Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX 78229, USA.
Department of Ophthalmology, The Second Xiangya Hospital, Central South University, Changsha 410000, China.
J Cell Sci. 2018 Mar 21;131(6):jcs212506. doi: 10.1242/jcs.212506.
Elevated oxidized stress contributes to lens cataracts, and gap junctions play important roles in maintaining lens transparency. As well as forming gap junctions, connexin (Cx) proteins also form hemichannels. Here, we report a new mechanism whereby hemichannels mediate transport of reductant glutathione into lens fiber cells and protect cells against oxidative stress. We found that Cx50 (also known as GJA8) hemichannels opened in response to HO in lens fiber cells but that transport through the channels was inhibited by two dominant-negative mutants in Cx50, Cx50P88S, which inhibits transport through both gap junctions and hemichannels, and Cx50H156N, which only inhibits transport through hemichannels and not gap junctions. Treatment with HO increased the number of fiber cells undergoing apoptosis, and this increase was augmented with dominant-negative mutants that disrupted both hemichannels formed from Cx46 (also known as GJA3) and Cx50, while Cx50E48K, which only impairs gap junctions, did not have such an effect. Moreover, hemichannels mediate uptake of glutathione, and this uptake protected lens fiber cells against oxidative stress, while hemichannels with impaired transport had less protective benefit from glutathione. Taken together, these results show that oxidative stress activates connexin hemichannels in the lens fiber cells and that hemichannels likely protect lens cell against oxidative damage through transporting extracellular reductants.
氧化应激升高可导致晶状体白内障，缝隙连接在维持晶状体透明性方面发挥重要作用。连接蛋白 (Cx) 不仅形成缝隙连接，还形成半通道。在这里，我们报告了一种新的机制，即半通道介导还原剂谷胱甘肽转运到晶状体纤维细胞中，并保护细胞免受氧化应激。我们发现 Cx50（也称为 GJA8）半通道在晶状体纤维细胞中对 HO 作出反应而打开，但通过 Cx50 的两种显性负突变体 Cx50P88S 和 Cx50H156N 抑制了通道转运，Cx50P88S 抑制了缝隙连接和半通道的转运，而 Cx50H156N 仅抑制半通道而不抑制缝隙连接。用 HO 处理增加了经历细胞凋亡的纤维细胞数量，而用破坏由 Cx46（也称为 GJA3）和 Cx50 形成的半通道和缝隙连接的显性负突变体处理会增加这种增加，而仅损害缝隙连接的 Cx50E48K 则没有这种效果。此外，半通道介导谷胱甘肽摄取，这种摄取可保护晶状体纤维细胞免受氧化应激，而转运受损的半通道从谷胱甘肽获得的保护作用较小。总之，这些结果表明氧化应激激活了晶状体纤维细胞中的连接蛋白半通道，并且半通道可能通过转运细胞外还原剂来保护晶状体细胞免受氧化损伤。
