Three family members compose the pannexin family of channel-forming glycoproteins (Panx1, Panx2 and Panx3). Their primary function is defined by their capacity to form single-membrane channels that are regulated by post-translational modifications, channel intermixing, and sub-cellular expression profiles. Panx1 is ubiquitously expressed in many mammalian tissues, while Panx2 and Panx3 appear to be more restricted in their expression. Paracrine functions of Panx1 as an ATP release channel have been extensively studied and this channel plays a key role, among others, in the release of "find-me" signals for apoptotic cell clearance. In addition Panx1 has been linked to propagation of calcium waves, regulation of vascular tone, mucociliary lung clearance, taste-bud function and has been shown to act like a tumor suppressor in gliomas. Panx1 channel opening can also be detrimental, contributing to cell death and seizures under ischemic or epileptic conditions and even facilitating HIV-1 viral infection. Panx2 is involved in differentiation of neurons while Panx3 plays a role in the differentiation of chondrocytes, osteoblasts and the maturation and transport of sperm. Using the available Panx1 knockout mouse models it has now become possible to explore some of its physiological functions. However, given the potential for one pannexin to compensate for another it seems imperative to generate single and double knockout mouse models involving all three pannexins and evaluate their interplay in normal differentiation and development as well as in malignant transformation and disease. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Pannexin1 (Panx1) is an integral membrane protein comprised of three species as follows: an unglycosylated core-Gly0, a high mannose-Gly1, and a complex glycosylated Gly2 species. Although Panx1 channels mediate several cellular responses, the domain regulating its oligomerization and cell surface trafficking and the mechanisms governing its internalization and degradation have not been identified. This study characterizes the role of the Panx1 C-tail domain by truncating the polypeptide at residue 307 and expressing the mutant in BICR-M1R k and HEK-293T cells. Enzymatic digestion and immunolabeling assays revealed that the Panx1 T307 -RFP was glycosylated primarily to the high mannose species consistent with its retention in the endoplasmic reticulum. Co-expression of Panx1 T307 -RFP with Panx1 followed by co-immunoprecipitation assays revealed that the mutant and Panx1 could interact, whereas biotinylation assays showed that this interaction inhibited Panx1 from maturing into the Gly2 species and reaching the cell surface. Additional inhibitor studies indicated that the degradation of the mutant was via proteasomes, whereas Panx1 was degraded by lysosomes. Analysis of the pathways important in Panx1 internalization revealed partial co-distribution of Panx1 with many molecular constituents of the endocytic machinery that include clathrin, AP2, dynamin II, caveolin-1, and caveolin-2. However, co-immunoprecipitation assays together with the disruption of lipid rafts by methyl-␤-cyclodextrin suggest that Panx1 does not engage this endocytic machinery. Furthermore, dominant-negative and pharmacological studies revealed that Panx1 internalization was dynamin II-independent. Collectively, these results indicate that the oligomerization and trafficking of Panx1 are regulated by the C-terminal domain, whereas internalization of long lived Panx1 channels occurs in a manner that is distinct from classical endocytic pathways.Pannexins were originally discovered due to their shared sequence homology with the invertebrate gap junction proteins, innexins (1). The pannexin family is comprised of three members as follows: Panx1, Panx2, and Panx3 (1). Although the scope of physiological significance of pannexin family members is only beginning to emerge, it has been reported that Panx1 holds importance in forming conduits for ATP release (2-4), propagation of Ca 2ϩ waves (5), and neuronal and immunological inflammasome (6 -8). Additionally, activation of Panx1 channels has been implicated in ischemic cell death (9) and seizure-like activities (10). Although Panx1 single channel conductances of 550 picosiemens (2) have been correlated with ionic dysregulation during ischemic conditions, subsequently leading to neuronal necrosis (9), the N-methyl-D-aspartate receptor-induced opening of Panx1 channels has been implicated in hippocampal epileptiform seizure-like activity (10). Other studies have associated Panx1 channel activation with apoptosis of Xenopus oocytes by forming a pore unit with the death complex of the P2X7 receptor (...