CD39, or vascular adenosine triphosphate diphosphohydrolase, has been considered an important inhibitor of platelet activation. Unexpectedly, cd39-deficient mice had prolonged bleeding times with minimally perturbed coagulation parameters. Platelet interactions with injured mesenteric vasculature were considerably reduced in vivo and purified mutant platelets failed to aggregate to standard agonists in vitro. This platelet hypofunction was reversible and associated with purinergic type P2Y1 receptor desensitization. In keeping with deficient vascular protective mechanisms, fibrin deposition was found at multiple organ sites in cd39-deficient mice and in transplanted cardiac grafts. Our data indicate a dual role for adenosine triphosphate diphosphohydrolase in modulating hemostasis and thrombotic reactions.

Nucleoside triphosphate diphosphohydrolases (NTPDases) are a recently described family of ectonucleotidases that differentially hydrolyze the ␥ and ␤ phosphate residues of extracellular nucleotides. Expression of this enzymatic activity has the potential to influence nucleotide P2 receptor signaling within the vasculature. We and others have documented that NTPDase1 (CD39, 78 kd) hydrolyzes both triphosphonucleosides and diphosphonucleosides and thereby terminates platelet aggregation responses to adenosine diphosphate (ADP). In contrast, we now show that NTPDase2 (CD39L1, 75 kd), a preferential nucleoside triphosphatase, activates platelet aggregation by converting adenosine triphosphate (ATP) to ADP, the specific agonist of P2Y 1 and P2Y 12 receptors. We developed specific antibodies to murine NTPDase1 and NTPDase2 and observed that both enzymes are present in the cardiac vasculature; NTPDase1 is expressed by endothelium, endocardium, and to a lesser extent by vascular smooth muscle, while NTPDase2 is associated with the adventitia of muscularized vessels, microvascular pericytes, and other cell populations in the subendocardial space. Moreover, NTPDase2 represents a novel marker for microvascular pericytes. IntroductionNucleoside triphosphate diphosphohydrolases (NTPDases) are a family of ectonucleotidases, previously classified as E-type ATPases, ATPDases, ecto-ATPases, or ecto-apyrases. 1-3 These enzymes differentially hydrolyze the terminal ␥ and ␤ phosphate residues of nucleotides, resulting in the rapid formation of the respective diphosphonucleosides and/or monophosphonucleosides. To date, 6 members of this NTPDase family have been identified. 3-13 NTPDase1, NTPDase2, and NTPDase3 are transmembrane proteins associated with the plasma membrane with an active site facing the extracellular space. 4,5,7,9,14 These NTPDase members have been demonstrated to differ in their substrate specificity. For example, NTPDase1 (CD39 [human] or cd39 [murine]) hydrolyzes both nucleoside triphosphates and diphosphates-eg, adenosine triphosphate (ATP) and adenosine diphosphate (ADP), 4,5 -whereas NTPDase2 (CD39L1) is a preferential nucleoside triphosphatase or ATPase. 7,[14][15][16] Extracellular nucleotides in various forms, and at different concentrations, activate multiple P2 receptors: ionotropic P2X and metabotropic P2Y receptors. [17][18][19][20] While NTPDases would be anticipated to generally terminate P2 receptor agonist signaling, we have also observed that NTPDase1 may prevent receptor desensitization by the catalysis of extracellular nucleotides. 21 NTPDase1 may then also facilitate subsequent responses to "pulses" of nucleotides. In addition, NTPDases in tandem with ecto-5Ј-nucleotidase may facilitate the salvage of nucleotides by the ultimate generation of dephosphorylated forms that are taken up by cells via specific transporters. 1,22 NTPDase1 is the major ectonucleotidase at the luminal surface of blood vessels. 5,21,23,24 By converting the ADP released from activated platelets, NTPDase1 modulates platel...