PU.1 is not essential for early myeloid gene expression but is required for terminal myeloid differentiation

Abstract

We have previously shown using gene targeting that PU.1 is essential for the development of lymphoid and myeloid lineages during fetal liver hematopoiesis. We now show that PU.1 is required for the maturation of yolk sac-derived myeloid progenitors and for the differentiation of ES cells into macrophages. The role of PU.1 in regulating target genes, thought to be critical in the development of monocytes and granulocytes, has been analyzed. Early genes such as GM-CSFR, G-CSFR, and myeloperoxidase are expressed in PU.1-/- embryos and differentiated PU.1-/- ES cells. However, the expression of genes associated with terminal myeloid differentiation (CD11b, CD64, and M-CSFR) is eliminated in differentiated PU.1-/- ES cells. Development of macrophages is restored with the introduction of a PU.1 cDNA regulated by its own promoter. The PU.1-/- ES cells represent an important model for analyzing myeloid cell development.

Role of PU.1 in Hematopoiesis

“…Similar results were obtained for day 8.5 and 10.5 yolk sacs, which represent earlier sites of hematopoiesis [17]. This indicates that a similar block in myeloid differentiation occurs during yolk sac hematopoiesis.…”

“…[17,18]. The failure of PU.1 -/-ES cells to differentiate into macrophages can be rescued by a PU.1 transgene under the control of its own promoter [17]. This result confirms the pivotal role that PU.1 serves in controlling macrophage differentiation.…”

“…The ability of a PU.1 transgene to rescue myeloid differentiation of PU.1 -/-(1)-ES cells would suggest that this explanation is highly unlikely [17]. This demonstrates that the observed phenotype for the PU.1 -/- (1) mutation is the direct result of the lack of PU.1 expression [17]. A more likely explanation for the differences between the two PU.1 knockout phenotypes is that PU.1 -/- (2) mice may express a truncated PU.1 protein that retains limited functional ability.…”

“…The kinetics and types of hematopoietic progenitors generated by differentiating ES cells are thought to mirror the developmental events occurring during yolk sac and fetal liver hematopoiesis. [17,18]. The failure of PU.1 -/-ES cells to differentiate into macrophages can be rescued by a PU.1 transgene under the control of its own promoter [17].…”

Blood cell progenitors: Insights into the properties of stem cells

“…PU.1 null progenitor cells are not responsive to GM-CSF, G-CSF, or M-CSF. Transduction of PU.1 null progenitor cells with the M-CSFR overcomes the block in M-CSF-induced cell proliferation but does not restore full myeloid cell differentiation to these progenitor cells (Olson et al, 1995;DeKoter et al, 1998). Further studies concluded that PU.1 controls cellular differentiation by regulating distinct proliferation and differentiation pathways.…”