The P2X1 receptor and platelet function

Abstract

Extracellular nucleotides are ubiquitous signalling molecules, acting via the P2 class of surface receptors. Platelets express three P2 receptor subtypes, ADP-dependent P2Y1 and P2Y12 G-protein-coupled receptors and the ATP-gated P2X1 non-selective cation channel. Platelet P2X1 receptors can generate significant increases in intracellular Ca(2+), leading to shape change, movement of secretory granules and low levels of α(IIb)β(3) integrin activation. P2X1 can also synergise with several other receptors to amplify signalling and functional events in the platelet. In particular, activation of P2X1 receptors by ATP released from dense granules amplifies the aggregation responses to low levels of the major agonists, collagen and thrombin. In vivo studies using transgenic murine models show that P2X1 receptors amplify localised thrombosis following damage of small arteries and arterioles and also contribute to thromboembolism induced by intravenous co-injection of collagen and adrenaline. In vitro, under flow conditions, P2X1 receptors contribute more to aggregate formation on collagen-coated surfaces as the shear rate is increased, which may explain their greater contribution to localised thrombosis in arterioles compared to venules within in vivo models. Since shear increases substantially near sites of stenosis, anti-P2X1 therapy represents a potential means of reducing thrombotic events at atherosclerotic plaques.

Fig. 1

Effects of P2X1 receptors on platelet activation studied in vitro. a Reversible shape change of human platelets following selective activation of P2X1 receptors with α,β-meATP. Scanning electron micrographs of washed platelets before stimulation (i), or 10 s (ii) and 60 s (iii) after addition of 10 μM α,β-meATP in the presence of 1 mM extracellular Ca²⁺. b Contribution of P2X1 receptors to aggregation at different collagen concentrations demonstrated using platelets from wild-type (black traces) and P2X1-deficient (grey traces) mice. c Enhancement of PAR1-dependent aggregation in human platelets by P2X1 receptor activation. The amplification of TRAP1 (SFLLRN)-induced aggregation is reduced as the time between P2X1 stimulation with α,β-meATP (1 μM) and TRAP (SFLLRN, threshold concentration) is increased from 0.25 to 3 min; additions at 15 s and 1 min are shown by the arrow heads, while the addition at 3 min is not shown. Figures reproduced from [38, 40, 71], with permission. The vertical axes in b and c have units of % light transmission; an upward deflection represents an increase in transmission and thus an aggregation. Traces in c have been inverted from the original reference to be consistent with those in b

Fig. 2

P2X1 receptor signalling and regulation in the platelet. Summary of the pathways whereby P2X1 receptors have been proposed to couple to functional responses in platelets, together with the mechanisms that regulate these ion channels. See text for explanation

Fig. 3

In vivo evidence for a contribution of platelet P2X1 receptors to thromboembolism and thrombosis. a Improved survival of P2X1-deficient mice following collagen/adrenaline injection due to reduced pulmonary thromboembolism. b Laser-induced thrombus development in murine cremaster muscle arterioles; (i) images showing the deposition of fluorescently tagged platelets in wild-type (WT) and P2X1^−/− mice following laser injury at the position marked by the arrow head; L indicates the arteriole lumen and the arrow shows the direction of blood flow (ii). Comparison of thrombus area development in WT, P2X1^−/−, P2Y1^−/− and P2Y12^−/−. Reproduced from references [40, 71], with permission