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

水通道称为水通道蛋白(AQP)，是一种膜蛋白，负责将水分子输送进和流出细胞。AQP4选择性地表达在快速抽动(II型)纤维的肌膜上，是人脑中含量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AQP4定位于肌肉和脑的细胞膜上(Yokota等人)。2000)。最近，我们发现，在渗透性休克或跑步机运动后，α-合成素缺乏的肌肉表现出肌肉容量调节和肌力恢复的受损(Yokota等人。2014年，Nichols等人。2015年)。我们推测：1)AQP4在运动中和运动后肌肉疲劳的恢复中起重要作用；2)AQP4参与运动中和运动后脑脊液流量的增加。这项研究的目的是通过分析突变的小鼠模型来破译AQP4的新作用。*1.AQP4缺陷肌肉的渗透调节*由于细胞内液体乳酸浓度的增加，快速抽动骨骼肌纤维的活性增加导致高渗透压。在其他组织中，水通道蛋白参与细胞体积调节，称为渗透压刺激时的调节性体积增加(RVI)或调节性体积减小(RVD)。骨骼肌是否具有容量调节能力，如RVI和RVD，以响应渗透条件的变化，目前仍存在争议。为了回答这个问题，我们将检查AQP4缺陷肌肉的肌肉容量调节和针对渗透性变化的力量恢复是否受损。我们将从突变小鼠身上分离肌纤维束，并通过测量体外比重来检测水分含量。此外，我们还将测量渗透休克后肌束产生的力量。2.AQP4在体内运动中的作用*我们最近证明，α-合成素缺乏的小鼠表现出从跑步机运动中恢复的延迟(Yokota等人)。肌肉神经，2014)。首先，我们将检查在AQP4缺陷的骨骼肌中是否观察到这些变化。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内脑脊液的形成，因为众所周知，运动可以自然增加脑脊液流量，而AQP4参与脑内脑脊液的形成。此外，我们还将检测野生型(WT)小鼠在运动过程中和运动后肌肉和脑中AQP4表达的变化。*这些目标的成功实现将清楚地表明，AQP4参与了运动中和运动后肌肉疲劳的恢复和大脑功能的维持。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.**