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

描述(申请人提供)：与所有其他器官不同，大脑和脊髓缺乏淋巴管。传统思想认为，尽管大脑的基础代谢率是所有器官中最高的，但在没有这样一个有组织的网络来清除间质液体携带的代谢废物的情况下，大脑可以发挥作用。我们质疑这一观点，试图定义大脑清除细胞活动的潜在有毒副产品的途径。我们基于活体双光子成像的初步分析表明，输送到脑脊液的低分子示踪剂在小鼠大脑中循环的速度惊人地快，而且是沿着确定的解剖路线进行的。这包括动脉旁流入路径、实质内间质血流路径和静脉旁流出路径。在间质内，星形胶质细胞支持对流流体，因为星形细胞水通道AQP4的缺失大大减少了沿着这些途径的示踪剂流动。鉴于液体沿这一途径的持续运动及其对星形胶质细胞液体运输的严重依赖，我们认为这个系统--我们称之为‘淋巴系统’--辅助了与外周淋巴系统类似的功能，并且对于清除中枢神经系统中的代谢废物是必不可少的。我们将测试这一具有挑衅性的假设，即在一个血管性痴呆的实验模型中，认知功能部分受到代谢废物积累的抑制。这一假说是基于在多腔性脑梗塞的小鼠模型中观察到的淋巴转运急剧减少，这导致小示踪剂在受损半球被广泛捕获。目的1将使用活体双光子显微镜来评估荧光团标记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。目的2推广老年小鼠淋巴系统功能显著下降的初步发现，并评价增龄相关的动脉壁搏动抑制和动脉旁通路对流流入丧失对淋巴系统功能的影响。目的3将扩大在多腔性脑梗塞小鼠模型中实质内液体运动减少的观察，并评估衰老是否导致额外的淋巴清除抑制。目的4将利用AQP4转基因小鼠可诱导的星形胶质细胞特异性缺失，并验证抑制多腔性脑梗塞小鼠的淋巴转运将损害其认知功能的假说，而不是缺血性损伤。 据我们所知，这些研究是第一次尝试在整个器官水平上系统地定义从大脑中清除代谢废物的机制。拟议的研究将为血管性痴呆的认知障碍提供新的基本见解，并可能提高我们对中风和头部创伤后脑损伤的病理生理学的理解。

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.