SCL knockouts in mouse megakaryocytes. Mus musculus

The bHLH transcription factor stem cell leukemia gene (Scl) is a master regulator for hematopoiesis essential for hematopoietic specification and proper differentiation of the erythroid and megakaryocyte lineages. However, the critical downstream targets of Scl remain undefined. Here, we identified a novel Scl target gene, transcription factor myocyte enhancer factor 2 C (Mef2C) from Sclfl/fl fetal liver progenitor cell lines. Analysis of Mef2C-/- embryos showed that Mef2C, in contrast to Scl, is not essential for specification into primitive or definitive hematopoietic lineages. However, adult VavCre+Mef2Cfl/fl mice exhibited platelet defects similar to those observed in Scl deficient mice. The platelet counts were reduced, while platelet size was increased and the platelet shape and granularity was altered. Furthermore, megakaryopoiesis was severely impaired in vitro. ChIP-on-chip analysis revealed that Mef2C is directly regulated by Scl in megakaryocytic cells, but not in erythroid cells. In addition, an Scl independent requirement for Mef2C in B-lymphoid homeostasis was observed in Mef2C-deficient mice, characterized as severe age-dependent reduction of specific B cell progenitor populations reminiscent of premature aging. In summary, this work identifies Mef2C as an integral member of hematopoietic transcription factors with distinct upstream regulatory mechanisms and functional requirements in megakaryocyte and B-lymphoid lineages. Overall design: Sclfl/fl hematopoietic progenitor lines were generated from fetal liver progenitors from E12.5 Sclfl/fl embryos9 by immortalization with Hox11 retrovirus.17 Sclfl/fl progenitor cells were cultured with IL3, and a clonal line containing cells with megakaryocyte morphology and acetylcholinesterase (AchE) activity was selected. The Sclfl/fl cell line was transduced with Cre- GFP retrovirus to generate the Scl Δ/Δ cell line, and the Scl Δ/Δ cell line was transduced with Scl retrovirus to re-introduce Scl expression (Scl Δ/Δ +Scl cell line). Megakaryocyte differentiation was enhanced by adding Tpo for 5 days before harvesting the cells. RNA was extracted with Trizol (Gibco BRL) and RNEasy (Qiagen) kits. Differential gene expression between Sclfl/fl, Scl Δ/Δ and Scl Δ/Δ +Scl cell lines was analyzed by Affymetrix MOE430_2 microarray in the microarray core facility at the Dana-Farber Cancer Institute.