Deciphering the role of water channel aquaporin-4 in muscle and brain

水通道被称为水通道蛋白（AQPs）是一种膜蛋白，可以将水分子导入和导出细胞。AQP 4选择性地表达于快速收缩（II型）纤维的肌膜，并且是人脑中最丰富的蛋白质之一。然而，它在肌肉和大脑中的作用仍然难以捉摸。我们以前发现AQP 4通过α-突触营养蛋白定位于肌肉和大脑的细胞膜上（Yokota et al. 2000）。最近，我们发现α-突触营养蛋白缺乏的肌肉在渗透压休克或跑步机运动后表现出受损的肌肉体积调节和肌力恢复（Yokota et al. 2014，Nichols et al. 2015）。我们假设1）AQP 4在运动期间和运动后肌肉疲劳的恢复中起重要作用，2）AQP 4参与运动期间和运动后脑脊液（CSF）流量增加。这项研究的目的是通过分析突变小鼠模型来破译AQP 4的新作用。1. AQP 4缺陷肌肉中的渗透调节 * 由于细胞内液乳酸盐浓度增加，快速收缩骨骼肌纤维的活性增加导致高渗透压。其他组织中的AQP参与细胞体积调节，称为渗透压休克时的调节性体积增加（RVI）或调节性体积减少（RVD）。骨骼肌是否能够响应渗透压条件的变化而进行容量调节（例如RVI和RVD）仍然存在争议。为了回答这个问题，我们将检查是否在AQP 4缺乏的肌肉中肌肉体积调节和针对渗透压变化的力恢复受损。我们将从突变小鼠分离肌纤维束，并通过测量离体比重来检查含水量。此外，我们将测量渗透压休克后肌束产生的力。我们预计，缺陷小鼠表现出受损的肌肉体积调节和减少的力量产生对渗透压休克。2. AQP 4在体内运动时的作用 * 我们最近证明，α-突触营养蛋白缺陷小鼠表现出从跑步机运动中恢复延迟（Yokota等人，Muscle Nerve，2014）。首先，我们将检查这些变化是否在AQP 4缺陷的骨骼肌中观察到。其次，我们将使用非侵入性近红外（NIR）光散射方法监测突变小鼠脑中CSF的形成，因为已知运动可以自然地增强CSF流动，并且AQP 4参与脑中CSF的形成。此外，我们将研究野生型（WT）小鼠在运动过程中和运动后肌肉和大脑中AQP 4表达的变化。* 这些目标的成功完成将清楚地表明，AQP 4参与肌肉疲劳的恢复和运动期间和运动后大脑功能的维持。这些结果将为基础肌肉生理学和脑科学提供有价值的知识。

Water channels called aquaporins (AQPs) are membrane proteins that conduct water molecules in and out of the cell. AQP4 is selectively expressed at the muscle membrane of fast twitch (type II) fibres, and one of the most abundant proteins in human brain. However, its roles in muscle and brain have remained elusive. We previously identified that AQP4 is localized by alpha-syntrophin at the cell membrane in muscle and brain (Yokota et al. 2000). More recently, we have shown that alpha-syntrophin deficient muscles exhibit impaired muscle volume regulation and muscle force recovery after osmotic shocks or treadmill exercise (Yokota et al. 2014, Nichols et al. 2015). We hypothesize that 1) AQP4 plays important roles in recovery from muscle fatigue during and after exercise, and 2) AQP4 is involved in increased cerebrospinal fluid (CSF) flow during and after exercise. The goal of this study is to decipher novel roles of AQP4 by analyzing mutant mouse models.******1. Osmotic Regulation in AQP4 Deficient Muscles ***Increased activity of fast twitch skeletal muscle fibres results in hyper-osmolality due to increased intracellular fluid lactate concentration. AQPs in other tissues are involved in cell volume regulation called regulatory volume increase (RVI) or regulatory volume decrease (RVD) upon osmotic shocks. It is still controversial whether the skeletal muscles are capable of volume regulation such as RVI and RVD in response to changes in osmotic conditions. To answer this question, we will examine if muscle volume regulation and force recovery against osmotic changes are impaired in the AQP4 deficient muscles. We will isolate muscle fibre bundles from the mutant mice, and examine the water contents by measuring the specific gravity ex-vivo. In addition, we will measure the force generated by muscle bundles after osmotic shock. We anticipate that the deficient mice show impaired muscle volume regulation and decreased force generation against osmotic shocks.******2. The role of AQP4 upon exercise in vivo***We recently demonstrated that alpha-syntrophin deficient mice exhibit delayed recovery from treadmill exercise (Yokota et al. Muscle Nerve, 2014). First, we will examine if these changes are observed in AQP4 deficient skeletal muscles. Second, we will monitor the CSF formation in brain in the mutant mice using non-invasive near-infrared (NIR) light-scattering method, as exercises are known to naturally enhance CSF flow and AQP4 is involved in the formation of CSF in brain. In addition, we will examine the changes of AQP4 expression in muscle and brain in wild-type (WT) mice during and after exercise. ***Successful completion of these aims will clearly demonstrate that AQP4 is involved in recovery from muscle fatigue and maintenance of brain function during and after exercise. These results will provide valuable knowledge of the basic muscle physiology and brain science.**