![]() ![]() Although astroglial Cxs are also involved in non-communicating functions ( Elias et al., 2007 Scemes, 2008 Cina et al., 2009), here I will limit my comments to glial wiring through networks of communicating astrocytes and thus only consider the GJC function of astroglial Cxs. ![]() The expression of functional Cx and/or Px HCs in astrocytes is still a matter of debate (see Scemes et al., 2007 Orellana et al., 2009). Finally, pannexins (Pxs), which constitute another family of membrane proteins homologous to the GJC forming proteins in invertebrates called innexins, were shown to form HCs but not GJCs in vertebrates. In defined conditions (free divalent solution, metabolic inhibition, inflammation), Cxs in astrocytes can also operate as hemichannels (HCs) defining an inside-outside signaling pathway (see Spray et al., 2006). These channels are poorly selective for ions and for small molecular weight signaling molecules (<1 − 1.2 kDa) ( Harris, 2007), providing an ionic and a biochemical or metabolic coupling, respectively. ![]() A gap junction channel (GJC) is made of two facing hexamers of Cxs, each of them inserted in the plasma membrane of neighboring cells ( Bruzzone et al., 1996). Gap junctions, at the level of which aggregate intercellular channels, represent a unique example of a direct cytoplasm-to-cytoplasm communication. However, in the CNS the highest level of Cxs expression occurs in glia, in particular astrocytes, where Cxs are detected from embryonic to adult stages ( Dermietzel et al., 1989 Nagy and Rash, 2000). In neurons Cxs provide the morphological basis for electrical synapses which are frequent in the developing brain and also remain in several structures in adults when chemical synapses prevail ( Roerig and Feller, 2000 Sohl et al., 2005). In the brain, 11 Cxs have been detected ( Theis et al., 2005) confirming that Cxs are widely expressed in the central nervous system where they may play important roles. Many of them play critical roles in development and are necessary for tissue functions since several Cx knock out mice are not viable ( Dobrowolski and Willecke, 2009). So far 20 and 21 different Cxs have been identified in rodents and humans, respectively ( Nagy et al., 2004 Theis et al., 2005). Gap junctions formed by connexins (Cxs), their molecular constituents, are ubiquitous membrane specializations found in all vertebrate tissues. ![]()
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