Hematopoietic stem cell (HSC) property changes profoundly during development. Fetal HSCs undergo rapid self-renewal divisions with massive expansion of the HSC pool. By contrast, the majority of adult HSCs are quiescent. Transition from actively dividing fetal HSCs to quiescent adult HSCs occurs 3-4 weeks after birth in mouse. The ability to enter and exit “cell cycle” is believed to be crucial for HSC function, as it allows establishment of normal hematopoiesis, and prevents HSC exhaustion or transformation. However, the molecular mechanisms underlying this change remain largely unknown. Here, using an inducible knockout model we found that C/EBPa, a transcription factor that is indispensable for myeloid differentiation and frequently disrupted in human acute myeloid leukemia, regulate HSC property. C/EBPa deficient adult HSCs underwent a pronounced expansion with enhanced proliferation and demonstrated advanced repopulating ability, mimicking characteristics of fetal HSCs. Consistently, transcription profiling of C/EBPa deficient HSCs revealed a gene expression programme similar to fetal liver HSCs. Furthermore, levels of C/EBPa mRNA in HSCs increased during development. Interestingly, there was a sharp 2-fold increase in C/EBPa expression in HSCs from 4-week old mice, compared to HSCs from 2-week old mice. Deleting C/EBPa in 1.5-week old and 4.5-week old C/EBPa conditional KO mice, respectively, demonstrated that levels of C/EBPa expression were indeed correlated with its inhibitory effect on HSC cell cycle progression. This was further supported by the finding that fetal liver HSCs with enforced C/EBPa expression showed an increased frequency of cells in the quiescent G0 phase. Mechanistically we identified N-Myc as C/EBPa downstream target. C/EBPa directly represses N-Myc transcription by binding to the proximal region of the N-Myc promoter. Loss of C/EBPa resulted in de-repression of N-Myc, which promoted HSC proliferation. In conclusion, we provide functional and genome-wide evidences that C/EBPa acts as a molecular switch for acquisition and maintenance of adult HSC properties and demonstrate a remarkable capacity of a single transcription factor in determination of HSC property.
Min Ye1, 2, 7, Hong Zhang1, 2, 7, Giovanni Amabile1, 2, Henry Yang3, Philipp B. Staber1, 2, 4, 5, Pu Zhang2, Elena Levantini1, 2, 6, Meritxell Alberich-Jordà1, 2, Junyan Zhang2, Akira Kawasaki3, and Daniel G. Tenen1, 3, 8
1Harvard Stem Cell Institute, Harvard Medical School, Boston, MA, USA
2Beth Israel Deaconess Medical Center, Boston, MA, USA
3Cancer Science Institute, National University of Singapore, Singapore
4Division of Hematology, Medical University Graz, Austria
5Division of Hematology and Hemostaseology, Medical University of Vienna, Vienna, Austria
6Institute of Biomedical Technologies, National Research Council, Italy
7These authors contributed equally to this work.