Coupling in Glial Cells: Who Is Coupled,and Why

Abstract

One of the features that distinguishes glial cells from most other brain cells is their ability to communicate with each other through cytoplasmic connections established by gap- junctions. Gap-junctions are cell-cell channels composed of two connexons, one con tributed by each communicating cell. These connexons are formed by six connexin mol ecules that assemble in a hexagonal array to form a central pore. As is common for channel proteins, connexins comprise a superfamily of at least 12 members, with molecular weights ranging from 26 to 50 kDa and overall sequence identities of 50%. In the brain, only two connexins are expressed to a significant degree. These are connexin 43 (Cx43), which is the predominant molecular unit of astrocytic gap-junctions, and connexin 32 (Cx32), which forms the gap-junctions in oligodendrocytes and some neurons. The pore formed by these connexins is permeant to ions and small hydrophilic molecules with molecular weights <1 kDa. Molecules believed to be trafficking through gap-junctions include metabolites and second messengers, but hydrophobic molecules and most pro teins are excluded. It is believed that large numbers of glial cells, and particularly astro cytes, participate in syncytia in which cells share a common cytoplasm. Gap-junctions are thought to integrate regional variances between cells and provide a diffusional pathway to facilitate the sequestration and redistribution of ions and neurotransmitters. Recent evidence suggests that astrocytic gap-junction may also allow the passage of cell-cell signals, permitting information to spread in the form of regenerative Ca²⁺ changes through a glial network. The Neuroscientist 1:188-191, 1995