Hechler and Gachet Purinergic Receptors : Thrombosis and Inflammation 2309

Extracellular nucleotides are the most ancient and widespread intercellular signaling system in living tissues and serve as intercellular or autocrine messengers regulating numerous physiological or pathological functions, including neurotransmission, muscle contraction, bone metabolism, liver glycogen metabolism, cardiac function, vascular tone, inflammation, and hemostasis and thrombosis. In most cases, extracellular nucleotides act as potent costimuli, which amplify and sustain the responses to neurotransmitters, hormones, cytokines, and other agents. They also play a role as autocrine regulators of immune responses, as a find-me-signal of apoptotic cells, and as activators of the inflammasome and are involved in both acute and chronic/adaptive situations. Extracellular nucleotides may be released through specific mechanisms involving different plasma membrane proteins, such as pannexins or connexin hemichannels or cell activation and granule secretion as in the case of blood platelets. They can also be passively released after mechanical stress or changes in the homeostatic conditions, including anoxia and ischemia-reperfusion injury. During inflammation, for example, adenosine 5′-triphosphate (ATP) released from dying cells and damaged tissues or in response to inflammatory stimuli acts as a danger signal or damage-associated molecular pattern, which displays potent immune-enhancing activity, whereas at sites of vascular injury, adenine nucleotides, massively released by activated platelets, contribute to both hemostasis and thrombus formation by greatly amplifying most of the platelet responses. Once in the extracellular compartment, nucleotides can be metabolized by various ectonucleotidases, such as ectonucleoside triphosphate diphosphohydrolase 1 (CD39), which converts ATP and adenosine 5′-diphosphate (ADP) to adenosine monophosphate; ecto-5′-nucleotidase (CD73), which converts adenosine monophosphate to adenosine; and nucleoside diphosphate kinase, which catalyzes the exchange of phosphate groups between different nucleoside diphosphates (Figure 1). This complex nucleotide metabolism probably explains why it has long been difficult to assign particular cell functions to the activation of specific receptors. Furthermore, the molecular identity of the nucleotide receptors remained unknown for a long time. The receptors for nucleotides belong to the P2 receptor family, whereas P1 receptors are activated by adenosine and comprise 4 subtypes A 1 , A 2A , A 2B , and A 3 . P2 receptors are subdivided into ligand-gated P2X cation channels and G protein–coupled P2Y receptors (Figure 1). The first P2 receptor subtype to be identified was cloned from a chick brain complementary DNA library in 1993. Currently, the known P2 receptor family comprises 7 subtypes of P2X receptors (P2X 1 , P2X 2 , P2X 3 , P2X 4 , P2X 5 , P2X 6 , and P2X 7 ) and 8 subtypes of P2Y receptors (P2Y 1 , P2Y 2 , P2Y 4 , P2Y 6 , P2Y 11 , P2Y 12 , P2Y 13 , and P2Y 14 ). These P2 receptor subtypes display some agonist selectivity. Thus, P2X receptors are activated principally by ATP, some P2Y receptors are activated by purine nucleotides alone (P2Y 1,12,13 ), and others by both purine and pyrimidine nucleotides (P2Y 2,4,6,11 ), whereas P2Y 14 is activated by nucleotide sugars. The various cell types involved in innate immunity, inflammation, and thrombosis all express multiple P2 receptor subtypes, which may play complementary roles depending on the relative affinities for their natural ligand and the downstream signaling pathways. However, a certain extent