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。传统观点认为，尽管大脑的基础代谢率是所有器官中最高的，但它可以在没有组织网络的情况下发挥作用，以清除组织间液中的代谢废物。我们对这一观点提出了质疑，试图确定大脑清除细胞活动中潜在有毒副产品的途径。我们的初步分析，在体内双光子成像的基础上，显示低分子量示踪剂交付到CSF循环令人惊讶地迅速通过小鼠大脑，并这样做沿着一个明确的解剖路径。在间质空间内，星形胶质细胞支持对流流体流，因为星形胶质细胞水通道AQP 4的缺失急剧减少了沿着这些路径的示踪剂流沿着。鉴于液体沿着这一途径的连续运动，以及其对星形胶质细胞液体转运的关键依赖性，我们认为该系统-我们将其命名为“胶质淋巴系统”-具有与外周淋巴系统同源的功能，并且对于从CNS清除代谢废物是必不可少的。我们将测试挑衅性的假设，即在血管性痴呆的实验模型中，认知功能部分被代谢废物的积累所抑制。这一假说是基于观察到胶质淋巴转运在多腔隙性梗死的小鼠模型中急剧减少，这导致小示踪剂在受损半球中广泛捕获。目标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。通过系统地比较示踪剂清除后的分子大小或表面电荷的修改的效果，我们将定义胶质淋巴系统的基本传输特性。目的2将扩展老年小鼠表现出胶质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搏动抑制和导致的沿动脉旁路径的对流流入沿着损失对胶质淋巴功能的影响。目的3将扩大观察，实质内的液体运动减少，在小鼠模型的多腔隙性梗死和评估是否老化引起额外的抑制胶质淋巴清除。目的4将利用AQP 4转基因小鼠的可诱导星形胶质细胞特异性缺失，并检验抑制多腔隙性梗死小鼠的胶质淋巴转运将损害其认知功能而不依赖于缺血性损伤的假设。 据我们所知，这些研究是首次尝试在整个器官水平上系统地定义大脑代谢废物清除的机制。拟议的研究将为血管性痴呆的认知障碍提供基本的新见解，并可能提高我们对中风和头部创伤后脑损伤病理生理学的理解。 公共卫生相关性：拟议的研究将测试的假设，代谢废物的积累有助于在多梗死性痴呆的小鼠模型的认知功能障碍。

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. PUBLIC HEALTH RELEVANCE: The proposed studies will test the hypothesis that accumulation of metabolic waste products contributes to impairment of cognitive functions in a murine model of multi-infarct dementia.