Ecto-nucleotidases of the CD39/NTPDase family modulate platelet activation and thrombus formation: Potential as therapeutic targets

Abstract

Extracellular nucleotide P2-receptor-mediated effects on platelets, leukocytes and endothelium are modulated by ecto-nucleotidases. These ecto-enzymes hydrolyze extracellular nucleotides to the respective nucleosides. The dominant ecto-nucleotidase expressed by the endothelium, by monocytes and vascular smooth muscle cells is CD39/NTPDase1. Ecto-nucleotidase biochemical activity of CD39 is lost at sites of acute vascular injury, such as in ischemia reperfusion and immune graft rejection. CD39L(Like)1/NTPDase2, a related protein, is associated with the basolateral surface of endothelium, the adventitia of vessels and microvascular pericytes. CD39/NTPDase1 hydrolyzes both tri- and diphosphonucleosides and blocks platelet aggregation responses to ADP. In contrast, CD39L1/NTPDase2, a preferential nucleoside triphosphatase, activates platelets by preferentially converting ATP to ADP, the major agonist of platelet P2 receptors. Spatial and temporal expression of NTPDases in the vasculature appears to control platelet activation, thrombus size and stability by regulating phosphohydrolytic activity and consequent P2 receptor signaling. Constitutively circulating microparticles appear to be associated with functional NTPDases, and accumulation of these at sites of vascular injury might influence local thrombus formation and evolution. The phenotype of the cd39-null mouse is in keeping with disordered thromboregulation with heightened susceptibility to inflammatory vasculary reactions, increased permeability and high levels of tissue fibrin. Paradoxically, these mutant mice also exhibit a bleeding phenotype with differential platelet P2Y1 desensitization. Over-expression of CD39 at sites of vascular injury and inflammation by adenoviral vectors, by transgenesis or by the use of pharmacological modalities with soluble derivatives has been shown to have major potential in several animal models tested to date. Future clinical applications will involve the development of new therapeutic strategies to various inflammatory vascular diseases and in transplantation.

Introduction

Vascular diseases are the number one public health issue in the developed world resulting in devastating symptoms relating to coronary artery occlusion, peripheral vascular insufficiency and cerebrovascular disorders. These diseases develop in millions of people annually, resulting in major morbidity and mortality. In these disease states, organ infarction is mediated by arterial thrombosis and is associated with vascular inflammation. Platelets and the endothelium have been recognized for decades as key pathological components of these processes [1].

In more recent years, extracellular nucleotides have become clearly recognized for the important role that they play in modulating a variety of processes linked to vascular inflammation and thrombosis [2]. In general, extracellular nucleotides tend to induce inflammation, whereas nucleosides largely result in cellular events that downregulate such activation responses. Selective pharmacological agents and experiments studying mutant mice bolster these observations. Clinical modalities to interfere with platelet-mediated arterial thrombosis have included successful strategies that target platelet receptors for extracellular nucleotides (e.g. inhibition of the platelet ADP receptor P2Y12 by the antithrombotic drug clopidogrel) [3], [4], [5].