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Megakaryocytes (MKs) and their progeny platelets function in a variety of biological processes including coagulation, hemostasis, inflammation, angiogenesis, and innate immunity. However, the divergent developmental and cellular landscape of adult MKs remains mysterious. Here, by deriving the single‐cell transcriptomic profiling of MKs from human adult bone marrow (BM), cellular heterogeneity within MKs is unveiled and an MK subpopulation with high enrichment of immune‐associated genes is identified. By performing the dynamic single‐cell transcriptomic landscape of human megakaryopoiesis in vitro, it is found that the immune signatures of MKs can be traced back to the progenitor stage. Furthermore, two surface markers, CD148 and CD48, are identified for mature MKs with immune characteristics. At the functional level, these CD148+CD48+ MKs can respond rapidly to immune stimuli both in vitro and in vivo, exhibit high‐level expression of immune receptors and mediators, and may function as immune‐surveillance cells. The findings uncover the cellular heterogeneity and a novel immune subset of human adult MKs and should greatly facilitate the understanding of the divergent functions of MKs under physiological and pathological conditions.

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Thrombocytopenia is a major complication in a subset of patients with multiple myeloma (MM). However, little is known about its development and significance during MM. Here, we show thrombocytopenia is linked to poor prognosis in MM. In addition, we identify serine, which is released from MM cells into the bone marrow microenvironment, as a key metabolic factor that suppresses megakaryopoiesis and thrombopoiesis. The impact of excessive serine on thrombocytopenia is mainly mediated through the suppression of megakaryocyte (MK) differentiation. Extrinsic serine is transported into MKs through SLC38A1 and downregulates SVIL via SAM-mediated tri-methylation of H3K9, ultimately leading to the impairment of megakaryopoiesis. Inhibition of serine utilization or treatment with TPO enhances megakaryopoiesis and thrombopoiesis and suppresses MM progression. Together, we identify serine as a key metabolic regulator of thrombocytopenia, unveil molecular mechanisms governing MM progression, and provide potential therapeutic strategies for treating MM patients by targeting thrombocytopenia.

Highlights d LGR4, but not LGR5, plays an essential role in mesoderm development and hematopoiesis d R-spondin family proteins exhibit differential dependencies on LGRs d ZNRF3 is required for the function of R-spondins during hematopoiesis d TGF-beta signaling acts as a novel effector of LGR4

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Ten-eleven translocation 2 (TET2) functions as a methylcytosine dioxygenase that catalyzes the iterative oxidation of 5-methylcytosine to 5-hydroxymethylcytosine, 5-formylcytosine and 5-carboxylcytosine. TET2 has been shown to be crucial for the maintenance and differentiation of hematopoietic stem cells, and its deletion and/or mutations results in the expansion of HSPCs, and leads to hematological malignancies. TET2 mutations were found in a variety of hematological disorders such as CMML (60%), MDS (30%), MPN (13%) and AML (20%). Interestingly, it was shown that CMML patients with TET2 mutation exhibited fewer platelets than CMML patients without TET2 mutation. However, the role and function of TET2 in platelet hemostasis and thrombogenesis is not well defined. Here in this study, using a genetically engineered Tet2 deletion mouse model, we found that the absence of Tet2 caused a decrease in the proportion of MEP cells and hyperploid megakaryocytes. Additionally, Tet2 -deficient mice displayed impaired platelet activation and aggregation under stimulation of ADP and low concentrations of thrombin, although the modestly compromised platelet function and MEP differentiation in Tet2 -deficient mice could be compensated without affecting blood coagulation function. Our study indicate that Tet2 deficiency leads to mild impairment of platelet function and thrombopoiesis in mice.

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