The glymphatic system, a new concept in glia biology

在躯体组织中，大脑和脊髓几乎是独一无二的，它们缺乏淋巴系统。尽管高 代谢活动和神经组织的脆弱性，目前还没有有效的了解的手段， 间质液和废物从CNS中除去。我们的初步分析，基于体内两个- 光子成像显示，递送到CSF的低分子量示踪剂令人惊讶地快速循环 穿过老鼠的大脑，并且沿着沿着一条令人惊讶的解剖学路线。这包括动脉旁流入 通道、间质流的跨胶质实质内通道和静脉旁流出通道。 在脑实质通路中，星形胶质细胞支持通过脑间质空间的对流流体流，如 星形胶质细胞水通道AQP 4的缺失急剧减少了沿这些路线的总示踪剂流沿着。鉴于 由这一途径支持的液体的连续运动，以及它对星形胶质细胞水的关键依赖 运输，我们提出，这个系统-我们在这里指定的“胶质淋巴系统”-有助于 功能与外周淋巴系统同源，并且对于代谢废物的清除至关重要 CNS的产品。目的1将使用双光子体内显微镜来评估空间动力学， 荧光团标记的示踪剂清除的时间动力学。通过系统地比较 示踪剂清除后分子大小或表面电荷的改变，我们将定义基本的运输 glymphatic系统的特性。目的2将扩大老年小鼠表现出一种 胶质淋巴系统功能显著下降，并评估与年龄相关的动脉壁抑制的作用 脉动并导致沿动脉旁路径的对流流入沿着减少和总体胶质淋巴功能。 目的3提出诱导敲除星形胶质细胞AQP 4水通道或缝隙连接（Cx43/Cx 30）， 将减慢实质对流流体流动并全面抑制示踪剂清除。目标4测试命题 AQP 4或Cx43/Cx 30缺失导致的跨星形胶质细胞液体运动的抑制将减缓 清除外源性A ²，从而增强年龄相关性淀粉样斑块的形成。我们预测 减缓星形胶质细胞实质液体流动将加速血管旁淀粉样蛋白沉积，这在一种饲料- 前向方式将进一步降低胶质淋巴系统清除废物的效率。 据我们所知，这些研究代表了首次尝试系统地定义机制， 在整个器官水平上参与清除大脑中的代谢废物。双光子 通过慢性颅窗的成像将允许真实的时间的示踪剂清除成像，而 关键星形胶质细胞膜蛋白可诱导缺失的转基因小鼠将建立 星形胶质细胞在胶质淋巴运输中的作用结合起来，这些研究将提供基本的洞察力， 与年龄相关的神经毒性代谢废物积累的机制，并定义 星形胶质细胞的新的、可能非常重要的功能特性。

Virtually unique among somatic tissues, the brain and spinal cord lack a lymphatic system. Despite the high metabolic activity and fragility of neural tissue, there exists no effective understanding of the means by which interstitial fluid and waste products are removed from the CNS. Our preliminary analysis, based on in vivo two- photon imaging, shows that low molecular weight tracers delivered to the CSF circulate surprisingly rapidly through the mouse brain, and do so along a surprising anatomical route. This consists of a para-arterial inflow path, a trans-glial intra-parenchymal path of interstitial flow, and a para-venous outflow path. Within the intra- parenchymal pathway, astrocytes support convective fluid currents through the brain interstitial space, as deletion of the astrocytic water channel AQP4 sharply reduces overall tracer flow along these routes. Given the continuous movement of fluid supported by this pathway, and its critical dependence upon astrocytic water transport, we propose that this system - which we designate here the 'glymphatic system' - subserves a function homologous to the peripheral lymphatic system, and is essential for the clearance of metabolic waste products from the CNS. Aim 1 will use 2-photon in vivo microscopy to assess the spatial dynamics and temporal kinetics of fluorophore-tagged tracer clearance. By systematically comparing the effect of modifications of molecular sizes or surface charge upon tracer clearance, we will define the basic transport properties of the glymphatic system. Aim 2 will extend the preliminary observation that aged mice exhibit a striking decline in glymphatic system function, and evaluate the role of age-related suppression of arterial wall pulsation and resulting reduced convective inflow along the para-arterial path and global glymphatic fucntion. Aim 3 proposes that induced knock-out of either astrocytic AQP4 water channels or gap junctions (Cx43/Cx30) will slow parenchymal convective fluid flow and globally suppress tracer clearance. Aim 4 tests the proposition that suppression of trans-astroglial fluid movement resulting from AQP4 or Cx43/Cx30 deletion will slow clearance of exogenous A¿ and thereby potentiate age-related amyloid plaque formation. We predict that slowing astrocytic parenchymal fluid flow will accelerate paravascular amyloid deposition, which in a feed- forward manner will further reduce the efficiency of clearance of waste products by the glymphatic system. To our knowledge, these studies represent the first attempt to systematically define the mechanisms involved in the clearance of metabolic waste products from the brain on a whole-organ level. Two-photon imaging of through chronic cranial windows will allow imaging of tracer clearance in real time, whereas transgenic mice with inducible deletion of key astroglial membrane proteins will establish the functional role of astrocytes in glymphatic transport. Combined, these studies will provide fundamental insight into the mechanisms contributing to age-related accumulation of neurotoxic metabolic waste products and define novel, and likely highly important, functional properties of astrocytes.