The Role of the In Vivo Microenvironment in Human Stem Cell Fate Decisions

The molecular basis for the unique proliferative and self-renewal properties that hierarchically distinguish human stem cells from progenitors and terminally differentiated cells remains largely unknown. We report a role for the Bcl-2 family member myeloid cell leukemia -1 (Mcl-1) as an indispensable regulator of self-renewal in human stem cells, and show that a functional dependence on Mcl-1 defines the human stem cell hierarchy. In vivo pharmacological targeting of the Bcl-2 family members in human hematopoietic stem cells (HSCs) and human leukemic stem cells (LSCs) reduced stem cell regenerative and self-renewal function. Subsequent protein expression studies revealed that among the Bcl-2 family members, only Mcl-1 was upregulated exclusively in the human HSC fraction upon in vivo regeneration of hematopoiesis. shRNAknockdown of Mcl-1 in human cord blood cells did not affect survival in the HSC or hematopoietic progenitor cell (HPC) fractions in vitro, but specifically reduced the in vivo self-renewal function of human HSCs. Moreover, knockdown of Mcl-1 in ontogenetically primitive human pluripotent stem cells (hPSCs) resulted in almost complete ablation of stem cell self-renewal function. Our findings reveal that Mcl-1 is an essential regulator of stem cell self-renewal in humans, and therefore represents an axis for therapeutic interventions.