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Connexins, innexins and pannexins: Bridging the communication gap.

By Jean-Claude Herve, Pauline Phelan, Roberto Bruzzone and Thomas W. White


Multicellular organisms have evolved different mechanisms of intercellular communication, the most direct and quickest of which is through channels that link the cytoplasms of adjacent cells. In plants, a cytoplasmic continuity exists through elongated cytoplasmic bridges termed plasmodesmata, which cross the thick cell walls surrounding plant cells. In metazoans, cells are interconnected by channels which span the two plasma membranes and the intercellular space; these result from the docking of two half channels, which are hexameric torus of junctional proteins around an aqueous pore. These densely packed channels, localised in gap junctions, provide a direct route by which cells can exchange ions and small molecules, including oligonucleotides, siRNAs, and second messengers such as Ca2+, inositol phosphates and cyclic nucleotides. Gap junctions are found in essentially all tissues at some stage of development hinting at an enormous diversity of functions beyond their traditional roles in coordinating electrical activity in excitable tissues. All junctional channels have a similar overall structure but, unlike many other membrane channels, different gene families encode the membrane proteins that form them in different animal phyla (see Fig. 1). For a long time, gap junction structure and functions were mainly investigated in the vertebrates, where they were thought to be formed solely by connexins (Cxs). Then, in Drosophila (an arthropod) and C. elegans (a nematode), which have no Cx genes, gap junctions were found to be composed of another gene family, the innexins (Inxs, invertebrate analogues of Cxs), which have no sequence homology to Cxs [1]. The list of animal phyla with identified Inx family members progressively extended to annelida, platyhelminthes, mollusca and coelenterata. Inxs were also identified in polydnaviruses, symbiotic proviruses of parasitic wasps; these functional genes appear to have originated from, and co-evolved with, host insect innexins (see [2] and [3]). Sequences with low similarity to the invertebrate innexins were identified in vertebrate chordates, leading some authors to suggest that the protein family be re-named pannexins (from the Greek “pan”, neuter of the adjective “pas”, which means all, entire, and nexus, connection ; [4] and [5]. At present the vertebrate proteins and a few invertebrate innexins are referred to as pannexins (abbreviated Panx; see Fig. 1). For clarity, this term will be used here only to refer to the chordate innexin-like sequences. It also emerged that Cx genes were not restricted to vertebrate animals but were also present in invertebrate chordates (e.g. in tunicates, ascidians and appendicularians; see [6] for an analysis of their relationship to vertebrate connexins)

Topics: Q
Year: 2005
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