Purinergic signaling plays distinct and important roles in the CNS, including the transmission of calcium signals between astrocytes. Gap junction hemichannels are among the mechanisms proposed by which astrocytes might release ATP; however, whether the gap junction protein connexin43 (Cx43) forms these "hemichannels" remains controversial. Recently, a new group of proteins, the pannexins, have been shown to form nonselective, high-conductance plasmalemmal channels permeable to ATP, thereby offering an alternative for the hemichannel protein. Here, we provide strong evidence that, in cultured astrocytes, pannexin1 (Panx1) but not Cx43 forms hemichannels. Electrophysiological and fluorescence microscope recordings performed in wild-type and Cx43-null astrocytes did not reveal any differences in hemichannel activity, which was mostly eliminated by treating Cx43-null astrocytes with Panx1-short interfering RNA [Panx1-knockdown (Panx1-KD)]. Moreover, quantification of the amount of ATP released from wild-type, Cx43-null, and Panx1-KD astrocytes indicates that downregulation of Panx1, but not of Cx43, prevented ATP release from these cells.

(Panx1), an ortholog to invertebrate innexin gap junctions, has recently been proposed to be the pore induced by P2X 7 receptor (P2X 7R) activation. We explored the pharmacological action of compounds known to block gap junctions on Panx1 channels activated by the P2X 7R and the mechanisms involved in the interaction between these two proteins. Whole cell recordings revealed distinct P2X 7R and Panx1 currents in response to agonists. Activation of Panx1 currents following P2X 7R stimulation or by membrane depolarization was blocked by Panx1 small-interfering RNA (siRNA) and with mefloquine Ͼ carbenoxolone Ͼ flufenamic acid. Incubation of cells with KN-62, a P2X 7R antagonist, prevented current activation by 2Ј(3Ј)-O-(4-benzoylbenzoyl)adenosine 5Ј-triphosphate (BzATP). Membrane permeabilization to dye induced by BzATP was also prevented by Panx1 siRNA and by carbenoxolone and mefloquine. Membrane permeant (TAT-P2X 7) peptides, provided evidence that the Src homology 3 death domain of the COOH-terminus of the P2X 7R is involved in the initial steps of the signal transduction events leading to Panx1 activation and that a Src tyrosine kinase is likely involved in this process. Competition assays indicated that 20 M TAT-P2X 7 peptide caused 50% reduction in Src binding to the P2X7R complex. Src tyrosine phosphorylation following BzATP stimulation was reduced by KN-62, TAT-P2X 7 peptide, and by the Src tyrosine inhibitor PP2 and these compounds prevented both large-conductance Panx1 currents and membrane permeabilization. These results together with the lack Panx1 tyrosine phosphorylation in response to P2X 7R stimulation indicate the involvement of an additional molecule in the tyrosine kinase signal transduction pathway mediating Panx1 activation through the P2X7R. gap junction blockers; permeabilization; P 2Z; src-tyrosine kinase THE IONOTROPIC PURINERGIC P2X 7 receptors (P2X 7 R) initially respond to agonists with the opening of a nonselective cation channel (24, 32). Prolonged exposure, however, leads to opening of a large pore that is permeable to molecules as large as 831 Da (44). Although this dual mode of function has been observed in many cell types, the pore has been reported to be absent in Xenopus oocytes in which P2X 7 cRNA is expressed (40), leading to the hypothesis that the small-conductance cation pathway may correspond to the P2X 7 R (whose other family members display such permeation upon ATP binding: 32), whereas the pore might be formed by another molecule that is recruited in the response. A candidate molecule recently proposed for the highly permeable pore is pannexin1 (Panx1) (30,38). This molecule, found in chordates, displays a low but significant degree of homology to innexins, the proteins that form gap junction channels in invertebrates (5, 52). Although Panx1 overexpression in Xenopus oocytes has been reported to result in a low degree of functional gap junction formation (10), formation of such intercellular channels by Panx1 in mammalian cells is unlikely (16,21) given the presence of ...

Pannexins (Panx1, 2 and 3) comprise a group of proteins expressed in vertebrates that share weak yet significant sequence homology with the invertebrate gap junction proteins, the innexins. In contrast to the other vertebrate gap junction protein family (connexin), pannexins do not form intercellular channels, but at least Panx1 forms non-junctional plasma membrane channels. Panx1 is ubiquitously expressed and has been shown to form large conductance (500pS) channels that are voltage-dependent, mechanosensitive and permeable to relatively large molecules, such as ATP. Pharmacological and knockdown approaches have indicated that Panx1 is the molecular substrate for the so-called “hemichannel” originally attributed to connexin43 (Cx43) and that Panx1 is the pore forming unit of the P2X7 receptor. Here, we describe, for the first time, conductance and permeability properties of Panx1-null astrocytes. The electrophysiological and fluorescence imaging analysis performed on these cells fully support our previous pharmacological and Panx1 knockdown studies that showed profoundly lower dye uptake and ATP release than wild-type untreated astrocytes. As a consequence of decreased ATP paracrine signaling, intercellular calcium wave spread is altered in Panx1-null astrocytes. Moreover, we found that in astrocytes as in Panx1 expressing oocytes, elevated extracellular K+ activates Panx1 channels independently of membrane potential. Thus, based on the present findings and our previous report, we propose that Panx1 channels serve as K+ sensors for changes in the extracellular milieu such as those occurring under pathological conditions.

The authors' laboratory has reported potent block of Pannexin1 (Panx1) currents by the antimalarial quinine derivative mefl oquine. However, other laboratories have found little or no mefl oquine sensitivity of Panx1 currents or processes attributable to these channels. In order to resolve this issue, the authors have performed extensive dose-response studies on Panx1-transfected neuroblastoma (Neuro2A) and rat insulinoma (Rin) cells, comparing mefl oquine obtained from three suppliers and also comparing the sensitivity to diastereomers. Results indicate a 20-fold difference in sensitivity to the (−)-threo-(11R/2R) diastereomer compared to the erythro enatiomers and much lower potency of (Ϯ)-erythro-(R*/S*)-mefl oquine obtained from one of the commercial sources. This markedly lower effi cacy presumably accounts for the disparity in results from different laboratories who have applied it in Panx1 studies.

The major neuronal gap junction protein Connexin 36 (Cx36) exhibits the remarkable property of “run-up”, in which junctional conductance typically increases by ten-fold or more within 5–10 min following cell break-in with patch pipettes. Such conductance “run-up” is a unique property of Cx36, as it has not been seen in cell pairs expressing other connexins. Because of the recent observation describing CaMKII binding and phosphorylation sites in Cx36 and evidence that calmodulin dependent protein kinase II (CaMKII) may potentiate electrical coupling in neurons of teleosts, we have explored whether CaMKII activates mammalian Cx36. Consistent with this hypothesis, certain Cx36 mutants lacking the CaMKII binding and phosphorylation sites or wild type Cx36 treated with certain cognate peptides corresponding to binding or phosphorylation sites blocked or strongly attenuated run-up of junctional conductance. Likewise, KN-93, an inhibitor of CaMKII, blocked run-up, as did a membrane permeable peptide corresponding to the CaMKII autoinhibitory domain. Furthermore, run-up was blocked by phosphatase delivered within the pipette and not affected by treatment with the phosphatase inhibitor okadaic acid. These results imply that phosphorylation by CaMKII strengthens junctional currents of Cx36 channels, thereby conferring functional plasticity on electrical synapses formed of this protein.