Thrombasthenic Platelets Do Not Bind Plasma Fibronectin

Plasma fibronectin (fn) binds to thrombin-stimulated normal platelets and may thus contribute to platelet participation in hemostasis. Glanzmann’s thrombasthenia is an inherited bleeding disorder in which the affected platelets do not aggregate in response to thrombin or other stimuli and have diminished ability to retract clots. To evaluate the ability of thrombasthenic platelets to bind fn, suspensions of gel filtered platelets and 2nM ¹²⁵I fn were incubated at 37°C in the presence or absence of 2 units/ml purified human thrombin. At various times, cells were centrifuged through 20% sucrose in microfuge tubes and bound ¹²⁵I fn measured in amputated tube tip. In the case of normal cells, in the presence of thrombin, there was time dependent uptake, plateauing by 20 min incubation. At equilibrium, greater than 95% of binding was saturable, and Scatchard plots indicated a single class of binding sites with Kd = 3×l0^-7 M and a maximum of 120,000 molecules/platelet. No binding was observed in the absence of thrombin. In contrast, platelets from 7 thrombasthenic individuals from 4 unrelated kindreds showed little or no detectable uptake of fn in the presence or absence of thrombin even after a 60 min incubation. Nevertheless, thrombin did induce serotonin secretion from these cells, showing that stimulation was occurring. Thrombasthenic platelets are defective in their ability to bind fibrinogen in the presence of ADP. These data indicate a defect in platelet interaction with another plasma protein in this disease. They also suggest the possibility that deficient fn-platelet interaction contributes to the thrombasthenic bleeding diathesis.