Nakamura-Ishizu A1, Takubo K2, Kobayashi H2, Suzuki-Inoue K3, Suda T1.
1 Cancer Science Institute, National University of Singapore, Singapore 117599; The Sakaguchi Laboratory, Department of Cell Differentiation, Keio University, Shinjuku-ku, Tokyo 160-8582; Japan International Research Center for Medical Sciences (IRCMS), Kumamoto University, Chuo-ku, Kumamoto City 860-0811, Japan
2 The Sakaguchi Laboratory, Department of Cell Differentiation, Keio University, Shinjuku-ku, Tokyo 160-8582, Japan.
3 Department of Clinical and Laboratory Medicine, Faculty of Medicine, University of Yamanashi, Chuo, Yamanashi 409-3898, Japan.
Hematopoietic stem cells (HSCs) depend on the bone marrow (BM) niche for their maintenance, proliferation, and differentiation. The BM niche is composed of nonhematopoietic and mature hematopoietic cells, including megakaryocytes (Mks). Thrombopoietin (Thpo) is a crucial cytokine produced by BM niche cells. However, the cellular source of Thpo, upon which HSCs primarily depend, is unclear. Moreover, no specific molecular pathway for the regulation of Thpo production in the BM has been identified. Here, we demonstrate that the membrane protein C-type lectin-like receptor-2 (CLEC-2) mediates the production of Thpo and other factors in Mks. Mice conditionally deleted for CLEC-2 in Mks (Clec2(MkΔ/Δ)) produced lower levels of Thpo in Mks. CLEC-2-deficient Mks showed down-regulation of CLEC-2-related signaling molecules Syk, Lcp2, and Plcg2. Knockdown of these molecules in cultured Mks decreased expression of Thpo. Clec2(MkΔ/Δ) mice exhibited reduced BM HSC quiescence and repopulation potential, along with extramedullary hematopoiesis. The low level of Thpo production may account for the decline in HSC potential in Clec2(MkΔ/Δ) mice, as administration of recombinant Thpo to Clec2(MkΔ/Δ) mice restored stem cell potential. Our study identifies CLEC-2 signaling as a novel molecular mechanism mediating the production of Thpo and other factors for the maintenance of HSCs.