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Blood, 19 March 2009, Vol. 113, No. 12, pp. 2826-2834. Prepublished online as a Blood First Edition Paper on January 9, 2009; DOI 10.1182/blood-2008-09-180596.
PLATELETS AND THROMBOPOIESIS Role for MKL1 in megakaryocytic maturation1 Department of Laboratory Medicine, Yale University, New Haven, CT; 2 111 Project, College of Bioengineering, Chongqing University, Chongqing, People's Republic of China, and Departments of 3 Pediatrics and 4 Pathology and 5 W. M. Keck Facility, Yale University, New Haven, CT; 6 Department of Medicine, Brigham and Women's Hospital, Boston, MA; 7 Departments of Pathology and Oncology, St Jude Children's Research Hospital, Memphis, TN; and 8 Yale Human Embryonic Stem Cell Core, Yale Stem Cell Center, Yale University, New Haven, CT Megakaryoblastic leukemia 1 (MKL1), identified as part of the t(1;22) translocation specific to acute megakaryoblastic leukemia, is highly expressed in differentiated muscle cells and promotes muscle differentiation by activating serum response factor (SRF). Here we show that Mkl1 expression is up-regulated during murine megakaryocytic differentiation and that enforced overexpression of MKL1 enhances megakaryocytic differentiation. When the human erythroleukemia (HEL) cell line is induced to differentiate with 12-O-tetradecanoylphorbol 13-acetate, overexpression of MKL1 results in an increased number of megakaryocytes with a concurrent increase in ploidy. MKL1 overexpression also promotes megakaryocytic differentiation of primary human CD34+ cells cultured in the presence of thrombopoietin. The effect of MKL1 is abrogated when SRF is knocked down, suggesting that MKL1 acts through SRF. Consistent with these findings in human cells, knockout of Mkl1 in mice leads to reduced platelet counts in peripheral blood, and reduced ploidy in bone marrow megakaryocytes. In conclusion, MKL1 promotes physiologic maturation of human and murine megakaryocytes.
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