Fig. 13

Schematic showing potential roles of astrocytic Cx43, hemichannels, and GJIC during OGD/R injury. Under normal conditions, astrocytic Cx43 is expressed in the plasma membrane and assembled into hemichannels that are normally closed. Hemichannel-hemichannel interactions induce the formation of GJIC between adjacent astrocytes, which permits the exchange of ions and small molecules; also, plasma membrane’s Cx43 was phosphorylated at Ser368 site. In such circumstances, astrocytes, together with those resting microglia, function as a supportive assistant for healthy neurons. OGD/R injury caused abnormal hemichannel opening and consequent substantial astrocytic ATP release. It also induced microglial activation with a predominance of the pro-inflammatory cytokine-releasing M1 subtype. Extracellular ATP induced further microglial activation and pro-inflammatory cytokine release, and these pro-inflammatory cytokines induced further opening of astrocytic hemichannels. SalB reversed these effects and thus provided protection against OGD/R injury. This suggests the existence of a vicious cycle in which astrocytic hemichannel opening and pro-inflammatory microglial activation reinforce each other following OGD/R injury. This vicious cycle may account for secondary injury and extended damage after OGD/R injury; OGD/R injury caused gap junction internalization, which may account for the astrocytic uncoupling events. It also decreased plasma membrane levels of Ser368-phosphorylated Cx43 while increasing plasma membrane levels of Ser373-phosphorylated Cx43, Ser265-phosphorylated Cx43, and Src’s Tyr416-phosphorylated activated form. The activated Src may well have phosphorylated Cx43 at Tyr265 and further induced gap junction internalization or autophagy. SalB directly inhibits Src, which may allow it to exert protective effects by attenuating Cx43 internalization. CBX, a non-selective hemichannel and GJIC inhibitor, did not apparently affect Cx43 phosphorylation, but it inhibited PKC and Src activity