Cutting Edge: Lymphomyeloid-Primed Progenitor Cell Fates Are Controlled by the Transcription Factor Tal1.

Lymphoid specification is the process by which hematopoietic stem cells (HSCs) and their progeny become restricted to differentiation through the lymphoid lineages. The basic helix-loop-helix transcription factors E2A and Lyl1 form a complex that promotes lymphoid specification. In this study, we demonstrate that Tal1, a Lyl1-related basic helix-loop-helix transcription factor that promotes T acute lymphoblastic leukemia and is required for HSC specification, erythropoiesis, and megakaryopoiesis, is a negative regulator of murine lymphoid specification. We demonstrate that Tal1 limits the expression of multiple E2A target genes in HSCs and controls the balance of myeloid versus T lymphocyte differentiation potential in lymphomyeloid-primed progenitors. Our data provide insight into the mechanisms controlling lymphocyte specification and may reveal a basis for the unique functions of Tal1 and Lyl1 in T acute lymphoblastic leukemia.

Inhibition of MEK/ERK signalling pathway promotes erythroid differentiation and reduces HSCs engraftment in ex vivo expanded haematopoietic stem cells.

The MEK/ERK pathway is found to be important in regulating different biological processes such as proliferation, differentiation and survival in a wide variety of cells. However, its role in self-renewal of haematopoietic stem cells is controversial and remains to be clarified. The aim of this study was to understand the role of MEK/ERK pathway in ex vivo expansion of mononuclear cells (MNCs) and purified CD34+ cells, both derived from human umbilical cord blood (hUCB). Based on our results, culturing the cells in the presence of an inhibitor of MEK/ERK pathway-PD0325901 (PD)-significantly reduces the expansion of CD34+ and CD34+  CD38- cells, while there is no change in the expression of stemness-related genes (HOXB4, BMI1). Moreover, in vivo analysis demonstrates that PD reduces engraftment capacity of ex vivo expanded CD34+ cells. Notably, when ERK pathway is blocked in UCB-MNCs, spontaneous erythroid differentiation is promoted, found in concomitant with increasing number of burst-forming unit-erythroid colony (BFU-E) as well as enhancement of erythroid glycophorin-A marker. These results are in total conformity with up-regulation of some erythroid enhancer genes (TAL1, GATA2, LMO2) and down-regulation of some erythroid repressor genes (JUN, PU1) as well. Taken together, our results support the idea that MEK/ERK pathway has a critical role in achieving the correct balance between self-renewal and differentiation of UCB cells. Also, we suggest that inhibition of ERK signalling could likely be a new key for erythroid induction of UCB-haematopoietic progenitor cells.