Failure of metabolite clearance in a model of multi-lacunar infarcts

描述（由申请人提供）：与所有其他器官不同，大脑和脊髓缺乏淋巴管。传统的观点认为，尽管大脑是所有器官中基础代谢率最高的，但没有这样一个有组织的网络来清除间质液体传播的代谢废物，大脑也能正常工作。我们质疑这一观点，试图确定大脑清除细胞活动潜在毒性副产品的途径。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，并且沿着确定的解剖路线进行。这包括动脉旁流入路径、实质内间质流动路径和静脉旁流出路径。在间隙内，星形胶质细胞支持对流流体流动，因为星形胶质细胞水通道AQP4的缺失急剧减少了沿着这些通道的示踪剂流动。考虑到液体沿着这一途径的持续运动，以及它对星形细胞液体运输的关键依赖，我们提出这个系统-我们称之为“淋巴系统”-具有与外周淋巴系统类似的功能，对于清除中枢神经系统的代谢废物至关重要。我们将在血管性痴呆的实验模型中测试认知功能部分受到代谢废物积累抑制的挑衅性假设。这一假设是基于观察到的，在小鼠多腔隙梗死模型中，淋巴转运急剧减少，导致小示踪剂在受损半球广泛捕获。目的1将使用体内双光子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将扩展老年小鼠淋巴系统功能显著下降的初步发现，并评估与年龄相关的动脉壁脉动抑制以及由此导致的沿动脉旁通道对流流入损失对淋巴功能的影响。目的3将扩展在小鼠多腔隙梗死模型中实质内液体运动减少的观察，并评估衰老是否会导致淋巴清除的额外抑制。Aim 4将利用AQP4转基因小鼠诱导的星形细胞特异性缺失，验证抑制多腔隙梗死小鼠的淋巴转运会独立于缺血性损伤损害其认知功能的假设。

DESCRIPTION (provided by applicant): Unlike all other organs, the brain and spinal cord lack lymphatic vessels. Traditional thought has averred that the brain - despite having the highest basal metabolic rate of any organ - can function without such an organized network for the removal of interstitial fluid-borne metabolic waste products. We questioned this position, seeking to define the pathways by which the brain removes the potentially toxic byproducts of cellular activity. 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 defined anatomical route. This consists of a para-arterial inflow path, an intra-parenchymal path of interstitial flow, and a para-venous outflow path. Within the interstitial space, astrocytes support convective fluid currents, as deletion of the astrocytic watr channel AQP4 sharply reduces tracer flow along these routes. Given the continuous movement of fluid along this pathway, and its critical dependence upon astrocytic fluid transport, we propose that this system - which we designate 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. We will test the provocative hypothesis that cognitive function in an experimental model of vascular dementia in part is suppressed by accumulation of metabolic waste products. This hypothesis is based on the observation that glymphatic transport is sharply reduced in a murine model of multi-lacunar infarcts, which results in widespread trapping of small tracers in the lesioned hemisphere. Aim 1 will use in vivo 2-photon 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 finding that aged mice exhibit a striking decline in glymphatic system function, and evaluate the effect that age-related suppression of arterial wall pulsation and resulting loss of convective inflow along the para-arterial path has on glymphatic function. Aim 3 will extend the observation that intra-parenchymal fluid movement is reduced in a mouse model of multi-lacunar infarcts and evaluate whether aging cause an additional suppression of glymphatic clearance. Aim 4 will take advantage of inducible astrocyte-specific deletion of AQP4 transgenic mice and test the hypothesis that suppressing glymphatic transport in mice with multi-lacunar infarcts will impair their cognitive functions independently of the ischemic injury. 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. The proposed studies will provide fundamental new insight into cognitive impairment in vascular dementia, and will likely also improve our understanding of the pathophysiology of brain injury following stroke and head trauma.