Mechanisms of activity-dependent myelination in the visual system

 描述（由申请人提供）：髓鞘完整性对视力的重要性在多发性硬化症（MS）或视神经肌萎缩症患者中表现得最为明显 (NMO)其中视神经的炎性脱髓鞘导致视力受损并最终失明。新生的少突胶质细胞祖细胞（OPCs）在病理条件下在成人视神经中增殖，但不能像正常发育期间那样改革髓鞘。该提案的目标是了解OPCs上的谷氨酸受体AMPAR如何响应轴突的活性依赖性谷氨酸释放，以启动髓鞘形成的转录调节。阐明这种神经元能信号传导机制将对成年患病视神经中新增殖的OPCs重新编程以重新髓鞘化具有重要意义。据了解，囊泡释放谷氨酸从恩passant轴突突触诱导AMPAR电流突触后OPC，但迄今为止，这种轴突神经胶质细胞信号的生理相关性仍然不明确。我们的初步数据表明，视网膜活动依赖性机制产生的动作电位引起轴突释放谷氨酸激活AMPARs的OPC。这种AMPAR激活激发了髓鞘形成所必需的丝裂原活化蛋白激酶（MAPK）信号级联。与这一想法一致，OPCs中AMPAR的诱导性缺失阻断了活性依赖性诱发的突触电流和髓鞘形成。我们推测，神经元的传递突触活动通过OPCs中AMPAR依赖的MAPK信号调节视神经轴突的髓鞘形成。已经确定，睁眼前（P1-P13）、睁眼前后（P11-P15）和睁眼后（P20-P27）的活动依赖性调节视觉可塑性;然而，尚未研究其对髓鞘形成的影响。我们开发了一种新的体内方法，通过（a）调节视网膜神经节细胞动作电位或（B）诱导缺失OPC上的AMPAR，将视网膜活动与视神经中的神经元轴突-OPC信号传导机械地联系起来，以评估：1）使用荧光激活细胞分选（FACS）分析，在OPC中AMPAR-MAPK途径的磷酸化的改变，2）髓鞘蛋白质的转录和翻译的变化，和3）使用电子显微镜在视神经中每轴突直径的髓鞘厚度的扰动。这项拨款提案将确定活动依赖性髓鞘形成所需的关键时期，建立视觉系统发展的基本机制。从这些研究中获得的知识将影响视神经炎性脱髓鞘疾病，提供重建神经纤维周围绝缘所必需的重要信号机制。

 DESCRIPTION (provided by applicant): The importance of myelin integrity in reference to vision is most clearly represented in patients with multiple sclerosis (MS) or neuromyelitis optica (NMO) where inflammatory demyelination of the optic nerve results in impaired vision and eventual blindness. Newly generated oligodendrocyte progenitor cells (OPCs) proliferate in adult optic nerves during pathological conditions, but fail to reform myelin as they would during normal development. The goal of this proposal is to understand how the glutamate receptor AMPAR on OPCs responds to activity-dependent glutamate release from axons to initiate transcriptional regulation of myelination. Elucidating this glutamatergic signaling mechanism will have important implications in reprogramming newly proliferated OPCs in the adult diseased optic nerve to remyelinate. It is known that vesicular release of glutamate from en passant axonal synapses induces AMPAR currents in postsynaptic OPCs but, to date, the physiological relevance of this axon-glial signaling remains ill-defined. Our preliminary data suggest that retinal activity-dependent mechanisms generate action potentials causing axonal release of glutamate which activates AMPARs on OPCs. This AMPAR activation elicits a mitogen-activated protein kinase (MAPK) signaling cascade necessary for myelination. Consistent with this idea, inducible deletion of AMPARs in OPCs blocked activity-dependent evoked synaptic currents and myelin formation. We hypothesize that neuronal en passant synaptic activity modulates myelination of optic nerve axons via AMPAR-dependent MAPK signaling in OPCs. It is established that activity-dependence before eye opening (P1-P13), around eye opening (P11-P15), and after eye opening (P20-P27) modulates visual plasticity; however, its effect on myelination has not been studied. We developed a novel in vivo approach to mechanistically link retinal activity with glutamatergic axon-OPC signaling in the optic nerve by either (a) regulating retinal ganglion cell action potentials or (b) inducibly deleting AMPARs on OPCs to evaluate: 1) alterations in phosphorylation of the AMPAR-MAPK pathway in OPCs using fluorescence activated cell sorting (FACS) analysis, 2) changes in transcription and translation of myelin proteins, and 3) perturbations in myelin thickness per axon diameter in the optic nerve using electron microscopy. This grant proposal will ascertain the critical periods necessary for activity-dependent myelination establishing a fundamental mechanism in the development of the visual system. Knowledge gained from these studies will impact optic nerve inflammatory demyelinating diseases providing an important signaling mechanism necessary to rebuild the insulation around nerve fibers.