FGFR1/2 KO)的小鼠中，OL祖细胞(OPC)能够正常增殖、分化和包裹轴突，但不能完全上调主要的髓鞘基因，并产生与轴突口径成比例的厚髓鞘(Furusho等人)。J.神经学2012年)。因此，这些研究揭示了OL中FGFR1/2信号转导的一个先前未知的功能，该功能有助于调节髓鞘厚度，并提示轴突的初始包膜和随后的髓鞘生长可能在中枢神经系统中受到明显的调控。在OL发育过程中，在体内FGFRs下游招募了哪些细胞内信号转导通路，髓鞘生长和维持以及配体的细胞来源是这里将使用遗传功能丧失和功能获得两种方法解决的关键问题。在AIM I中，我们将确定是否可以通过提高OL中ERK1/2的活性来挽救FGFR1/2 KO中减弱的髓鞘生长，以测试两者在活体环境中是否在功能上存在联系。我们还将检验这一假设 ERK1/2和FGFR1/2信号在OPC成熟的早期阶段对OPC的扩张具有重要意义，但在出生后CNS的后期阶段变得不可或缺。在AIM II中，我们将从基因上将FGFRs与FRS2(成纤维细胞生长因子受体底物-2)或PLC？(成纤维细胞生长因子受体的直接下游靶标)的结合解偶联，以分析它们在髓鞘形成调节中的单独作用。此外，我们将完全去除OL系细胞中的FRS2，以测试FRS2在OL中作为关键的“细胞内控制中心”的假设，整合和放大来自髓鞘生长因子受体子集的信号，主要是FGFRs和Trks。在目标III中，我们将在转基因小鼠的神经元中过表达FGF1或FGF2，以检验一个潜在的范式转换假说，即在轴突-神经胶质界面的FGFR相互作用是调节轴突导向的髓鞘径向生长的重要机制 中枢神经系统中的鞘。总体而言，更好地了解刺激正常髓鞘扩张的信号机制与刺激人类脱髓鞘疾病有效再髓鞘形成的最终目标高度相关，例如多发性硬化症，再髓鞘作用往往效率低下，导致髓鞘比正常薄。

DESCRIPTION (provided by applicant): A key question in myelin biology is how oligodendrocytes (OLs) and Schwann cells are instructed to myelinate axons and what molecular mechanisms control myelin growth in order to allow efficient nerve conduction. While much progress has been made to define transcription factors that are essential for myelination, the signal transduction pathways that govern OL development, myelin growth and maintenance remain poorly understood. In the PNS, axonal neuregulin-1-typeIII has emerged as a 'master-regulator' of Schwann cell development and myelination. However, its role for myelinating CNS axons has been questioned. Our recent studies have revealed that FGFR1/2 (Fibroblast Growth Factor Receptor-1 & -2) signaling plays a significant role in the control of myelin growth in the CNS. We found that in mice lacking Fgfr1/2 (Fgfr1/2 KO), OL progenitors (OPCs) were able to proliferate, differentiate, and ensheath axons normally but were unable to fully upregulate major myelin genes and generate thick myelin sheaths in proportion to axon caliber (Furusho et al. J. Neurosci. 2012). Thus, these studies have uncovered a previously unrecognized function of FGFR1/2 signaling in OLs that contributes to the regulation of myelin sheath thickness and suggests that initial ensheathment of axons and subsequent myelin growth is likely to be distinctly regulated in the CNS. What intracellular signal transduction pathways are recruited downstream of the FGFRs in vivo during OL development, myelin growth and maintenance and the cellular source of the ligand are key questions that will be addressed here using both genetic loss-and gain-of-function approaches. In AIM I we will determine whether attenuated myelin growth in the Fgfr1/2 KO can be rescued by genetically elevating ERK1/2 activity in OLs, to test if the two are functionally linked in the in vivo context. We will also test the hypothesis that ERK1/2 and FGFR1/2 signaling is significant for OPC expansion at earliest stages of OPC maturation but becomes dispensable at later stages in the postnatal CNS. In Aim II, we will genetically uncouple binding of FGFRs with either FRS2 (FGF Receptor Substrate-2) or PLC?, immediate downstream targets of FGF-receptors, to parse their individual contributions in the regulation of myelinogenesis. In addition, we will completely ablate FRS2 in OL-lineage cells, to test the hypothesis that FRS2 serves as a key "intracellular control center" in OLs, integrating and amplifying signals from a subset of promyelinating growth factor receptors, primarily FGFRs and Trks. In Aim III we will over-express FGF1 or FGF2 postnatally in neurons of transgenic mice to test a potentially paradigm shifting hypothesis that FGF/FGFR interaction at the axon-glial interface is a significant mechanism for regulating axon-directed radial growth of the myelin sheath in the CNS. Overall, a better understanding of the signaling mechanisms that stimulate normal myelin sheath expansion are highly relevant to the ultimate goal of stimulating efficient remyelination in human demyelinating disorders, such as Multiple Sclerosis, where remyelination is often inefficient leading to myelin sheath that are thinner than normal.