P2X1: definitely not an ADP receptor

The story of platelets and purine nucleotides is one of the longest and most exciting in the biology and pathology of hemostasis. It began in the 1960s with the discovery of the role of adenosine diphosphate (ADP) as an important platelet agonist; like a good novel this story follows large avenues, but also dead ends, and turnabouts. Among the large avenues was the use of cloning technology, production, and analysis of knock-out and/or transgenic mice, along with the careful clinical, biologic, and molecular genetic analysis of patients suffering from ADP-dependent platelet defects, which led to the identification of the 3 purine receptors P2X₁, P2Y₁, and P2Y₁₂ in platelets. The combination of all 3 receptors appears to correspond to the formerly termed P2T receptor. P2Y₁ and P2Y₁₂ are both G-protein-coupled receptors (GPCRs) specific for ADP. Acting in concert, P2Y₁ mediates Ca²⁺ mobilization through a Gαq pathway, while P2Y₁₂ elicits a Gαi-dependent pathway, both acting concurrently to activate integrin αIIbβ3 and initiate platelet aggregation.¹  One specific feature of P2X₁ is that it is not a GPCR, but a purine-dependent ligand-gated ion channel (hence, X in the P2 subfamily of purine receptors). P2X₁ appears to trigger a rapid initial extracellular Ca²⁺ intake by platelets, necessary for P2Y₁ and P2Y₁₂ to promote platelet aggregation.¹  A second surprise is that P2X₁ was shown to bind adenosine triphosphate (ATP) instead of ADP, which is contrary to what was originally thought. The role of ATP in hemostasis is a long-standing controversial issue, but consensus has been reached on the fact that it is an antagonist of ADP for P2Y₁ and an agonist for P2X₁. However, the issue of ADP versus ATP as a P2X₁-specific ligand was recently “reactivated” by the identification of an alternate P2X₁ receptor termed P2X_1del, which is deleted of 17 amino acids within the cytoplasmic tail. P2X_1del was found to mediate ADP-evoked inward Ca²⁺ currents and to be present in cell lines of the megakaryocytic lineage, as well as in platelets.²  Strong controversy was raised by this work,³  and the issue has remained unsettled. In a new study, Vial and colleagues (page 3646) provide clear and definitive evidence that P2X_1del is not surface expressed when transfected alone, but that it can be surface expressed only as an heteromeric P2X₁/P2X_1del receptor, which then mediates ATP- and not ADP-evoked inward currents. Finally, these authors show that in platelets the level of total P2X_1del is most likely too low to account for a physiologic response, and that ATP but not ADP has potential to evoke the P2X₁-specific rapid Ca²⁺ rise in platelets in conditions where P2Y₁ response is blunted. These authors therefore conclude that ATP is the sole P2X₁ agonist and reinforce the idea that this purine nucleotide is probably an important physiologic platelet activator.