Division of Anatomy, Department of Molecular Medicine, Institute of Basic Medical Sciences, University of Oslo, Oslo, Norway.
Brain Struct Funct. 2017 Dec;222(9):3959-3972. doi: 10.1007/s00429-017-1448-5. Epub 2017 May 27.
Aquaporin-4 (AQP4) is the predominant water channel in the brain and is expressed in high density in astrocytes. By fluxing water along osmotic gradients, AQP4 contributes to brain volume and ion homeostasis. Here we ask whether deletion of Aqp4 leads to upregulation of the gap junctional proteins connexin-43 (Cx43) and connexin-30 (Cx30). These molecules couple adjacent astrocytes to each other and allow water and ions to redistribute within the astrocyte syncytium. Immunogold analysis of parietal cortex and hippocampus showed that the number of gap junctions per capillary profile is increased in AQP4 knockout (AQP4 KO) mice. The most pronounced changes were observed for Cx43 in hippocampus where the number of connexin labeled gap junctions increased by 100% following AQP4 KO. Western blot analysis of whole tissue homogenates showed no change in the amount of Cx43 or Cx30 protein after AQP4 KO. However, AQP4 KO led to a significant increase in the amount of Cx43 in a Triton X-100 insoluble fraction. This fraction is associated with connexin assembly into gap junctional plaques in the plasma membrane. In line with our immunoblot data, RT-qPCR showed no significant increase in Cx43 and Cx30 mRNA levels after AQP4 KO. Our findings suggest that AQP4 KO leads to increased aggregation of Cx43 into gap junctions and provide a putative mechanistic basis for the enhanced tracer coupling in hippocampi of AQP4 KO mice. The increased number of gap junctions in AQP4 deficient mice may explain why Aqp4 deletion has rather modest effects on brain volume and K homeostasis.
水通道蛋白 4（AQP4）是大脑中主要的水通道，在星形胶质细胞中高表达。通过沿着渗透梯度流动水，AQP4 有助于脑容量和离子内稳态。在这里，我们询问 Aqp4 的缺失是否会导致缝隙连接蛋白连接蛋白 43（Cx43）和连接蛋白 30（Cx30）的上调。这些分子将相邻的星形胶质细胞彼此连接，并允许水和离子在星形胶质细胞合胞体中重新分布。顶叶皮层和海马体的免疫金分析表明，AQP4 敲除（AQP4 KO）小鼠中每个毛细血管轮廓的缝隙连接数量增加。在海马体中观察到最明显的变化是 Cx43，AQP4 KO 后，连接蛋白标记的缝隙连接数量增加了 100％。全组织匀浆的 Western blot 分析显示，AQP4 KO 后 Cx43 或 Cx30 蛋白的量没有变化。然而，AQP4 KO 导致 Triton X-100 不溶性级分中 Cx43 的量显着增加。该级分与质膜中连接蛋白组装成缝隙连接斑有关。与我们的免疫印迹数据一致，RT-qPCR 显示 AQP4 KO 后 Cx43 和 Cx30 mRNA 水平没有显着增加。我们的研究结果表明，AQP4 KO 导致 Cx43 聚集到缝隙连接中增加，并为 AQP4 KO 小鼠海马体中示踪剂偶联增强提供了潜在的机制基础。AQP4 缺陷小鼠中缝隙连接数量的增加可能解释了为什么 Aqp4 缺失对脑容量和 K 内稳态的影响相当温和。
