Expression and Roles of Pannexins in ATP Release in the Pituitary Gland

Abstract

Pannexins are a newly discovered three-member family of proteins expressed in the brain and peripheral tissues that belong to the superfamily of gap junction proteins. However, in mammals pannexins do not form gap junctions, and their expression and function in the pituitary gland have not been studied. Here we show that the rat pituitary gland expresses mRNA and protein transcripts of pannexins 1 and 2 but not pannexin 3. Pannexin 1 was more abundantly expressed in the anterior lobe, whereas pannexin 2 was more abundantly expressed in the intermediate and posterior pituitary. Pannexin 1 was identified in corticotrophs and a fraction of somatotrophs, the S100-positive pituicytes of the posterior pituitary and AtT-20 (mouse pituitary adrenocorticotropin-secreting cells) and rat immortalized pituitary cells secreting prolactin, whereas pannexin 2 was detected in the S100-positive folliculostellate cells of the anterior pituitary, melanotrophs of the intermediate lobe, and vasopressin-containing axons and nerve endings in the posterior lobe. Overexpression of pannexins 1 and 2 in AtT-20 pituitary cells enhanced the release of ATP in the extracellular medium, which was blocked by the gap junction inhibitor carbenoxolone. Basal ATP release in At-T20 cells was also suppressed by down-regulating the expression of endogenous pannexin 1 but not pannexin 2 with their short interfering RNAs. These results indicate that pannexins may provide a pathway for delivery of ATP, which is a native agonist for numerous P2X cationic channels and G protein-coupled P2Y receptors endogenously expressed in the pituitary gland.

Pannexin 1, an ATP Release Channel, Is Activated by Caspase Cleavage of Its Pore-associated C-terminal Autoinhibitory Region

“…In other cases of ball-and-chain inhibition, there has been no physiological mechanism described for cutting the chain to permanently remove channel inhibition, as we have found for caspase-mediated cleavage and activation of hPANX1. Although this cleavage-based mechanism seems appropriate for a terminal process such as apoptosis, we expect that more subtle mechanisms will account for other forms of PANX1 activation associated with different physiological conditions (10,26,27). Nevertheless, because C-terminal inhibition in the intact channel depends on its association with the pore, our data suggest that these other PANX1 activation mechanisms will also require some disruption of the C terminus from its inhibitory interaction site within the channel pore.…”

S-Nitrosylation Inhibits Pannexin 1 Channel Function

“…Since their identification in 2000, Panx1 channels have been characterized as ATP release channels that may play a pivotal role in supporting purinergic signaling in a multitude of cell types (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Importantly, Panx1 channels mediate ATP release from vascular smooth muscle and endothelial cells (7,14), circulating erythrocytes (11,15), airway epithelial cells (13), astrocytes (5,16,17) and T-cells (6,8).…”

Pannexin1 Drives Multicellular Aggregate Compaction via a Signaling Cascade That Remodels the Actin Cytoskeleton

“…Instead, recent evidence suggests that Panx1 channels primarily facilitate the release of ATP, which then binds to purinergic receptors (26). The expression of Panx1 in Xenopus oocytes (36) has been found to increase ATP release, whereas siRNA directed against Panx1 suppresses basal ATP release in mouse pituitary At-t20 cells (37).…”