The Lin28b–let-7–Hmga2 axis determines the higher self-renewal potential of fetal haematopoietic stem cells

TLDR: Mouse haematopoietic stem cells undergo a postnatal transition in several properties, including a marked reduction in their self-renewal activity, and it is shown that the developmentally timed change in this key function of HSCs is associated with their decreased expression of Lin28b and an accompanying increase in their let-7 microRNA levels.

Abstract: Mouse haematopoietic stem cells (HSCs) undergo a postnatal transition in several properties, including a marked reduction in their self-renewal activity. We now show that the developmentally timed change in this key function of HSCs is associated with their decreased expression of Lin28b and an accompanying increase in their let-7 microRNA levels. Lentivirus-mediated overexpression of Lin28 in adult HSCs elevates their self-renewal activity in transplanted irradiated hosts, as does overexpression of Hmga2, a well-established let-7 target that is upregulated in fetal HSCs. Conversely, HSCs from fetal Hmga2−/− mice do not exhibit the heightened self-renewal activity that is characteristic of wild-type fetal HSCs. Interestingly, overexpression of Hmga2 in adult HSCs does not mimic the ability of elevated Lin28 to activate a fetal lymphoid differentiation program. Thus, Lin28b may act as a master regulator of developmentally timed changes in HSC programs with Hmga2 serving as its specific downstream modulator of HSC self-renewal potential. Fetal haematopoietic stem cells (HSCs) display higher self-renewal potential than their adult counterparts. Eaves and colleagues show that adult HSCs express lower levels of Lin28b and higher levels of let-7 miRNA. They demonstrate that Lin28 overexpression, as well as that of Hmga2 (a target of let-7), induces fetal properties in adult HSCs. Conversely, HMGA2 loss in fetal HSCs results in premature induction of adult HSC self-renewal properties.