Mouse anti-Human MIXL1 monoclonal antibody, clone 5E22

Mixl1, the sole murine homologue of the Xenopus Mix/Bix family of homeobox transcription factors, is essential for the patterning of axial mesendodermal structures during early embryogenesis. Gene targeting and overexpression studies have implicated Mixl1 as a regulator of hematopoiesis arising in differentiating embryonic stem cells. To assess the role of Mixl1 in the regulation of adult hematopoiesis, we overexpressed Mixl1 in murine bone marrow using a retroviral transduction/transplantation model. Enforced expression of Mixl1 profoundly perturbed hematopoietic lineage commitment and differentiation, giving rise to abnormal myeloid progenitors and impairing erythroid and lymphoid differentiation. Moreover, all mice reconstituted with Mixl1-transduced bone marrow developed fatal, transplantable acute myeloid leukemia with a mean latency period of 200 days. These observations establish a link between enforced Mixl1 expression and leukemogenesis in the mouse.

We postulate that exit from pluripotency involves intermediates that retain pluripotency while simultaneously exhibiting lineage-bias. Using a MIXL1 reporter, we explore mesoderm lineage-bias within the human pluripotent stem cell compartment. We identify a substate, which at the single cell level coexpresses pluripotent and mesodermal gene expression programmes. Functionally these cells initiate stem cell cultures and exhibit mesodermal bias in differentiation assays. By promoting mesodermal identity through manipulation of WNT signalling while preventing exit from pluripotency using lysophosphatidic acid, we 'trap' and maintain cells in a lineage-biased stem cell state through multiple passages. These cells correspond to a normal state on the differentiation trajectory, the plasticity of which is evidenced by their reacquisition of an unbiased state upon removal of differentiation cues. The use of 'cross-antagonistic' signalling to trap pluripotent stem cell intermediates with different lineage-bias may have general applicability in the efficient production of cells for regenerative medicine. Application of pluripotent cells in regenerative medicine requires an understanding of how they exit pluripotency. Here the authors demonstrate support for the idea that pluripotency exit involves pluripotent intermediates that exhibit lineage bias by identifying and trapping a mesoderm biased sub-state in culture.