Professor University of Kentucky Lexington, Kentucky, United States
Background: The platelets are small cellular fragments important for hemostasis, thrombosis, inflammation, and infection. These functions are carried out by a diverse cargo in platelet granules which fuse to the plasma membrane upon platelet activation. Various complexes such as SNAREs and Exocyst facilitate this granule fusion. The exocyst complex is an octameric tethering complex known for its role in exocytosis, cell polarity, cytokinesis, and tumorigenesis.
Aims: All eight subunits are present in platelets, but it is unclear how they operate or whether they contribute to platelet function.
Methods: To study the role of exocyst complex, we generated a megakaryocyte/platelet-specific Sec10/EXOC5-/- mouse model. Western blot analysis was performed to confirm the deletion of Sec10 and the levels of other components in these platelets. Using ATP and serotonin release (dense), PF4/P-selectin (alpha), β-hexosaminidase/LAMP1 (Lysosome) release as cargo markers, we studied kinetics and the extent of granule secretion. Tail bleeding assay, FeCl3 assay and jugular puncture assays were used to study arterial and venous hemostasis.
Results: The animals were morphologically sound and fertile. Sec10/EXOC5-/- animals were thrombocytopenic with increased mean platelet volume suggesting defective platelet biogenesis. Other Exocyst subunits: Exo70/EXOC7 and Exo84/EXOC8, were diminished in Sec10/EXOC5-/- platelets. Though the secretion of α-granule surface protein- P-selectin was normal, the exocytosis of soluble cargo, PF4, was reduced. Secretion from dense granules and lysosomes was unaffected. Additionally, Sec10/EXOC5-/- platelets showed slightly increased surface levels of integrins but expression of the collagen receptor, GPVI was decreased. The Sec10/EXOC5-/- animals had normal tail-bleeding times indicating that the deficiency in α-granule secretion did not affect hemostasis.
Conclusion(s): Our findings suggest that the deletion of Sec10 perturbs the exocyst complex assembly but it has minimal effect on platelet secretion and hemostasis remains unaffected. Hematological data suggest that the exocyst complex may play an important role in both platelet biogenesis and function.
