Yamamura Hisao, Ugawa Shinya, Ueda Takashi, Nagao Masataka, Shimada Shoichi
Department of Molecular Morphology, Graduate School of Medical Sciences, Nagoya City University, 1 Kawasumi Mizuhocho Mizuhoku, Nagoya 467-8601, Japan.
J Biol Chem. 2004 Mar 26;279(13):12529-34. doi: 10.1074/jbc.M400274200. Epub 2004 Jan 15.
The amiloride-sensitive epithelial Na(+) channel (ENaC) controls Na(+) transport into cells and across epithelia. So far, four homologous subunits of mammalian ENaC have been isolated and are denoted as alpha, beta, gamma, and delta. ENaCdelta can associate with beta and gamma subunits and generate a constitutive current that is 2 orders of magnitude larger than that of homomeric ENaCdelta. However, the distribution pattern of ENaCdelta is not consistent with that of the beta and gamma subunits. ENaCdelta is expressed mainly in the brain in contrast to beta and gamma subunits, which are expressed in non-neuronal tissues. To explain this discrepancy, we searched for novel functional properties of homomeric ENaCdelta and investigated the detailed tissue distribution in humans. When human ENaCdelta was expressed in Xenopus oocytes and Chinese hamster ovary cells, a reduction of extracellular pH activated this channel (half-maximal pH for an activation of 5.0), and the acid-induced current was abolished by amiloride. The most striking finding was that the desensitization of the acid-evoked current was much slower (by approximately 10% 120 s later), dissociating from the kinetics of acid-sensing ion channels in the degenerin/epithelial Na(+) channel family, which were rapidly desensitized during acidification. RNA dot-blot analyses showed that ENaCdelta mRNA was widely distributed throughout the brain and was also expressed in the heart, kidney, and pancreas in humans. Northern blotting confirmed that ENaCdelta was expressed in the cerebellum and the hippocampus. In conclusion, human ENaCdelta activity is regulated by protons, indicating that it may contribute to the pH sensation and/or pH regulation in the human brain.
氨氯吡脒敏感的上皮钠离子通道（ENaC）控制着钠离子进入细胞并穿过上皮组织的转运过程。到目前为止，已分离出哺乳动物ENaC的四个同源亚基，分别命名为α、β、γ和δ。ENaCδ可与β和γ亚基结合，并产生一种组成性电流，其大小比同聚体ENaCδ产生的电流大2个数量级。然而，ENaCδ的分布模式与β和γ亚基并不一致。与在非神经组织中表达的β和γ亚基不同，ENaCδ主要在大脑中表达。为了解释这种差异，我们寻找了同聚体ENaCδ的新功能特性，并研究了其在人类中的详细组织分布。当人ENaCδ在非洲爪蟾卵母细胞和中国仓鼠卵巢细胞中表达时，细胞外pH值的降低会激活该通道（激活的半数最大pH值为5.0），且酸诱导电流会被氨氯吡脒消除。最显著的发现是，酸诱发电流的脱敏过程要慢得多（120秒后约为10%），这与退化素/上皮钠离子通道家族中的酸敏感离子通道的动力学不同，后者在酸化过程中会迅速脱敏。RNA斑点印迹分析表明，ENaCδ mRNA广泛分布于整个大脑，在人类的心脏、肾脏和胰腺中也有表达。Northern印迹证实ENaCδ在小脑和海马体中表达。总之，人ENaCδ的活性受质子调节，这表明它可能在人类大脑的pH感知和/或pH调节中发挥作用。
