The effect of phosphoinositol-4-kinase inhibition on spreading in human platelets

Platelets, although for a long time only associated with the process of hemostasis, are now gaining scientific interest because of their emerging roles in processes like the modulation of the immune system, viral infections and cancer. However, platelets’ signaling network is still not completely understood because of the numerous activatory, inhibitory and negative feedback signals all interconnected in a finely regulated system responsible for platelet function and responsiveness to their environment. Platelet cytoskeletal rearrangements during activation are mediated by well-known lipid second messengers, phosphatidylinositol 4, 5‐bisphosphate (PIP2) and phosphatidylinositol 3, 4, 5‐trisphosphate (PIP3), however, the role of phosphatidylinositol 4‐ phosphate (PI4P) has not been studied thus far. Therefore, our aim was to investigate the role of phosphatidylinositol 4-kinase (PI4K), an enzyme responsible for producing PI4P, a lipid molecule long thought to be only a precursor for PIP2 and PIP3. Transcriptomic and proteomic data show high levels of several PI4K isoforms in mouse and human platelets. We confirmed the presence of PI4K type II (α and β) in human platelets isolated from blood of several different donors as well as in megakaryocytic cell line DAMI using Western blot analysis. Also, we show the presence of PI4K type II (α and β) by immunostaining, as well as their product, PI4P in human platelets. Finally, we determined that PI4Kα is essential for platelet spreading on glass by specific pharmacological inhibition of PI4Kα. Inhibition of PI4Kα dose dependently decreased F-actin staining, and increased number of platelets with filopodia and actin nodules, characteristic of early platelet activation events. We conclude that PI4Kα is an essential part of the platelet signaling network, specifically the activatory pathways responsible for platelet spreading with roles in modulating actin cytoskeleton.