Function and Regulation of Pannexin 1 Channels in the Vascular Endothelium

Abstract

The nucleotide adenosine 5'-triphosphate (ATP) has classically been considered the cell's primary energy currency; however, a novel role for ATP as an extracellular autocrine/paracrine signaling molecule has evolved over the past century. Purinergic signaling is now known to regulate a plethora of physiological and pathophysiological processes in almost every organ system. In the vasculature, ATP and its metabolites elicit dual control over blood vessel tone and tissue perfusion, and purinergic signaling events have been implicated in vascular pathologies including atherosclerosis and inflammation.While the importance of extracellular ATP in the vascular system is well recognized, the mechanism(s) mediating the regulated release of the purine from vascular cells are less well understood and are a key target of current investigation. One such ATP-liberation mechanism has been ascribed to the recently identified pannexin (Panx) channels, namely Panx1. Initially, we characterized the expression and localization profiles of Panx isoforms across the systemic vasculature, identifying predominant representation by the Panx1 isoform in both smooth muscle (SMC) and endothelial cells (EC) comprising the blood vessel wall, with more heterogeneous expression profiles observed in specialized vascular systems including the heart, lung and kidney. In particular, the Panx1 isoform is highly expressed in ECs regardless of blood vessel type or size, while Panx1 expression is limited to SMCs of small arteries and arterioles. Panx1 forms hexameric channels in the plasma membrane of ECs, functioning to release ATP in response to a number of stimuli. However, prolonged Panx1 channel activity is extremely detrimental to cell viability. Here we report a novel negative regulatory mechanism that may govern the activity of Panx1 channels in ECs, by which the bioactive gas nitric oxide (NO) covalently modifies two cysteine (Cys) residues in the channel by a process termed S- reinforce the dual effect of purines on peripheral resistance and blood pressure. Purinergic Control of Vascular InflammationWhile the foundation for purinergic control of vascular functions was initially characterized extensively in the context of vascular reactivity and overall blood flow and pressure regulation, it has become increasingly clear that extracellular ATP also plays important regulatory roles in the vascular inflammatory response. Vascular inflammation can be characterized as acute (physiological) or chronic (pathological) in nature.The acute vascular inflammatory response is an innate process of inflammatory cell homing to infected or damaged tissues resulting from integrated cross-talk between circulating leukocytes and the vascular endothelium, primarily at the level of postcapillary venules [61]. This process has been highly studied and is now known to occur in a step-wise manner beginning with local recruitment, rolling (fast and slow), adhesion and final extravasation into the surrounding tissue (reviewed in [62...