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.

During acute myelosuppression or thrombocytopenia, bone marrow (BM) hematopoietic cells respond rapidly to replenish peripheral blood platelets. While the cytokine thrombopoietin (Thpo) both regulates platelet production and maintains HSC potential, whether Thpo controls megakaryocyte (Mk)-lineage differentiation of HSCs is unclear. Here, we show that Thpo rapidly upregulates mitochondrial activity in HSCs, an activity accompanied by differentiation to an Mk lineage. Moreover, in unperturbed hematopoiesis, HSCs with high mitochondrial activity exhibit Mk-lineage differentiation in vitro and myeloid lineage-biased reconstitution in vivo. Furthermore, Thpo skewed HSCs to express the tetraspanin CD9, a pattern correlated with mitochondrial activity. Mitochondria-active HSCs are resistant to apoptosis and oxidative stress upon Thpo stimulation. Thpo-regulated mitochondrial activity associated with mitochondrial translocation of STAT3 phosphorylated at serine 727. Overall, we report an important role for Thpo in regulating rapid Mk-lineage commitment. Thpo-dependent changes in mitochondrial metabolism prime HSCs to undergo direct differentiation to an Mk lineage.

Just as normal stem cells require niche cells for survival, leukemia-initiating cells (LICs) may also require niche cells for their maintenance. Chronic myeloid leukemia (CML) is caused by the activity of BCR-ABL, a constitutively active tyrosine kinase. CML therapy with tyrosine kinase inhibitors is highly effective; however, due to the persistence of residual LICs, it is not curative. Several factors are known to support CML LICs, but purification of LICs and a thorough understanding of their niche signals have not yet been achieved. Using a CML-like mouse model of myeloproliferative disease, we demonstrate that CML LICs can be divided into CD25⁺FcεRIα⁻ Lineage marker (Lin)⁻ Sca-1⁺c-Kit⁺ (F⁻LSK) cells and CD25⁻F⁻LSK cells. The CD25⁺F⁻LSK cells had multilineage differentiation capacity, with a preference toward cytokine-producing mast cell commitment. Although cells interconverted between CD25⁻F⁻LSK and CD25⁺F⁻LSK status, the CD25⁺F⁻LSK cells exhibited higher LIC capacity. Our findings suggest that interleukin-2 derived from the microenvironment and CD25 expressed on CML LICs constitute a novel signaling axis. The high levels of CD25 expression in the CD34⁺CD38⁻ fraction of human CML cells indicate that CD25⁺ LICs constitute an “LIC-derived niche” that could be preferentially targeted in therapy for CML. Key Points CD25⁺ CML LICs have high LIC capacity and secrete cytokines that constitute the LIC niche. Targeting the IL-2/CD25 axis effectively eliminates CML LICs and improves the survival of CML model mice.

Hematopoietic stem cells (HSCs) reside in a hypoxic microenvironment that enables glycolysis-fueled metabolism and reduces oxidative stress. Nonetheless, metabolic regulation in organelles such as the mitochondria and lysosomes as well as autophagic processes have been implicated as essential for the determination of HSC cell fate. This review encompasses the current understanding of anaerobic metabolism in HSCs as well as the emerging roles of mitochondrial metabolism and lysosomal regulation for hematopoietic homeostasis.

Hematopoietic stem cells (HSCs) undergo self-renewal or differentiation to sustain lifelong hematopoiesis. HSCs are preserved in quiescence with low mitochondrial activity. Recent studies indicate that autophagy contributes to HSC quiescence through suppressing mitochondrial metabolism. However, it remains unclear whether autophagy is involved in the regulation of neonatal HSCs, which proliferate actively. In this study, we clarified the role of autophagy in neonatal HSCs using 2 types of autophagy-related gene 7 (Atg7)-conditional knockout mice: Mx1-Cre inducible system and Vav-Cre system. Atg7-deficient HSCs exhibited excess cell divisions with enhanced mitochondrial metabolism, leading to bone marrow failure at adult stage. However, Atg7 deficiency minimally affected hematopoiesis and metabolic state in HSCs at neonatal stage. In addition, Atg7-deficient neonatal HSCs exhibited long-term reconstructing activity, equivalent to wild-type neonatal HSCs. Taken together, autophagy is dispensable for stem cell function and hematopoietic homeostasis in neonates and provide a novel aspect into the role of autophagy in the HSC regulation.