The glymphatic system, a new concept in glia biology

描述（由申请人提供）：在躯体组织中，大脑和脊髓缺乏淋巴系统，这几乎是独一无二的。尽管神经组织具有高代谢活性和脆弱性，但对于间质液和废物是如何从中枢神经系统排出的，目前还没有有效的了解。我们基于体内双光子成像的初步分析表明，传递到脑脊液的低分子量示踪剂通过小鼠大脑的循环速度惊人，并且沿着令人惊讶的解剖路线进行。这包括动脉旁流入路径，间质间流的跨胶质实质内路径和静脉旁流出路径。在实质内通路中，星形胶质细胞支持对流流体通过脑间质空间，因为星形胶质细胞水通道AQP4的缺失会急剧减少沿这些通路的整体示踪剂流动。考虑到这一途径支持液体的持续运动，以及它对星形细胞水运输的关键依赖，我们认为这一系统——我们在这里称之为“淋巴系统”——具有与外周淋巴系统类似的功能，对中枢神经系统代谢废物的清除至关重要。目的1将使用双光子体内显微镜来评估荧光团标记的示踪剂清除的空间动力学和时间动力学。通过系统地比较分子大小或表面电荷的改变对示踪剂清除的影响，我们将定义淋巴系统的基本运输性质。Aim 2将扩展老龄小鼠淋巴系统功能显著下降的初步观察，并评估年龄相关抑制动脉壁脉动的作用，从而减少沿动脉旁通路的对流流入和整体淋巴功能。Aim 3提出，诱导敲除星形细胞AQP4水通道或间隙连接（Cx43/Cx30）将减缓实质对流流体流动，并整体抑制示踪剂清除。目的4验证了AQP4或Cx43/Cx30缺失导致的星形胶质液体运动抑制将减缓外源性A¿的清除，从而增强与年龄相关的淀粉样斑块形成的观点。我们预测星形细胞实质液体流动的减慢将加速血管旁淀粉样蛋白的沉积，这将以前反馈的方式进一步降低淋巴系统清除废物的效率。据我们所知，这些研究首次尝试系统地确定大脑在全器官水平上清除代谢废物的机制。通过慢性颅窗的双光子成像将允许实时成像示踪剂清除，而诱导缺失关键星形胶质膜蛋白的转基因小鼠将建立星形胶质细胞在淋巴运输中的功能作用。综上所述，这些研究将为研究与年龄相关的神经毒性代谢废物积累机制提供基础见解，并定义星形胶质细胞的新颖且可能非常重要的功能特性。

DESCRIPTION (provided by applicant): 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 function. 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. PUBLIC HEALTH RELEVANCE: The proposed studies will define the function of astroglia fluid transport in clearance of metabolic waste.