Gap junctions and hemichannels in glia

Gap junctions (GJs) are unique large channels that connect the cytoplasm of two adjacent cells. Half GJs, called connexons or hemichannels (HC), expressed by individual cells may also be functionally active, and when open would connect a cell's cytoplasm to extracellular space. Gap junctions mediate a primitive and important form of intercellular communication that is seen across phyla. Discovered in 1952 in heart muscle cells, GJs are expressed in most mammalian cells; mature skeletal muscle cells, spermatozoa, and erythrocytes are the only exceptions. Gap junction mediated coupling between glial cells was first noted in the leech central nervous system (CNS) by Stephen Kuffler and colleagues in 1964. In the mammalian CNS, connexins, the proteins that form GJs and HCs, are most abundantly expressed by glial cells. All subtypes of glia express connexins and GJs, with the possible exception of the newly defined NG2 cell. The characteristics and possible functions of GJs and HCs in glial cells are the focus of this review. First, however, some important general principles about these channels and the proteins that make them will be discussed. © 2009 Elsevier Ltd All rights reserved.