Genetic modeling of the Hippo pathway in hematopoietic stem cells

Abstract: Hematopoiesis is the process of blood formation from a limited pool of hematopoietic stem cells (HSCs). These rare stem cells can both self-renew to maintain the HSC pool, and differentiate to continuously replenish lost blood cells. The mechanisms of HSC regulation are not fully known. The aim of this thesis was to study the role of the Hippo signaling pathway in HSCs. The Hippo pathway is a newly discovered signaling pathway, which regulates organ size in Drosophila. Hippo signaling has further been implicated in regulation of mammalian stem cells. In Article I we developed a new way of modeling genetic changes by combining genetic engineering of murine ES cells with blastocyst complementation. This approach avoids the cost and time constraints associated with the creation of standard transgenic mouse strains while taking advantage of the sophisticated site-directed manipulations that are possible in ES cells. In Article II we studied YAP1, the downstream effector in the Hippo pathway. We created a transgenic model with inducible YAP1 expression exclusively within the hematopoietic system using the blastocyst complementation approach developed in article I. When investigating the effect of overexpressing YAP1 in HSCs we detected no effect on HSC function during steady state or regenerative stress. This is contrast to effects seen in other tissue stem cells and suggests tissue specific functions of YAP1 in regulation of stem cells. In Article III we investigated a knockout model for the other Hippo effector Taz. Adult mice deficient in Taz display no changes in hematopoietic parameters but are born below mendelian ratios. Taz thus seems dispensable for adult hematopoiesis but may influence embryonic development. Taken together, using both novel and traditional genetic engineering approaches in mice, we have taken the first steps to understand the role of the Hippo pathway in hematopoiesis.