Cutting edge Seminar
Speaker: Naotaka Tsutsumi (Assistant Professor, Faculty of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University)
Title: Cryo-EM structure of the thrombopoietin-MPL receptor complex guides design of the cytokine variants that bias hematopoiesis
※This seminar can also be attended through ZOOM. Please check the URL on “HIGO Cutting-Edge Seminar” at Moodle.
https://md.kumamoto-u.ac.jp/course/view.php?id=106543
Abstract:
Cytokines regulate the balance between self-renewal and differentiation of hematopoietic stem cells (HSCs) for blood cell homeostasis. Thrombopoietin (TPO) is one of the vital cytokines in HSC maintenance and a major factor responsible for megakaryopoiesis and platelet production in vivo (1). Abnormal activation of the TPO receptor (MPL) causes myeloproliferative neoplasms, while impaired function leads to thrombocytopenia and aplastic anemia (2). Furthermore, the TPO-MPL signaling is important for ex vivo HSC expansion aimed at bone marrow transplantation and gene therapy (3). Despite the clinical importance of MPL signaling, the structural mechanism of TPO-induced MPL activation was not elucidated due to the difficulty of producing the recombinant MPL.
We purified TPO bound to two chains of detergent-solubilized MPLs from HEK293 cells and visualized the extracellular region of the signaling complex by cryo-electron microscopy (4). In the structure, the TPO:MPL interfaces were clearly resolved, enabling the design of TPO mutants that would alter the stability of the receptor dimers. We screened cell growth and intracellular signaling induced by TPO mutants and found partial agonistic TPO variants activate specific downstream signaling pathways more selectively than the wild-type. We then probed the functional outcomes of these mutations on cells and in mice, demonstrating that the partial agonists could separate the two conflicting functions of TPO, HSC maintenance and myeloid cell differentiation, to some extent. The results indicate the potential utility of the modified TPO in fine control of platelet production in the difficult cases of immune thrombocytopenia (5) and clinical HSC transplantation in conjunction with the state-of-the-art HSC expansion protocols (6).
Reference：
1. C. C. Zhang, H. F. Lodish, Cytokines regulating hematopoietic stem cell function. Curr Opin Hematol 15, 307-311 (2008).
2. P. Guglielmelli, L. Calabresi, The MPL mutation. Int Rev Cell Mol Biol 365, 163-178 (2021).
3. J. A. Rubio-Lara et al., Expanding hematopoietic stem cell ex vivo: recent advances and technical considerations. Exp Hematol 125-126, 6-15 (2023).
4. N. Tsutsumi et al., Structure of the thrombopoietin-MPL receptor complex is a blueprint for biasing hematopoiesis. Cell 186, 4189-4203.e4122 (2023).
5. W. Ghanima et al., Bone marrow fibrosis in 66 patients with immune thrombocytopenia treated with thrombopoietin-receptor agonists: a single-center, long-term follow-up. Haematologica 99, 937-944 (2014).
6. M. Sakurai et al., Chemically defined cytokine-free expansion of human haematopoietic stem cells. Nature 615, 127-133 (2023).
