Evidence for a role for G alpha(i1) in mediating weak agonist-induced platelet aggregation in human platelets: reduced G alpha(i1) expression and defective G(i) signaling in the platelets of a patient with a chronic bleeding disorder

We have examined platelet functional responses and characterized a novel signaling defect in the platelets of a patient suffering from a chronic bleeding disorder. Platelet aggregation responses stimulated by weak agonists such as adenosine diphosphate (ADP) and adrenaline were severely impaired. In comparison, both aggregation and dense granule secretion were normal following activation with high doses of collagen, thrombin, or phorbol-12 myristate-13 acetate (PMA). ADP, thrombin, or thromboxane A(2) (TxA(2)) signaling through their respective G(q)- coupled receptors was normal as assessed by measuring either mobilization of intracellular calcium, diacylglycerol (DAG) generation, or pleckstrin phosphorylation. In comparison, G(i)-mediated signaling induced by either thrombin, ADP, or adrenaline, examined by suppression of forskolin-stimulated rise in cyclic AMP (cAMP) was impaired, indicating dysfunctional Galpha(i) signaling. Immunoblot analysis of platelet membranes with specific antiserum against different Galpha subunits indicated normal levels of Galpha(i2), Galpha(i3), Galpha(z), and Galpha(q) in patient platelets. However, the Galpha(i1) level was reduced to 25% of that found in normal platelets. Analysis of platelet cDNA and gDNA revealed no abnormality in either the Galpha(i1) or Galpha(i2) gene sequences. Our studies implicate the minor expressed Galpha(i) subtype Galpha(i1) as having an important role in regulating signaling pathways associated with the activation Of alpha(IIb)beta(3) and subsequent platelet aggregation by weak agonists. (Blood. 2003;101:4828-4835).