Platelet aging and desialylation increase apoptotic priming and BCL-X_L dependence

Platelets are short-lived anucleate cells essential for primary hemostasis and recognized for their functions in thrombosis, immunity, antimicrobial defense, neurodegeneration, as well as cancer growth and metastasis. Their brief lifespan in circulation is controlled by the removal of sialic acid residues from the platelet surface (desialylation) and also the mitochondrial apoptosis pathway, with high expression of the anti-apoptotic protein BCL-X_L being required for platelet survival. This dependence on BCL-X_L has prevented the clinical deployment of recently developed small molecule inhibitors of BCL-X_L, which have promising activity in solid as well as liquid cancers but cause on-target thrombocytopenia. Here, we investigate the functional relationship between platelet desialylation and apoptosis to determine how cross-talk between these mechanisms may impact platelet lifespan. We find that platelets progressively lose sialic acid residues and become more primed for apoptosis while in circulation, resulting in aged platelets that are desialylated and highly prone to undergoing apoptosis. In addition, platelet desialylation via endogenous or exogenous factors directly increases their BCL-X_L dependence and accelerates apoptosis, which can be reversed by treatment with the sialidase inhibitor DANA (2,3-dehydro-2-deoxy-N-acetylneuraminic acid). Notably, young platelets recently released into circulation are less primed for apoptosis and less dependent on BCL-X_L for survival. Consistent with these changes in priming, platelets aged in vitro exhibit increasing expression of multiple pro-apoptotic proteins including BIM, BAK and PUMA along with increasing cleaved caspase 3. Leveraging the lower BCL-X_L dependence of young platelets, stimulation of de novo platelet production with the thrombopoietin receptor agonist romiplostim prevents BH3 mimetic-induced thrombocytopenia in vivo and may prevent severe platelet loss in patients treated with BCL-X_L inhibitors.