Cell fate can be controlled through asymmetric division and segregation of protein determinants. But the regulation of this process in the hematopoietic system is poorly understood. Here we show that the dynein binding protein Lis1 (Pafah1b1) is critically required for blood formation and hematopoietic stem cell function. Conditional deletion of Lis1 in the hematopoietic system led to a severe bloodless phenotype, depletion of the stem cell pool and embryonic lethality. Further, the loss of Lis1 accelerated cell differentiation, in part through defects in spindle positioning and inheritance of cell fate determinants. Finally, deletion of Lis1 blocked propagation of myeloid leukemia and led to a marked improvement in animal survival, suggesting that Lis1 is also required for oncogenic growth. These data identify a key role for Lis1 in hematopoietic stem cells, and mark the directed control of asymmetric division as a critical regulator of normal and malignant hematopoietic development.

Summary Acute Myelogenous Leukemia (AML) is an aggressive cancer that strikes both adults and children, and is frequently resistant to therapy. Thus, identifying signals needed for AML propagation is a critical step toward developing new approaches for treating this disease. Here we show that Tetraspanin3 is a target of the RNA binding protein Musashi2, which plays a key role in AML. We generated Tspan3 knockout mice which were born without overt defects. However, Tspan3 deletion impaired leukemia stem cell self-renewal and disease propagation, and markedly improved survival in mouse models of AML. Additionally, Tspan3 inhibition blocked growth of AML patient samples suggesting that Tspan3 is also important in human disease. As part of the mechanism, we show that Tspan3-deficiency disabled responses to CXCL12/SDF-1, and led to defects in AML localization within the niche. These identify Tspan3 as an important regulator of aggressive leukemias and highlight a role for Tspan3 in oncogenesis.

Although we know a great deal about the phenotype and function of haematopoietic stem/progenitor cells, a major challenge has been mapping their dynamic behaviour within living systems. Here we describe a strategy to image cells in vivo with high spatial and temporal resolution, and quantify their interactions using a high-throughput computational approach. Using these tools, and a new Msi2 reporter model, we show that haematopoietic stem/progenitor cells display preferential spatial affinity for contacting the vascular niche, and a temporal affinity for making stable associations with these cells. These preferences are markedly diminished as cells mature, suggesting that programs that control differentiation state are key determinants of spatiotemporal behaviour, and thus dictate the signals a cell receives from specific microenvironmental domains. These collectively demonstrate that high-resolution imaging coupled with computational analysis can provide new biological insight, and may in the long term enable creation of a dynamic atlas of cells within their native microenvironment.