Akt signaling maintains survival and proliferation in immature T cells and hematopoietic stem cells

Hematopoietic development and cell fate choices are regulated by signals from the extracellular environment. This study focuses on two developmental checkpoints in the hematopoietic system that are characterized by unique proliferative stages: (1) The DN3 (double negative 3) to DP (double positive) transition, otherwise known as β-selection, in T cell development and (2) The regulation of self-renewal in long-term hematopoietic stem cells. This thesis tests the hypothesis that Akt, a serine/threonine kinase, is required for both of these proliferative stages of hematopoietic development. The first set of experiments test whether a constitutively active Akt molecule is sufficient to rescue survival, proliferation, and differentiation in a mouse model that fails to generate mature T cells due to a block at the DN3 stage. In the second set of experiments, we ask if Akt is necessary for T cell development, specifically at the DN3 to DP transition. After defining the biological role for Akt signaling in thymocyte development, we next determine whether Akt also has a similar role in the development of other hematopoietic lineages. The hematopoietic stem cell pool is another exquisitely regulated developmental stage in hematopoiesis that is dependent on cell survival and proliferation. In the last set of experiments, we test the hypothesis that Akt would also be required for maintenance of the most primitive hematopoietic stem cell. In summary, we found that Akt is sufficient to promote differentiation to the DP stage in the absence of signals from the pre-TCR, and that Akt1 and Akt2 are also required for differentiation at the same pre-TCR mediated checkpoint. The biological role of Akt in T cell development is similar to its function in long-term hematopoietic stem cells (LT-HSCs). LT-HSCs require Akt signals to maintain self-renewal and survival of more mature progenitor cells. In conclusion, these studies provide evidence that Akt is required in proliferating hematopoietic cells during development, but is not required at the more quiescent stage of hematopoiesis