Abstract
Human platelets and their phylogenetic counterparts, avian thrombocytes, play a key role in primary hemostasis. Based upon extensive studies in mammals, platelet cohesion resulting in the formation of the "hemostatic plug" is known to be mediated by the mammalian platelet glycoprotein IIb-IIIa complex in concert with fibrinogen and calcium. The immunological and biochemical technology already developed for the analyses of mammalian platelet glycoproteins has never been applied to avian thrombocytes. By indirect immunofluorescence, we now show that a polyclonal rabbit antibody specific for human glycoproteins IIb plus IIIa and the well-characterized murine monoclonal anti-IIb-IIIa complex antibody, AP2, both crossreact with IIb and IIIa analogs on intact chicken thrombocytes. By two-dimensional polyacrylamide gel electrophoresis, we also demonstrate that chicken thrombocytes will incorporate [35S]methionine into several proteins, including the glycoprotein IIb and IIIa analogs during short-term (4 hr) incubation in vitro. This finding indicates that peripheral blood nucleated thrombocytes of the chicken, unlike their mammalian counterparts, retain the capacity to synthesize protein. The significance of these findings is 2-fold. First, we provide biochemical and immunological evidence that those proteins responsible for platelet cohesion in humans are structurally conserved in cells of analogous function in chickens despite the fact that these species have diverged from a common ancestor more than 200-250 million years ago. Second, we identify chicken thrombocytes as a readily available source of messenger RNA encoding numerous proteins analogous to those already characterized in human platelets, including glycoproteins IIb and IIIa.