SCL/TAL1 is a hematopoietic-specific transcription factor of the basic helix-loop-helix (bHLH) family that is essential for erythropoiesis. Here we identify the erythroid cell-specific glycophorin A gene (GPA) as a target of SCL in primary hematopoietic cells and show that SCL occupies the GPA locus in vivo. GPA promoter activation is dependent on the assembly of a multifactorial complex containing SCL as well as ubiquitous (E47, Sp1, and Ldb1) and tissue-specific (LMO2 and GATA-1) transcription factors. In addition, our observations suggest functional specialization within this complex, as SCL provides its HLH protein interaction motif, GATA-1 exerts a DNA-tethering function through its binding to a critical GATA element in the GPA promoter, and E47 requires its N-terminal moiety (most likely entailing a transactivation function). Finally, endogenous GPA expression is disrupted in hematopoietic cells through the dominant-inhibitory effect of a truncated form of E47 (E47-bHLH) on E-protein activity or of FOG (Friend of GATA) on GATA activity or when LMO2 or Ldb-1 protein levels are decreased. Together, these observations reveal the functional complementarities of transcription factors within the SCL complex and the essential role of SCL as a nucleation factor within a higher-order complex required to activate gene GPA expression.

Gene expression programs are established by networks of interacting transcription factors. The basic helix-loop-helix factor SCL and the LIM-only protein LMO2 are components of transcription factor complexes that are essential for hematopoiesis. Here we show that LMO2 and SCL are predominant interaction partners in hematopoietic cells and that this interaction occurs through a conserved interface residing in the loop and helix 2 of SCL. This interaction nucleates the assembly of SCL complexes on DNA and is required for target gene induction and for the stimulation of erythroid and megakaryocytic differentiation. We also demonstrate that SCL determines LMO2 protein levels in hematopoietic cells and reveal that interaction with SCL prevents LMO2 degradation by the proteasome. We propose that the SCL-LMO2 interaction couples protein stabilization with higher order protein complex assembly, thus providing a powerful means of modulating the stoichiometry and spatiotemporal activity of SCL complexes. This interaction likely provides a rate-limiting step in the transcriptional control of hematopoiesis and leukemia, and similar mechanisms may operate to control the assembly of diverse protein modules.

The very small embryonic-like stem cells (VSELs) are known as a subset of adult pluripotent stem cells able to differentiate to all three germ layers. However, their small number and quiescence restrict the possibility of their use in cell therapy. In the present study, we first delineate different subpopulation of VSELs from human cord blood CD34+ cells to define their purity. We next determine genes expression levels in the whole transcriptome of VSELs expressing the pluripotent marker NANOG and control cells under the steady state condition. We found that more than a thousand of genes are downregulated in VSELs, as well as many membrane receptors, cells signaling molecules and CDKs mRNAs. In addition, we observed discordance in some pluripotent genes expression levels with embryonic stem cells (ESCs), which could explain VSELs quiescence. We then evaluate VSELs capacity to expand and differentiate in vitro in specific and appropriate media. After 12 days culture in specific medium containing a pyrimidoindole derivative (UM171), VSELs were significantly expanded for the first time without feeder cells and importantly preserve their capacities to differentiate into hematopoietic and endothelial cells. Interestingly, this stimulation of VSELs self-renewal restores the expression of some downregulated genes known as key regulators of cell proliferation and differentiation. The properties of such pluripotent expanded cells make them a potential candidate in regenerative medicine.Electronic supplementary materialThe online version of this article (10.1007/s12015-018-9821-1) contains supplementary material, which is available to authorized users.