Department of Molecular Medicine, Graduate School of Pharmaceutical Sciences, Kumamoto University, Kumamoto, Japan.
Exp Cell Res. 2009 Nov 15;315(19):3294-300. doi: 10.1016/j.yexcr.2009.09.023. Epub 2009 Sep 30.
Epithelial sodium channel (ENaC) is a heteromultimeric Na(+) channel at the apical membrane in the kidney, colon, and lung. Because ENaC plays a crucial role in regulating Na(+) absorption and extracellular fluid volume, its dysregulation causes severe phenotypes including hypertension, hypokalemia, and airway obstruction. Despite the importance of ENaC, its protein quality control mechanism remains less established. Here we firstly show the role of calreticulin (CRT), a lectin-like molecular chaperone in the endoplasmic reticulum (ER), on the regulation of ENaC. Overexpression and knockdown analyses clearly indicated that CRT positively affects the expression of each ENaC subunit (alpha, beta and gamma). CRT overexpression also up-regulated the cell surface expression of alpha-, beta- and gamma-ENaC. Moreover, we found that CRT directly interacts with each ENaC subunit. Although CRT knockdown did not affect the de novo synthesis of ENaC subunits, CRT overexpression decreased alpha-, beta- and gamma-ENaC expression in the detergent (RIPA)-insoluble fraction, suggesting that CRT enhanced the solubility of ENaC subunits. Consistent with the increased intracellular and cell surface expression of ENaC subunits, increased channel activity of ENaC was also observed upon overexpression of CRT. Our study thus identifies CRT as an ER chaperone that regulates ENaC expression and function.
上皮钠离子通道（ENaC）是肾脏、结肠和肺顶膜中的一种异源多聚体 Na(+) 通道。由于 ENaC 在调节 Na(+) 吸收和细胞外液量方面起着至关重要的作用，其功能失调会导致严重的表型，包括高血压、低钾血症和气道阻塞。尽管 ENaC 非常重要，但它的蛋白质质量控制机制仍不太明确。在这里，我们首先展示了内质网（ER）中一种分子伴侣钙网蛋白（CRT）在调节 ENaC 中的作用。过表达和敲低分析清楚地表明，CRT 可正向影响每个 ENaC 亚基（α、β和γ）的表达。CRT 过表达还上调了α-、β-和γ-ENaC 的细胞表面表达。此外，我们发现 CRT 可直接与每个 ENaC 亚基相互作用。虽然 CRT 敲低并不影响 ENaC 亚基的从头合成，但 CRT 过表达降低了 RIPA 不可溶性部分中α-、β-和γ-ENaC 的表达，表明 CRT 增强了 ENaC 亚基的可溶性。与 ENaC 亚基的细胞内和细胞表面表达增加一致，CRT 过表达也增加了 ENaC 的通道活性。因此，我们的研究确定 CRT 是一种内质网伴侣，可调节 ENaC 的表达和功能。
