Platelets are prototypic adhesive cells that have several major membrane glycoproteins which bind a variety of plasma and subendothelial molecules involved in maintaining vascular integrity and initiating hemostasis or thrombosis. We have continued studies of a rare hemorrhagic disorder, thrombasthenia, in which platelets have deficient or defective fibrinogen-binding and aggregation-promoting glycoprotein, GPIIb/IIIa. We found one patient had markedly reduced GPIIb mRNA and variant splicing with a partial deletion of exon 28, that proved on population studies to be an unusual alternative of a normal polymorphism. There were no sequence defects in the putative promoter/enhancer regions nor previously reported point mutations. Complete genomic DNA for GPIIb is currently being sequenced for insight into abnormalities of this major integrin, unique to platelets, which is pivotal in modulating platelet function. Alzheimer's disease is associated with brain deposition of plaques of a peptide called beta-amyloid. Beta-amyloid is normally present in plasma at levels of 225-625 pM. There is no function attributed to the soluble peptide. We found that soluble beta-amyloid at concentrations found in plasma augmented ADP-induced platelet aggregation 2-fold. The activation process was blocked by RGD inhibition of GPIIb/IIIa fibrinogen-binding sites. Partial beta-amyloid sequence (amino acids 1-16) was weakly active; the reverse sequence (40-1) was inactive. Beta-amyloid alone induced tyrosine phosphorylation of a 180 kD platelet protein, suggesting that physiologic plasma levels of this peptide may act through a specific tyrosine kinase-linked receptor. Hemorrhagic disorders due to alloantibodies against polymorphic antigens on platelet membrane GPs occur after transfusions and in neonates of sensitized mothers. Diagnosis and prophylaxis of these disorders depend on accurate pheno-or geno-typing. We developed restriction enzyme assays to allotype two different platelet antigen groups, Yuk and Br, using genomic DNA as a time -saving alternative to existing methods requiring platelet cDNA and Southern blotting.
