Peracchia C, Bernardini G
Fed Proc. 1984 Sep;43(12):2681-91.
In most tissues neighboring cells communicate directly with each other by exchanging ions and small metabolites via cell-to-cell channels located at the intermembrane particles of gap junctions. Evidence indicates that the channels close when the [Ca2+]i or [H+]i increases. The channel occlusion (cell-to-cell uncoupling) is mainly a safety device by which cells can isolate themselves from damaged neighboring cells ("healing-over" process). Despite our knowledge of uncoupling agents, the uncoupling mechanism is still poorly understood. Uncoupling treatments have been shown to cause structural changes in gap junctions, characterized by an increase in tightness and regularity (crystallization) of particle packing and a decrease in particle size. Recently these changes have been shown to be induced by Ca2+ or H+ in isolated lens junctions and by Ca2+ in liver junctions, which suggests a close relationship between structural changes and uncoupling, but preliminary studies indicate that the junctional changes may not be synchronous with uncoupling but may lag behind it. However, recent X-ray diffraction data show that the channels of crystalline gap junctions (typical of uncoupled cells) are indeed closed, because they are inaccessible to sucrose (a gap junction permeant). Thus it seems that crystalline junctions are indeed in a non-permeable state, but the occlusion of the channels may precede the crystallization process. In the lens, junction crystallization is inhibited by a calmodulin (CaM) inhibitor, trifluoperazine (TFP). Is CaM involved in the uncoupling mechanism? To test this hypothesis, TFP and calmidazolium (CDZ), the most specific CaM inhibitor, were used on amphibian embryonic cells electrically uncoupled by CO2. Both TFP and CDZ effectively protect the cells from uncoupling, which suggests that CaM participates in the process. As a hypothesis, we propose that channel occlusion follows a CaM-mediated conformational change in the junctional protein. Particle crystallization may follow the conformational changes and result from a modification in electrostatic repulsion among the particles.
在大多数组织中，相邻细胞通过位于间隙连接膜间颗粒处的细胞间通道交换离子和小代谢物，从而直接相互通讯。有证据表明，当细胞内钙离子浓度（[Ca2+]i）或氢离子浓度（[H+]i）升高时，这些通道会关闭。通道阻塞（细胞间解偶联）主要是一种安全机制，通过该机制细胞能够将自身与受损的相邻细胞隔离开来（“愈合”过程）。尽管我们了解解偶联剂，但解偶联机制仍知之甚少。已表明解偶联处理会导致间隙连接发生结构变化，其特征是颗粒堆积的紧密性和规则性（结晶）增加，颗粒尺寸减小。最近，这些变化已被证明在分离的晶状体连接中由Ca2+或H+诱导，在肝连接中由Ca2+诱导，这表明结构变化和解偶联之间存在密切关系，但初步研究表明连接变化可能与解偶联不同步，而是可能滞后于解偶联。然而，最近的X射线衍射数据表明，结晶性间隙连接（未偶联细胞的典型特征）的通道确实是关闭的，因为蔗糖（一种间隙连接通透剂）无法进入这些通道。因此，似乎结晶性连接确实处于非通透状态，但通道阻塞可能先于结晶过程。在晶状体中，连接结晶受到钙调蛋白（CaM）抑制剂三氟拉嗪（TFP）的抑制。CaM是否参与解偶联机制？为了验证这一假设，将TFP和最特异的CaM抑制剂卡马西平（CDZ）用于通过二氧化碳进行电解偶联的两栖类胚胎细胞。TFP和CDZ均能有效保护细胞免于解偶联，这表明CaM参与了该过程。作为一种假设，我们提出通道阻塞是由连接蛋白中CaM介导的构象变化引起的。颗粒结晶可能跟随构象变化，并由颗粒间静电排斥的改变导致。
