The process of lymphocyte development is regulated by the complex interplay of signaling pathways and changing sets of transcription factors that are present in maturing lymphocytes. We are currently interested in the role played by the Myb family of transcription factors, particularly c-myb, in regulating T and B-lymphocyte development. Proper expression of the c-myb locus is crucial for normal adult hematopoiesis and traditional null mutations are embryonic lethal. c-myb null embryos die between day 14 and 15 of embryogenesis do to severe anemia. In addition to the crucial role played by c-myb during hematopoiesis c-myb also appears to play a significant role in the transformation of hematopoietic cells and has recently been implicated in tumors of the breast and gut epithelium as well as tumors of the nervous system. The c-myb locus encodes a highly conserved transcription factor that is highly expressed in immature hematopoietic cells and expression is turned off during the maturation of each hematopoietic lineage. During lymphocyte development, c-myb is highly expressed during the immature stages of lymphocyte development but is down regulated at or near the point of repertoire selection. Interestingly, c-myb activity increases in mature B and T lymphocytes following activation in response to antigen. Due to the embryonic lethality of the c-myb null mutation we have developed conditional mutants at the c-myb locus using the cre/loxP technology. With these mice, we are able induce deletion at the c-myb locus a specific times during lymphocyte development that allow us to study c-myb function both during antigen independent development and during differentiation of mature effector function in response to antigen. We are currently focused on determining the key points during lymphocyte development that are dependent on c-myb activity as well as the role of c-myb in regulating the population dynamics of mature B and T cell populations. In addition, these mice provide models to examine the regulation of c-myb activity by physiologically relevant signaling pathways, to determine the structural features on c-myb that are important at different times during lymphocyte development and to identify downstream effectors of c-myb function.
Given the importance of appropriately regulated c-myb expression, our laboratory has been interested in determining the mechanisms that regulate c-myb expression and activity. We have demonstrated the primary mechanism that regulates expression of c-myb mRNA is a conditional block to transcription elongation (attenuation) that occurs in the first intron of the c-myb locus. We are particularly interested characterizing signaling pathways that impact on the efficiency of this block to transcription elongation during lymphocyte development and induction of effector function. As potential targets of c-myb activity have been identified it has become apparent that c-myb is involved in the regulation of lineage and stage specific genes as well as genes that are expressed in most cell types. Thus, identifying mechanisms that regulate c-myb activity poses an interesting problem. We have identified and characterized a major site of phosphorylation on c-myb. Significantly, this site is located in a region of the c-myb protein that is deleted in transforming forms of c-myb and serves to differentially regulate c-myb on some target promoters but not others. We will examine the potential role of this phosphorylation site in activating c-myb transforming potential as well as in regulating c-myb activity during hematopoiesis. Phosphorylation does not modulate the ability of c-myb to bind DNA and we postulate that it likely regulates interaction between c-myb and other proteins that serve to modulate c-myb activity. We have identified several candidate interaction partners and are in the process of characterizing them.