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

PU.1 functions in a cell-autonomous manner to control the differentiation of multipotential lymphoid-myeloid progenitors

Transcription factor PU.1 is required for the development of lymphoid and myeloid progenitors during fetal hematopoiesis. By generating chimeric animals using PU.1-/- ES cells or PU.1(-/-) hematopoietic progenitors, we demonstrate that PU.1 functions in an exclusively cell-autonomous manner to regulate the development of the lymphoid-myeloid system. Multipotential lymphoid-myeloid progenitors (AA4.1+, Lin-) are significantly reduced in PU.1(-/-) embryos and fail to differentiate into B lymphoid or myeloid cells in vitro. These results suggest that the lymphoid and myeloid lineages develop in the fetal liver from a common hematopoietic progenitor not shared with erythrocytes and megakaryocytes. Finally, the Ikaros gene is expressed in PU.1 mutant embryos, suggesting that PU.1 and Ikaros are independently required for specification of embryonic lymphoid cell fates.

Transcription factor GATA-3 is required for development of the T-cell lineage

THE zinc-finger transcription factor GATA-3 is expressed in haematopoietic cells and in the developing kidney and nervous system. Within the haematopoietic lineages, expression of GATA-3 is restricted to thymocytes and T cells. Functionally important GATA-3 binding sites have been identified in multiple T-cell-specific genes. Mice containing homozygous null mutations of the GATA-3 gene die on embryonic day 12, precluding a detailed assessment of the role of GATA-3 in haematopoietic development. Here we have used murine embryonic stem (ES) cells containing homozygous mutations in the GATA-3 gene (GATA-3(-/-)) in conjunction with the RAG-2(-/-) (ref. 10) and C57BL/6 complementation systems to study the role of GATA-3 in mammalian haematopoiesis. Our results show that GATA-3(-/-) ES cells can contribute to the development of the mature erythroid, myelomonocytic and B-cell lineages, but fail to give rise to thymocytes or mature peripheral T cells. The differentiation of GATA-3(-/-) T cells is blocked at or before the earliest double-negative (CD4-/CD8-) stage of thymocyte development, such that the GATA-3(-/-) ES cells are unable to contribute measurably to the double-negative thymocyte population. These findings suggest that GATA-3 is an essential and specific regulator of early thymocyte development.

Pip, a lymphoid-restricted IRF, contains a regulatory domain that is important for autoinhibition and ternary complex formation with the Ets factor PU.1

Pip is a lymphoid-restricted IRF transcription factor that is recruited to composite elements within immunoglobulin light-chain gene enhancers through a specific interaction with the Ets factor PU.1. We have examined the transcriptional regulatory properties of Pip as well as the requirements for its interaction with PU.1 and DNA to form a ternary complex. We demonstrate that Pip is a dichotomous regulator; it specifically stimulates transcription in conjunction with PU.1, but represses alpha/beta-interferon-inducible transcription in the absence of PU.1. Thus, during B-cell activation and differentiation, Pip may function both as an activator to promote B cell-specific gene expression and as a repressor to inhibit the antiproliferative effects of alpha/beta-interferons. Mutational analysis of Pip reveals a carboxy-terminal segment that is important for autoinhibition of DNA binding and ternary complex formation. A domain of Pip containing this segment confers autoinhibition and PU.1-dependent binding activity to the DNA-binding domain of the related IRF family member, p48. On the basis of these and other data we propose a model for PU.1/Pip ternary complex formation.

The T cell leukemia oncoprotein SCL/tal-1 is essential for development of all hematopoietic lineages

The T cell leukemia oncoprotein SCL/tal-1, a basic-helix-loop-helix transcription factor, is required for production of embryonic red blood cells in the mouse yolk sac. To define roles in other lineages, we studied the hematopoietic potential of homozygous mutant SCL/tal-1 -/- embryonic stem cells upon in vitro differentiation and in vivo in chimeric mice. Here we show that in the absence of SCL/tal-1, hematopoiesis, Including the generation of red cells, myeloid cells, megakaryocytes, mast cells, and both T and B lymphoid cells, is undetectable. These findings suggest that SCL/tal-1 functions very early in hematopoietic development, either in specification of ventral mesoderm to a blood cell fate, or in formation or maintenance of immature progenitors.

The transcription factor PU.1 is involved in macrophage proliferation

PU.1 is a tissue-specific transcription factor that is expressed in cells of the hematopoietic lineage including macrophages, granulocytes, and B lymphocytes. Bone marrow-derived macrophages transfected with an antisense PU.1 expression construct or treated with antisense oligonucleotides showed a decrease in proliferation compared with controls. In contrast, bone marrow macrophages transfected with a sense PU.1 expression construct displayed enhanced macrophage colony-stimulating factor (M-CSF)-dependent proliferation. Interestingly, there was no effect of sense or antisense constructs of PU.1 on the proliferation of the M-CSF-independent cell line, suggesting that the response was M-CSF dependent. This was further supported by the finding that macrophages transfected with a sense or an antisense PU.1 construct showed, respectively, an increased or a reduced level of surface expression of receptors for M-CSF. The enhancement of proliferation seems to be selective for PU.1, since transfections with several other members of the ets family, including ets-2 and fli-1, had no effect. Various mutants of PU.1 were also tested for their ability to affect macrophage proliferation. A reduction in macrophage proliferation was found when cells were transfected with a construct in which the DNA-binding domain of PU.1 was expressed. The PEST (proline-, glutamic acid-, serine-, and threonine-rich region) sequence of the PU.1 protein, which is an important domain for protein-protein interactions in B cells, was found to have no influence on PU.1-enhanced macrophage proliferation when an expression construct containing PU.1 minus the PEST domain was transfected into bone marrow-derived macrophages. In vivo, PU.1 is phosphorylated on several serine residues. The transfection of plasmids containing PU.1 with mutations at each of five serines showed that only positions 41 and 45 are critical for enhanced macrophage proliferation. We conclude that PU.1 is necessary for the M-CSF-dependent proliferation of macrophages. One of the proliferation-relevant targets of this transcription factor could be the M-CSF receptor.