FUBP1 IS AN ESSENTIAL REGULATOR OF HEMATOPOIETIC STEM CELL SELF-RENEWAL

Analyzing the regulatory mechanisms that control self-renewal and differentiation of hematopoietic stem cells (HSCs) is important to understand the pathophysiology of certain hematopoietic diseases and to succeed in expanding multipotent HSCs for therapy ex vivo. In a gene trap mouse model, the functional impairment of the transcriptional regulator Far Upstream Element Binding Protein 1 (FUBP1) led to embryonic lethality at day E15.5. Homozygous Fubp1-/- embryos showed largely reduced numbers of long-term repopulating (LT-) HSCs in the fetal liver, and lower engraftment of Fubp1-/- fetal liver cells was observed in competitive transplantation experiments. However, all hematopoietic lineages were produced in the absence of FUBP1, indicating an HSC-specific function. Eventually, all Fubp1-/- LT-HSCs were exhausted in the transplanted animals, suggesting an intrinsic self-renewal defect in these cells. Similarly, the transplantation of adult Fubp1 knockdown (kd) LT-HSCs resulted in low engraftment and their eventual loss. Fubp1 kd in ex vivo cultured LT-HSCs led to reduced cell expansion, and single cell tracking following time-lapse microscopy revealed prolonged generation times and increased apoptosis upon Fubp1 kd, confirming the essential role of FUBP1 in fetal and adult HSC self-renewal and expansion. Furthermore, our groups aim at the identification of HSC-relevant FUBP1 target genes, as well as transcription factors (TFs) involved in the upstream regulation of Fubp1 expression. Transcriptome profiling of Fubp1 kd HSCs revealed potential FUBP1 target genes that might account for its pro-proliferative and anti-apoptotic function. In silico analyses and genome-wide ChIP-sequencing data predicted a putative binding of TAL1/SCL, an important TF in early hematopoiesis, within the Fubp1 promoter region. This could be confirmed by ChIP experiments using murine bone marrow mononuclear cells. Further analyses will show whether TAL1 is indeed involved in the regulation of Fubp1 expression in early hematopoiesis. Our studies have demonstrated the requirement of FUBP1 for HSC self-renewal, and the ongoing experiments will further our understanding of how FUBP1 is implicated in the regulatory network that controls the development and maintenance of the hematopoietic system.