c-Myb maintains glucose metabolism and regulates differentiation across the pre-BCR checkpoint

The transcription factor c-Myb is critical for normal adult hematopoiesis. However, analysis of c-Myb function is difficult due to the embryonic lethality of Myb null mutations. We previously utilized conditional inactivation at the Myb locus to demonstrate that c-Myb is absolutely required for the differentiation and survival of pro-B cells and that c-Myb coordinates the survival of pro-B cells with the expression of genes required for transition across the pre-BCR checkpoint to the large pre-B cell stage of differentiation. However, it is unclear what additional functions c-Myb and mediators of c-Myb activity have within the pro-B cell compartment and whether c-Myb is important for the proliferative expansion of large pre-B cells or subsequent differentiation into small pre-B cells. We demonstrate that c-Myb expression is critical to maintain glucose metabolism and fuel proliferation across the pre-BCR checkpoint at the pro-B and large pre-B cells stages of development. c-Myb-deficient pro-B and large pre-B cells exhibit decreased glucose uptake and Glut1 mRNA expression, decreased hexokinase activity and Hk1 mRNA expression, and are ultimately hypoproliferative and apoptotic. In pro-B cells, IL-7 signaling through the Plcg/Pkcb/mTORC1 signaling pathway promotes pro-B cell glucose metabolism and proliferation. Microarray differential gene expression analysis revealed that c-Myb regulates multiple components of the Plcg/Pkcb/mTORC1 signaling pathway as well as additional pathways dependent on glucose uptake and utilization, which are critical to maintain homeostatic metabolism in the pro-B cell compartment. In large pre-B cells, exit from the cell cycle and decreased metabolic activity accompany differentiation. We demonstrate that in addition to defects in proliferation and glucose utilization, c-Myb-deficient large pre-B cells exhibit premature expression of Ikaros and Aiolos, which act downstream of pre-BCR signaling and put into place a gene expression program essential to drive the large to small pre-B cell transition. To assess c-Myb-dependent changes in gene expression in large pre-B cells, we performed RNA-seq and found a large proportion of c-Myb-dependent gene expression changes are Ikaros targets. However, c-Myb regulates glucose metabolism in an Ikaros-independent manner and due to loss of glucose-dependent survival and proliferation, c-Myb-deficient large pre-B cells fail to differentiate and undergo apoptotic cell death. Overall, this thesis demonstrates that c-Myb regulates glucose uptake and utilization in pro-B and large pre-B cells, which is critical to maintain proliferation and survival across the pre-BCR checkpoint.