The role of AGK in thrombocytopoiesis and possible therapeutic strategies.

Abnormal megakaryocyte development and platelet production lead to thrombocytopenia or thrombocythemia and increase the risk of hemorrhage or thrombosis. Acylglycerol kinase (AGK) is a mitochondrial membrane kinase that catalyzes the formation of phosphatidic acid and lysophosphatidic acid. Mutation of AGK has been described as the major cause of Sengers syndrome, and the patients with Sengers syndrome have been reported to exhibit thrombocytopenia. In this study, we found that megakaryocyte/platelet-specific AGK-deficient mice developed thrombocytopenia and splenomegaly, mainly caused by inefficient bone marrow thrombocytopoiesis and excessive extramedullary hematopoiesis, but not by apoptosis of circulating platelets. It has been reported that the G126E mutation arrests the kinase activity of AGK. The AGK G126E mutation did not affect peripheral platelet counts or megakaryocyte differentiation, suggesting that the involvement of AGK in megakaryocyte development and platelet biogenesis was not dependent on its kinase activity. The Mpl/Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (Stat3) pathway is the major signaling pathway regulating megakaryocyte development. Our study confirmed that AGK can bind to JAK2 in megakaryocytes/platelets. More interestingly, we found that the JAK2 V617F mutation dramatically enhanced the binding of AGK to JAK2 and greatly facilitated JAK2/Stat3 signaling in megakaryocytes/platelets in response to thrombopoietin. We also found that the JAK2 JAK homology 2 domain peptide YGVCF617CGDENI enhanced the binding of AGK to JAK2 and that cell-permeable peptides containing YGVCF617CGDENI sequences accelerated proplatelet formation. Therefore, our study reveals critical roles of AGK in megakaryocyte differentiation and platelet biogenesis and suggests that targeting the interaction between AGK and JAK2 may be a novel strategy for the treatment of thrombocytopenia or thrombocythemia.

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