Regulation of intrinsic activity of the Epithelial Na+ Channel

描述(由申请人提供)：上皮性钠通道或ENaC是限制肾上皮细胞钠吸收的速率。通道活动决定了最终的尿钠浓度，并部分决定了钠的排泄。通道活动会影响身体的钠和水平衡，从而影响血压。该通道主要存在于质膜上的电非活性池中。最近，有研究表明，丝氨酸蛋白酶可以通过切割两个通道亚基来激活这个不活跃的池。因此，这种通道调节模式为表达ENaC的肾上皮细胞提供了快速响应钠负荷、肾滤过和体内钠平衡变化的巨大能力。蛋白水解酶激活沉默的通道涉及从通道的α和伽马亚基中去除短的抑制域。然而，我们的数据清楚地表明，非切割的通道可以是活跃的，而一些切割的通道亚基不形成活跃的通道。我们证明Po可以受到多种机制的调节，包括通道处理，膜脂刚性，通道分配到脂筏，以及ENaC亚单位抑制域之间的相互作用。我们认为ENaC亚基之间的多重相互作用决定了通道Po。通道亚基的切割提供了一种消除或改变其中一些相互作用的手段，并导致通道激活。我们还提出，通道亚基的相互作用会影响亚基的加工、膜的分割和稳定性。统一的假设是，未处理的通道在膜上是稳定的，但本质上是不活跃的，除非存在减少亚单位-亚单位抑制的条件，而且，蛋白水解会通过永久消除这种亚单位间的相互作用来激活或启动通道。蛋白水解会使通道亚基内化，除非加工的亚基在RAFT膜结构域中受到保护。为了解决通道活性控制的机制，我们开发了新的工具，利用1)工程裂解位点2)最近发现的组成ENaC亚单位(Epsilon)3)通道到ASIC1晶体结构的同源映射和4)通道划分到膜域的分析。我们的数据将为理解航道PO的调节以及进一步修改该PO的每一个下游过程铺平道路。 公共卫生相关性：我们研究了控制肾上皮钠通道自发活动的机制。这种蛋白的活性对于确定肾脏的盐分排泄，进而决定体内的盐分滞留以及血容量和血压是很重要的。了解这种蛋白质的自发活动将有助于理解血压对盐敏感的基础，并对设计高血压患者的治疗方案很重要。

DESCRIPTION (provided by applicant): The Epithelial Na+ Channel or ENaC is rate limiting to sodium absorption in renal epithelia. Channel activity sets the final urinary sodium concentration and determines in part sodium excretion. Channel activity affects body sodium and water balance and therefore blood pressure. This channel exists at the plasma membrane largely in an electrically inactive pool. Recently, it was demonstrated that this inactive pool can be activated by cleavage of two of the channel subunits by serine proteases. Thus, this mode of channel regulation affords ENaC expressing renal epithelia an enormous capacity to rapidly respond to changes of sodium load, renal filtration, and body sodium balance. Activation of silent channels by proteases is proposed to involve the removal of short inhibitory domains from the channel's alpha and gamma subunits. However, our data clearly demonstrate that non-cleaved channels can be active, while some cleaved channel subunits do not form active channels. We demonstrate that Po can be regulated by multiple mechanisms that include channel processing, membrane lipid rigidity, channel partitioning into lipid rafts, and interaction between inhibitory domains of the ENaC subunits. We propose that multiple interactions between the ENaC subunits determine channel Po. Cleavage of the channel subunits provides a mean of removing or altering some of these interactions and leads to channel activation. We also propose that channel subunit interactions affect subunit processing, membrane partitioning and stability. The unifying hypothesis is that unprocessed channels are stable at the membrane but inherently inactive unless conditions exist to reduce subunit-subunit inhibition, and that moreover, proteolysis activates or primes the channels for activation by permanently removing such inter-subunit interactions. Proteolysis then predisposes the channel subunits for internalization unless processed subunits are protected in raft membrane domains. To address the mechanism of control of channel activity, we develop new tools that utilize 1) engineered cleavage sites 2) a recently identified constitutively ENaC subunit (epsilon) 3) homology mapping of the channel to the crystal structure of ASIC1 and 4) analysis of channel partitioning into membrane domains. Our data will pave the way for understanding channel Po regulation and every downstream process that further modify this Po. PUBLIC HEALTH RELEVANCE: We examine the mechanisms that control the spontaneous activity of the renal epithelial sodium channel. The activity of this protein is important in determining kidney salt excretion and consequently salt retention by the body and blood volume and pressure. Understanding the spontaneous activity of this protein would help understand the basis of salt sensitivity of blood pressure and is important in designing therapy for individuals with high blood pressure.