PU.1 is an Ets family transcription factor that is essential for fetal liver hematopoiesis. We have generated a PU.1 gfp reporter strain that allowed us to examine the expression of PU.1 in all hematopoietic cell lineages and their early progenitors. Within the bone marrow progenitor compartment, PU.1 is highly expressed in the hematopoietic stem cell, the common lymphoid progenitor, and a proportion of common myeloid progenitors (CMPs). Based on Flt3 and PU.1 expression, the CMP could be divided into three subpopulations, Flt3+ PU.1hi, Flt3− PU.1hi, and Flt3− PU.1lo CMPs. Colony-forming assays and in vivo lineage reconstitution demonstrated that the Flt3+ PU.1hi and Flt3− PU.1hi CMPs were efficient precursors for granulocyte/macrophage progenitors (GMPs), whereas the Flt3− PU.1lo CMPs were highly enriched for committed megakaryocyte/erythrocyte progenitors (MEPs). CMPs have been shown to rapidly differentiate into GMPs and MEPs in vitro. Interestingly, short-term culture revealed that the Flt3+ PU.1hi and Flt3− PU.1hi CMPs rapidly became CD16/32high (reminiscent of GMPs) in culture, whereas the Flt3− PU.1lo CMPs were the immediate precursors of the MEP. Thus, down-regulation of PU.1 expression in the CMP is the first molecularly identified event associated with the restriction of differentiation to erythroid and megakaryocyte lineages.

The transcription factor Pax5 is essential for B cell commitment in the mouse, where it represses lineage-inappropriate gene expression while simultaneously activating the B cell gene expression program. In this study we have performed a global gene expression screen of wild-type and Pax5-deficient pro-B cells in an attempt to identify the crucial Pax5 targets in early B lymphopoiesis. These studies have identified 109 Pax5 targets comprising 61% activated and 39% repressed genes. Interestingly, Pax5 directly regulates the genes encoding a number of transcription factors that are required at the pre-B cell stage of differentiation, including Irf8, Spib, and Ikzf3 (Aiolos), suggesting that a key function of Pax5 is to activate secondary transcription factors that further reinforce the B cell program. Pax5 is also required for the expression of many genes known to be involved in adhesion and signaling, indicating that Pax5 modulates the homing and or migration properties of B cell progenitors. Finally, Pax5 also represses a cohort of genes that are involved in multiple biological processes, many of which are not typically associated with B cells. These include the repression of the adhesion molecule Embigin, which is expressed in bone marrow progenitors, T cells, and myeloid cells but is specifically repressed by Pax5 in B cells.

No abstract

Xenopus laevis has been a particularly useful model organism for identifying factors involved in the induction and patterning of the mesoderm, however, much remains to be learned about how these factors interact. The myogenic transcription factor Xmyf-5 is the earliest known gene to be expressed specifically in the dorsolateral mesoderm of the gastrula, a domain that is established by the interaction of dorsal and ventral signals. For this reason, we have begun to investigate how the expression of Xmyf-5 is regulated. We have identified a 7.28 kb Xenopus tropicalis Xmyf-5 (Xtmyf-5) genomic DNA fragment that accurately recapitulates the expression of the endogenous gene. Deletion and mutational analysis has identified HBX2, an essential element, approximately 1.2 kb upstream from the start of transcription, which is necessary for both activation and repression of Xtmyf-5 expression, implying that positional information is integrated at this site. Electrophoretic mobility shift assays demonstrate that HBX2 specifically interacts with gastrula stage embryonic extracts and that in vitro translated Xvent-1 protein binds to one of its functional motifs. Combined with gain- and loss-of-function experiments, the promoter analysis described here suggests that Xvent-1 functions to repress Xmyf-5 expression in the ventral domain of the marginal zone. Furthermore, the identification of HBX2 provides a tool with which to identify other molecules involved in the regulation of Xmyf-5 expression during gastrulation.