Department of Cell Biology, Yale University School of Medicine, New Haven, CT, USA.
Blood. 2012 Sep 13;120(11):2317-29. doi: 10.1182/blood-2012-04-420828. Epub 2012 Jul 17.
Serum response factor and its transcriptional cofactor MKL1 are critical for megakaryocyte maturation and platelet formation. We show that MKL2, a homologue of MKL1, is expressed in megakaryocytes and plays a role in megakaryocyte maturation. Using a megakaryocyte-specific Mkl2 knockout (KO) mouse on the conventional Mkl1 KO background to produce double KO (DKO) megakaryocytes and platelets, a critical role for MKL2 is revealed. The decrease in megakaryocyte ploidy and platelet counts of DKO mice is more severe than in Mkl1 KO mice. Platelet dysfunction in DKO mice is revealed by prolonged bleeding times and ineffective platelet activation in vitro in response to adenosine 5'-diphosphate. Electron microscopy and immunofluorescence of DKO megakaryocytes and platelets indicate abnormal cytoskeletal and membrane organization with decreased granule complexity. Surprisingly, the DKO mice have a more extreme thrombocytopenia than mice lacking serum response factor (SRF) expression in the megakaryocyte compartment. Comparison of gene expression reveals approximately 4400 genes whose expression is differentially affected in DKO compared with megakaryocytes deficient in SRF, strongly suggesting that MKL1 and MKL2 have both SRF-dependent and SRF-independent activity in megakaryocytopoiesis.
血清反应因子及其转录共激活因子 MKL1 对于巨核细胞成熟和血小板形成至关重要。我们发现 MKL2，MKL1 的同源物，在巨核细胞中表达，并在巨核细胞成熟中发挥作用。利用巨核细胞特异性 Mkl2 敲除（KO）小鼠在常规 Mkl1 KO 背景下产生双重 KO（DKO）巨核细胞和血小板，揭示了 MKL2 的关键作用。与 Mkl1 KO 小鼠相比，DKO 小鼠的巨核细胞倍性和血小板计数减少更为严重。DKO 小鼠的血小板功能障碍通过延长出血时间和体外对二磷酸腺苷的无效血小板激活来揭示。DKO 巨核细胞和血小板的电子显微镜和免疫荧光表明，细胞骨架和膜组织异常，颗粒复杂性降低。令人惊讶的是，与巨核细胞中缺乏血清反应因子（SRF）表达的小鼠相比，DKO 小鼠的血小板减少更为严重。基因表达的比较显示，大约有 4400 个基因的表达在 DKO 与缺乏 SRF 的巨核细胞之间存在差异影响，强烈表明 MKL1 和 MKL2 在巨核细胞生成中既有 SRF 依赖性又有 SRF 非依赖性活性。
