Despite the importance of blood platelets in health and disease, the mechanisms regulating their formation within megakaryocytes are unknown. We generated mice lacking the hematopoietic subunit (p45) of the heterodimeric erythroid transcription factor NF-E2. Unexpectedly, NF-E2-/- mice lack circulating platelets and die of hemorrhage; their megakaryocytes show no cytoplasmic platelet formation. Though platelets are absent, serum levels of the growth factor thrombopoietin/MGDF are not elevated above controls. Nonetheless, NF-E2-/- megakaryocytes proliferate in vivo in response to thrombopoietin administration. Thus, as an essential factor for megakaryocyte maturation and platelet production, NF-E2 must regulate critical target genes independent of the action of thrombopoietin. These findings provide insight into the genetic analysis of megakaryocyte maturation and thrombopoiesis.

Many receptors activate phospholipase Cgamma1 or -gamma2. To assess the role of PLCgamma2, we derived enzyme-deficient mice. The mice are viable but have decreased mature B cells, a block in pro-B cell differentiation, and B1 B cell deficiency. IgM receptor-induced Ca2+ flux and proliferation to B cell mitogens are absent. IgM, IgG2a, and IgG3 levels are reduced, and T cell-independent antibody production is absent. The similarity to Btk- or Blnk-deficient mice demonstrates that PLCgamma2 is downstream in Btk/Blnk signaling. FcRgamma signaling is also defective, resulting in a loss of collagen-induced platelet aggregation, mast cell FcepsilonR function, and NK cell FcgammaRIII and 2B4 function. The results define a signal transduction pathway broadly utilized by immunoglobulin superfamily receptors.

The coactivators CBP (Cre-element binding protein (CREB)-binding protein) and its paralogue p300 are thought to supply adaptor molecule and protein acetyltransferase functions to many transcription factors that regulate gene expression. Normal development requires CBP and p300, and mutations in these genes are found in haematopoietic and epithelial tumours. It is unclear, however, which functions of CBP and p300 are essential in vivo. Here we show that the protein-binding KIX domains of CBP and p300 have nonredundant functions in mice. In mice homozygous for point mutations in the KIX domain of p300 designed to disrupt the binding surface for the transcription factors c-Myb and CREB, multilineage defects occur in haematopoiesis, including anaemia, B-cell deficiency, thymic hypoplasia, megakaryocytosis and thrombocytosis. By contrast, age-matched mice homozygous for identical mutations in the KIX domain of CBP are essentially normal. There is a synergistic genetic interaction between mutations in c-Myb and mutations in the KIX domain of p300, which suggests that the binding of c-Myb to this domain of p300 is crucial for the development and function of megakaryocytes. Thus, conserved domains in two highly related coactivators have contrasting roles in haematopoiesis.

We show here that a zinc finger transcriptional repressor, Slug, which is aberrantly upregulated by the E2A-HLF oncoprotein in pro-B cell acute leukemia, functions as an antiapoptotic factor in normal hematopoietic progenitor cells. Slug(-/-) mice were much more radiosensitive than wild-type mice, dying earlier and showing accentuated decreases in peripheral blood cell counts, as well as abundant microhemorrhages and widely disseminated bacterial microabscesses throughout the body. Slug expression was detected in diverse subsets of hematopoietic progenitors, but not in more differentiated B and T lymphoid cells, and there was a significant increase in apoptotic (TUNEL-positive) bone marrow progenitor cells in irradiated Slug(-/-) mice compared to wild-type controls. These results implicate Slug in a novel survival pathway that protects hematopoietic progenitors from apoptosis after DNA damage.

Transcription of immediate-early genes-as well as multiple genes affecting muscle function, cytoskeletal integrity, apoptosis control, and wound healing/angiogenesis-is regulated by serum response factor (Srf). Extracellular signals regulate Srf in part via a pathway involving megakaryoblastic leukemia 1 (Mkl1, also known as myocardin-related transcription factor A [Mrtf-a]), which coactivates Srf-responsive genes downstream of Rho GTPases. Here we investigate Mkl1 function using gene targeting and show the protein to be essential for the physiologic preparation of the mammary gland during pregnancy and the maintenance of lactation. Lack of Mkl1 causes premature involution and impairs expression of Srf-dependent genes in the mammary myoepithelial cells, which control milk ejection following oxytocin-induced contraction. Despite the importance of Srf in multiple transcriptional pathways and widespread Mkl1 expression, the spectrum of abnormalities associated with Mkl1 absence appears surprisingly restricted.Megakaryoblastic leukemia 1 (MKL1) was initially identified in acute megakaryoblastic leukemias (AMKLs) that occur in infants and young children due to its involvement in the RBM15-MKL1 fusion protein created by the t(1;22) chromosomal translocation found uniquely in these leukemias (41, 48). As a result of this translocation, the MKL1 gene (alternatively known as MAL [megakaryocytic acute leukemia], MRTF-A