Pannexins, a family of gap junction proteins expressed in brain

Database search has led to the identification of a family of proteins, the pannexins, which share some structural features with the gap junction forming proteins of invertebrates and vertebrates. The function of these proteins has remained unclear so far. To test the possibility that pannexins underlie electrical communication in the brain, we have investigated their tissue distribution and functional properties. Here, we show that two of these genes, pannexin 1 (Px1) and Px2, are abundantly expressed in the CNS. In many neuronal cell populations, including hippocampus, olfactory bulb, cortex and cerebellum, there is coexpression of both pannexins, whereas in other brain regions, e.g., white matter, only Px1-positive cells were found. On expression in Xenopus oocytes, Px1, but not Px2 forms functional hemichannels. Coinjection of both pannexin RNAs results in hemichannels with functional properties that are different from those formed by Px1 only. In paired oocytes, Px1, alone and in combination with Px2, induces the formation of intercellular channels. The functional characteristics of homomeric Px1 versus heteromeric Px1/Px2 channels and the different expression patterns of Px1 and Px2 in the brain indicate that pannexins form cell type-specific gap junctions with distinct properties that may subserve different functions. Gap junctions are collections of intercellular channels that, in vertebrates, are formed by connexins, a multigene family of which 20 members have been identified in humans (1). It is generally accepted that gap junctions between neurons represent the anatomical substrate of electrical synapses (reviewed in refs. 2 and 3). Although the incidence of electrical coupling relative to chemical synapses in the adult is relatively low, several studies have demonstrated that different types of interneurons of the hippocampus and neocortex communicate by means of electrical synapses in a cell-specific manner (4-11). These observations suggest that this additional form of intercellular communication is more widespread than previously imagined and delineates independent networks of coupled cells. Besides the undisputed role of chemical transmission in network oscillations, both computer simulations and electrophysiological recordings have recently emphasized a key role for electrical synapses in the generation of synchronous activity in the hippocampus and neocortex (6, 12-17). The identification of connexin36 (Cx36) as the main neuronal connexin expressed in several areas of the brain (18), suggested that it may be an important component of gap junctions involved in the synchronization of large-scale neuronal networks. This possibility has been directly tested in mice with a targeted ablation of Cx36, which exhibit impaired electrical coupling in several brain regions (15, 19-23). Loss of this gap-junction protein abolishes electrical coupling between hippocampal interneurons and disrupts ?-frequency network oscillations in vitro and in vivo (15, 24). The specificity of this impairment was indicated by the finding that high-frequency rhythms in hippocampal pyramidal cells are unaffected by the lack of Cx36 (15). These observations raise two possibilities: either a different connexin is specifically deployed throughout the pyramidal cell network or, alternatively, another class of molecules expressed in the mammalian brain forms electrical junctions between pyramidal cells. The latter hypothesis has received theoretical support from the discovery, in the database, of a family of genes for which the name pannexin (Px) has been proposed (25). Because they share structural features with gap junction proteins of invertebrates and vertebrates, we investigated their tissue distribution and analyzed their ability to form functional channels.