Purinergic Receptors in Human Blood Platelets: Chemical Modification and Cloning Investigations

Abstract

Platelet aggregation is important for maintaining normal hemostasis. However, aberrant platelet aggegation plays a major role in acute coronary artery diseases, myocardial infarction, unstable angina, and stroke. ADP is one of the earliest and most important platelet agonists. ADP induces platelet aggregation, shape change, secretion, influx and intracellular mobilization of Ca²⁺, and inhibition of the adenylyl cyclase stimulated by prostaglandins. Binding of ADP to purinergic receptor(s) is required for elicitation of the ADP-induced platelet responses. But the platelet ADP receptor(s) has not been purified, largely due to the unavailability of the reagents that can be used to selectively label the platelet ADP receptor. The ADP receptor responsible for the ADP-induced platelet aggregation and inhibition of stimulated adenylyl cyclase activity has not been cloned due to difficulties in screening responsive clones generated from a cDNA library. Since the purified ADP-receptor protein is not available, antibodies that can be used as alternative tools to purify the ADP receptor or screen the clones expressing the receptor could not be made. In addition, the problem may be compounded by the low copy number and the susceptibility of the receptor to proteolysis. Therefore, signal transduction mechanisms underlying biochemical transformations in ADP-induced platelet responses remain less well defined and/less well understood. In the past decade efforts have been made to identify a platelet ADP receptor(s) by photoaffinity as well as affinity labeling by the ADP-affinity analogs. More recently efforts have been directed to clone the platelet ADP receptors. These investigations, however, have not produced definite results. The purpose of this review is to examine the results obtained by the photoaffinity- and affinity-labeling investigations and cloning experiments to identify a platelet ADP receptor(s).