FLI-1 is an ETS family transcription factor which is overexpressed in

Members of the MEF2 family of transcription factors bind as homo-and heterodimers to the MEF2 site found in the promoter regions of numerous muscle-specific, growth-or stress-induced genes. We showed previously that the transactivation activity of MEF2C is stimulated by p38 mitogen-activated protein (MAP) kinase. In this study, we examined the potential role of the p38 MAP kinase pathway in regulating the other MEF2 family members. We found that MEF2A, but not MEF2B or MEF2D, is a substrate for p38. Among the four p38 group members, p38 is the most potent kinase for MEF2A. Threonines 312 and 319 within the transcription activation domain of MEF2A are the regulatory sites phosphorylated by p38. Phosphorylation of MEF2A in a MEF2A-MEF2D heterodimer enhances MEF2-dependent gene expression. These results demonstrate that the MAP kinase signaling pathway can discriminate between different MEF2 isoforms and can regulate MEF2-dependent genes through posttranslational activation of preexisting MEF2 protein.The transactivation activity of many transcription factors is regulated by phosphorylation (2). The mitogen-activated protein (MAP) kinase family of serine/threonine kinases has been shown to play important roles in regulating gene expression via transcription factor phosphorylation (5,10,16,38,40,42). Unique structural features, specific activation pathways, and different substrate specificities provide evidence to support the contention that different MAP kinases are independently regulated and control different cellular responses to extracellular stimuli (7,38,40,44).p38 MAP kinase was first identified in studies designed to explore how bacterial endotoxin induces cytokine expression (11,13,23). Following the initial description of p38 (p38␣), three additional isoforms of this MAP kinase group have been cloned and characterized: p38␤ (18), p38␥ (also termed ERK6 or SAPK3) (22,24,30), and p38␦ (also termed SAPK4) (4, 17, 41). p38␣ and p38␤ are sensitive to pyridinyl imidazole derivatives, whereas p38␥ and p38␦ are not (4). In mammalian cells, these closely related p38 isoforms are activated coordinately by a broad panel of stimuli which include physical-chemical stresses and proinflammatory cytokines (17, 36). Two MAP kinase kinases (MKK), MKK3 and MKK6, are the upstream activators of the p38 group MAP kinases (6,12,14,37). Several proteins including transcription factors such as CHOP 10 (GADD153) (42), Sap1 (16), MEF2C (10), enzymes such as cPLA2 (20), and the protein kinases MAPKAPK2/3 (27, 29, 39), MNK1/2 (8, 45), and p38-regulated/activated protein kinase (33) have been shown by us and others to be substrates of p38.We showed that MEF2C, a member of the MEF2 family of transcription factors, is phosphorylated by p38 and that this event regulates the transactivation activity of MEF2C (10). Our studies showed that p38 specifically phosphorylates serine 387 and threonines 293 and 300 within the MEF2C transactivation domain (10). MEF2C phosphorylation by p38 was shown to play an important role in regulation of c-Jun ...

In order to elucidate the function of the cytoplasmic core (or rachis: a structure specific of the nematode gonads), we have carried out a cytological study of this structure in the free-living nematode Caenorhabditis elegans, in wild-type and in several mutant strains showing an abnormal gametogenesis. We also performed an ultrastructural radioautographic study of RNA synthesis during oogenesis in order to examine the part played by the rachis in the transport of nutritive substances. Our results evidence for the first time a metabolite transfer from the germ cells to the cytoplasmic core and lead us to assign to the core a trophic role linked to oogenesis. A statistical analysis of silver grain distribution has led us to conclude that there is no accumulation of RNA labelling in any part of the cytoplasmic core. In addition, our studies performed on sterile mutant strains suggest that the cytoplasmic core may have a specific function in oogenesis determination.

This study investigated the role of the ETS transcription factor Fli-1 in adult myelopoiesis using new transgenic mice allowing inducible Fli-1 gene deletion. Fli-1 deletion in adult induced mild thrombocytopenia associated with a drastic decrease in large mature megakaryocytes number. Bone marrow bipotent megakaryocyticerythrocytic progenitors (MEPs) increased by 50% without increase in erythrocytic and megakaryocytic common myeloid progenitor progeny, suggesting increased production from upstream stem cells. These MEPs were almost unable to generate pure colonies containing large mature megakaryocytes, but generated the same total number of colonies mainly identifiable as erythroid colonies containing a reduced number of more differentiated cells. Cytological and fluorescenceactivated cell sorting analyses of MEP progeny in semisolid and liquid cultures confirmed the drastic decrease in large mature megakaryocytes but revealed a surprisingly modest (50%) reduction of CD41-positive cells indicating the persistence of a megakaryocytic commitment potential. Symmetrical increase and decrease of monocytic and granulocytic progenitors were also observed in the progeny of purified granulocytic-monocytic progenitors and common myeloid progenitors. In summary, this study indicates that Fli-1 controls several lineages commitment decisions at the stem cell, MEP, and granulocytic-monocytic progenitor levels, stimulates the proliferation of committed erythrocytic progenitors at the expense of their differentiation, and is a major regulator of late stages of megakaryocytic differentiation. (Blood. 2010;116(23):4795-4805) IntroductionMature blood cells in adults are permanently regenerated from a limited pool of pluripotent hematopoietic stem cells (HSCs) localized in the bone marrow. This process occurs through the hierarchical generation of intermediate progenitors with more and more restricted differentiation and proliferation potential that can be prospectively purified. According to current models, myeloid lineages are derived from a common myeloid progenitor (CMP) generating 2 distinct bipotent progenitors, the megakaryocyticerythrocytic progenitors (MEPs) and the granulocytic-monocytic progenitors (GMP). 1 MEPs generate in turn erythrocytic and megakaryocytic monopotent progenitors, whereas GMP generate granulocytic and monocytic monopotent progenitors. Increasing data also indicate an alternative pathway generating MEPs directly from HSCs. [2][3][4] All this process is controlled by permanent crosstalk between extracellular signals and intracellular regulatory networks of transcription factors. Although having no instructive role, erythropoietin (EPO) is the major cytokine controlling erythropoiesis through the stimulation of proliferation and survival of committed erythrocytic progenitors expressing the specific receptor EPOR. 5 Thrombopoietin (TPO) is the major cytokine controlling megakaryopoiesis through the stimulation of proliferation and differentiation of committed megakaryocytic progenitors expressing the sp...