Differential Regulation of α-Granule and Dense Granule Secretion by an Actin Cytoskeletal Barrier.

Platelet granule secretion is an essential component of normal arterial thrombus formation. Stimulation of platelets with strong agonists results in centralization of cytoplasmic organelles and loss of granules. These observations have lead to the supposition that cytoskeletal contraction facilitates granule secretion. Yet, the influence of the actin cytoskeleton in controlling membrane fusion events required for granule secretion remains largely unknown. Initial studies using electron microscopy revealed that the actin disrupting agents latrunculin A (4 μM) or cytochalasin E (4 μM) prevented pseudopod formation and granule centralization in platelets exposed to SFLLRN or PMA, but did not prevent degranulation. We next determined the effects of disruption of the actin cytoskeleton on α-granule secretion by monitoring P-selectin expression and β-thromboglobulin release. Incubation of platelets with either latrunculin A or cytochalasin E failed to stimulate α-granule secretion, but increased the rate of SFLLRN-induced α-granule secretion by 3.5-fold. Cytoskeletal disruption also augmented the degree of SFLLRN-induced α-granule secretion by 41±18% and reduced the amount of SFLLRN required to cause half-maximal stimulation by 2-fold. Incubation with latrunculin A stimulated α-granule secretion by the weak secretogues epinephrine or ADP by 7.6-fold and 5.4-fold, respectively. Cytoskeletal disruption also facilitated β-thromboglobulin release in response to SFLLRN, epinephrine, or ADP. In platelets permeabilized in the absence of ATP, exposure to 2 μM latrunculin A resulted in a 6.5- and 3.5-fold increase in α-granule release induced by Ca²⁺- or GTP-γ-S, respectively. Antibodies directed at a SNARE protein termed vesicle-associated fusion protein (VAMP) inhibited latrunculin A-dependent α-granule secretion. Thus, disruption of the actin cytoskeletal barrier by latrunculin A supports SNARE protein-dependent membrane fusion. Since actin acts as a barrier to α-granule secretion, we evaluated α-granules purified by subcellular fractionation for the presence of F-actin. Purified α-granules, but not phospholipid micelles, bound the F-actin probe FITC-phalloidin as determined by flow cytometry. FITC-phalloidin binding was inhibited in a dose-dependent manner by latrunculin A. These data indicated that α-granules are coated with F-actin that could serve a barrier function. We next evaluated the effects of cytoskeletal disruption on dense granule secretion by monitoring ADP/ATP release using a luciferin-luciferase based assay and by quantifying [³H]serotonin release. Cytoskeletal disruption by 4 μM latrunculin A failed to affect the degree of dense granule secretion from platelets stimulated by either SFLLRN, epinephrine, or ADP. Yet, 200 μM latrunculin A stimulated substantial dense granule release in the absence of agonist exposure and augmented SFLLRN-induced dense granule release by 2-fold. In contrast, 200 μM latrunculin A abolished SFLLRN-induced α-granule secretion. These observations indicate that the cytoskeleton differentially regulates α-granule and dense granule secretion. Our results also suggest that while some degree of actin polymerization is required for α-granule secretion, dense granule secretion is not dependent on actin polymerization.