Regulation of Blood Pressure Homeostasis by Pannexin 1 Channels

The homeostatic regulation of blood pressure (BP) is multifaceted and temporally controlled by overlapping neural, vascular, and renal mechanisms. These mechanisms operate acutely in time to synchronize cardiovascular function with BP responses on a minute-to-minute basis, but also chronically over long-time scales to maintain BP within physiological limits. The integration of multivariate control mechanisms is heavily dependent on purinergic signaling, which utilizes adenosine 5’-triphosphate (ATP) as an autocrine/paracrine signal to coordinate cellular responses. The regulated release of cellular ATP and its accumulation in the extracellular space is a rate limiting step; yet the mechanisms responsible for ATP release are not well understood. Nevertheless, a primary role has been ascribed to pannexin (Panx) channels, specifically the Panx1 isoform in the vasculature. We initially focused on a role Panx1 channels in the peripheral vasculature where smooth muscle Panx1 influences acute hemodynamics involved in BP regulation. We report that phosphorylation of the intracellular loop YLK motif is directly and constitutively phosphorylated by Src kinase at tyrosine 198 (Y198). Using a Panx1-Y198 specific antibody and chemical/genetic modulators of Src kinase, we found that Src-mediated Y198 phosphorylation correlated with ATP release in vitro and ex vivo, and was necessary to support normal adrenergic vasoconstriction responses. We further demonstrated that Y198 phosphorylation was required for the presence of Panx1 at the plasma membrane, which was enriched in the smooth muscle layer of hypertensive human arteries suggestive of a contribution to hypertensive phenotypes. This discovery connects a significant purinergic vasoconstriction pathway with a previously identified, yet unexplored, tyrosine kinase-based adrenergic constriction mechanism, that causes dysregulation of vascular hemodynamics in hypertensive disease. Delving further into a potential regulation pathway for Panx1-mediated ATP release and adrenergic vasoconstriction in smooth muscle cells, we identified a novel interaction between Panx1 and the membrane scaffold protein caveolin-1 in resistance arteries. We found that the onset of this adrenergic-stimulus dependent interaction was rapid and occurred in proximity to areas of sympathetic nerve innervation. The functional importance of this interaction was further elucidated in vivo, in which cell-specific caveolin-1 genetic deletion resulted in significantly blunted adrenergic-stimulated ATP release, impaired vasoconstriction, and a significant reduction in mean arterial pressure (MAP). These studies implicate plasma membrane Panx1 as a necessary component of hemodynamic control in resistance arteries. In comparison to the hemodynamic regulation of Panx1 in the peripheral vasculature, we also assessed renovascular Panx1 from renin-lineage cells, which require purinergic signaling for negative feedback control of chronic BP responses. Using a novel renin-cell Panx1 knockout model, we found that Panx1 deletion significantly alters RAAS activation and significantly increases steady-state plasma renin concentrations. In Panx1 deficient animals, high renin levels correlated with increased aldosterone levels, reduced urine volume, and a significant increase in MAP. RAAS activity and BP were only partially dependent on angiotensin type 1 receptor (AT1R) activation. Finally, we uncovered a novel Panx1-dependent function in renin-lineage cells from both renovascular and adrenocortical cell. Panx1 deficient animals exhibited impairments in adaptive cell responses that typically maintain physiological BP setpoints (renal renin recruitment and adrenal cell trans-differentiation). Thus, Panx1 channels are novel regulators of cardiovascular feed-back mechanisms that are important in BP homeostasis.