Xiao Fanrong, Hrabe Jan, Hrabetova Sabina
Department of Cell Biology, State University of New York, Downstate Medical Center, Brooklyn, New York.
Department of Cell Biology, State University of New York, Downstate Medical Center, Brooklyn, New York; Medical Physics Laboratory, Nathan S. Kline Institute, Orangeburg, New York.
Biophys J. 2015 May 5;108(9):2384-95. doi: 10.1016/j.bpj.2015.02.034.
Extracellular space (ECS) is a major channel transporting biologically active molecules and drugs in the brain. Diffusion-mediated transport of these substances is hindered by the ECS structure but the microscopic basis of this hindrance is not fully understood. One hypothesis proposes that the hindrance originates in large part from the presence of dead-space (DS) microdomains that can transiently retain diffusing molecules. Because previous theoretical and modeling work reported an initial period of anomalous diffusion in similar environments, we expected that brain regions densely populated by DS microdomains would exhibit anomalous extracellular diffusion. Specifically, we targeted granular layers (GL) of rat and turtle cerebella that are populated with large and geometrically complex glomeruli. The integrative optical imaging (IOI) method was employed to evaluate diffusion of fluorophore-labeled dextran (MW 3000) in GL, and the IOI data analysis was adapted to quantify the anomalous diffusion exponent dw from the IOI records. Diffusion was significantly anomalous in rat GL, where dw reached 4.8. In the geometrically simpler turtle GL, dw was elevated but not robustly anomalous (dw = 2.6). The experimental work was complemented by numerical Monte Carlo simulations of anomalous ECS diffusion in several three-dimensional tissue models containing glomeruli-like structures. It demonstrated that both the duration of transiently anomalous diffusion and the anomalous exponent depend on the size of model glomeruli and the degree of their wrapping. In conclusion, we have found anomalous extracellular diffusion in the GL of rat cerebellum. This finding lends support to the DS microdomain hypothesis. Transiently anomalous diffusion also has a profound effect on the spatiotemporal distribution of molecules released into the ECS, especially at diffusion distances on the order of a few cell diameters, speeding up short-range diffusion-mediated signals in less permeable structures.
细胞外空间（ECS）是大脑中运输生物活性分子和药物的主要通道。这些物质的扩散介导运输受到ECS结构的阻碍，但其阻碍的微观基础尚未完全理解。一种假说认为，这种阻碍很大程度上源于死腔（DS）微区的存在，这些微区可以短暂地保留扩散分子。由于之前的理论和建模工作报道了在类似环境中存在反常扩散的初始阶段，我们预期DS微区密集分布的脑区会表现出反常的细胞外扩散。具体来说，我们将目标对准大鼠和龟小脑的颗粒层（GL），这些颗粒层中充满了大且几何形状复杂的肾小球。采用整合光学成像（IOI）方法评估荧光团标记的葡聚糖（分子量3000）在GL中的扩散，并对IOI数据分析进行调整，以从IOI记录中量化反常扩散指数dw。在大鼠GL中，扩散明显反常，dw达到4.8。在几何结构更简单的龟GL中，dw升高但并非强烈反常（dw = 2.6）。实验工作得到了在包含肾小球样结构的几个三维组织模型中对反常ECS扩散进行的数值蒙特卡罗模拟的补充。结果表明，瞬时反常扩散的持续时间和反常指数都取决于模型肾小球的大小及其包裹程度。总之，我们在大鼠小脑的GL中发现了反常的细胞外扩散。这一发现支持了DS微区假说。瞬时反常扩散对释放到ECS中的分子的时空分布也有深远影响，特别是在扩散距离为几个细胞直径量级时，它会加快在渗透性较低结构中的短程扩散介导信号。
