- Email: [email protected]
Somite-derived cells replace ventral aortic hemangioblasts and provide aortic smooth muscle cells of the trunk
ABSTRACTS / Blood Cells, Molecules, and Diseases 38 (2007) 120 – 191
55 Somite-derived cells replace ventral aortic hemangioblasts and provide aortic smooth muscle cells of the trunk C. Pouget, R. Gautier, M.-A. Teillet, T. Jaffredo Universite´ Pierre et Marie Curie-Paris6, CNRS UMR7622, Laboratoire de Biologie du De´veloppement, Bat C, 6e`me e´tage; Case 24; 75252 Paris Cedex 05, France E-mail: [email protected] We have shown previously that endothelial cells of the aortic floor give rise to hematopoietic cells revealing the existence of an aortic hemangioblast. It was proposed that the restriction of hematopoiesis to the aortic floor was based on the existence of two different and complementary endothelial lineages that form the vessel: one originating from the somite would contribute to the roof and sides, another originating from the splanchnopleura would contribute to the floor. Using quail/chick orthotopic transplantations of paraxial mesoderm, we have traced the distribution of somite-derived endothelial cells during aortaassociated hematopoiesis. We show that before and during this period, the aortic endothelium undergoes two successive waves of remodelling by cells of somite origin; one when the splanchnopleura-derived aortae are still paired, during which the initial roof and sides of the vessels are renewed; a second, contemporary with aorta-associated hematopoiesis, in which the floor hemangioblasts are progressively replaced by somite endothelial cells. Thus the aortic floor appears as a temporary structure that becomes spend out and replaced during the first 4 days of development. In addition, the somite contributes to smooth muscle cells of the aorta. Our in vivo lineage tracing experiments with nonreplicative retroviral vectors showed that endothelial cells do not give rise to smooth muscle cells. However when cultured in vitro, purified endothelial cells may acquire smooth muscle cells characteristics. Taken together these data point to the critical role of the somite in shaping the aorta and also give an explanation for the short life of aortic hematopoiesis. doi:10.1016/j.bcmd.2006.10.066
56 The protein methyltransferase PRMT5 links histone H4 methylation to Dnmt3a-dependent DNA methylation and transcriptional silencing Quan Zhao 1, Gerhard Rank 1, Loretta Cerruti 1, David J. Curtis 1, John M. Cunningham 2, Stephen M. Jane 1,3 1 Rotary Bone Marrow Research Laboratories, Melbourne Health Research Directorate, Royal Melbourne Hospital, Parkville, VIC 3050, Australia 2 Division of Experimental Hematology, St Jude Children’s Research Hospital, 332 Nth Lauderdale St, Memphis, TN 38101, USA 3 Department of Medicine, University of Melbourne, Parkville, VIC 3050, Australia Direct mechanistic links between DNA methylation and histone modifications are emerging in the context of gene
147
silencing. In mammals, and in plants, restrictive heterochromatin is associated with hypermethylation of DNA at CG sites, and with specific histone modifications, such as methylation of histone H3 at lysine 9. We now report a direct link between the repressive histone modification, symmetric methylation of Arg3 on histone H4 (H4R3me2s) induced by the methyltransferase PRMT5 with other repressive epigenetic modifications, including Dnmt3a-induced DNA methylation. Assembly of a repression complex containing PRMT5, Dnmt3a, casein kinase 2, Suv4-20h1/2, and components of the MBD2/NuRD complex on the human fetal (g) globin gene promoters resulted in transcriptional silencing associated with multiple repressive epigenetic modifications, including H4K20 trimethylation, phosphorylation of H4S1, and DNA methylation. Knock down of PRMT5 in an erythroid cell line resulted in complete loss of H4R3me2s coincident with loss of all repressive markers at the g-promoters, and enhanced g-gene expression. Our findings provide a novel link between histone methylation and the establishment of an array of repressive epigenetic markers including DNA methylation that culminate in gene silencing. doi:10.1016/j.bcmd.2006.10.067
57 Differential sensitivities of transcription factor target genes underlie cell type-specific gene expression profiles Kirby D. Johnson, Shin-Il Kim, Megan E. Boyer, Emery H. Bresnick University of Wisconsin Medical School, Department of Pharmacology, Madison, WI, USA The hematopoietic transcription factor GATA-1 is a critical regulator of erythroid differentiation via direct transcriptional control of numerous erythroid-specific genes, including the globin genes. GATA-1 levels are low in hematopoietic stem cells but rise during erythropoiesis, yet little is known regarding how changes in GATA-1 levels or activity affect the full ensemble of GATA-1 target genes. We have investigated the relationship between the activity of GATA1, chromatin occupancy, and target gene sensitivity. In GATA1-null G1E erythroid precursor cells, graded activation of GATA-1 fused to the estrogen receptor hormone binding domain (ER-GATA-1) revealed high, intermediate, and low sensitivity targets. GATA-1 activity requirements for transcription often correlated with ER-GATA-1 occupancy at regulatory regions within loci. The sensitivity of regulatory regions to occupancy by ER-GATA-1 could not be predicted based solely upon the number of GATA motifs. Having categorized GATA-1 targets based upon sensitivity to ERGATA-1 activity, we further investigated the mechanisms that determine the sensitivity of individual GATA-1 targets. In particular, we define a novel role for the GATA-1 cofactor FOG-1 in limiting GATA-1 occupancy at a low-sensitivity GATA-1 target gene. Thus, by either facilitating or opposing GATA-1 chromatin occupancy in a context-dependent man-
55 Somite-derived cells replace ventral aortic hemangioblasts and provide aortic smooth muscle cells of the trunk C. Pouget, R. Gautier, M.-A. Teillet, T. Jaffredo Universite´ Pierre et Marie Curie-Paris6, CNRS UMR7622, Laboratoire de Biologie du De´veloppement, Bat C, 6e`me e´tage; Case 24; 75252 Paris Cedex 05, France E-mail: [email protected] We have shown previously that endothelial cells of the aortic floor give rise to hematopoietic cells revealing the existence of an aortic hemangioblast. It was proposed that the restriction of hematopoiesis to the aortic floor was based on the existence of two different and complementary endothelial lineages that form the vessel: one originating from the somite would contribute to the roof and sides, another originating from the splanchnopleura would contribute to the floor. Using quail/chick orthotopic transplantations of paraxial mesoderm, we have traced the distribution of somite-derived endothelial cells during aortaassociated hematopoiesis. We show that before and during this period, the aortic endothelium undergoes two successive waves of remodelling by cells of somite origin; one when the splanchnopleura-derived aortae are still paired, during which the initial roof and sides of the vessels are renewed; a second, contemporary with aorta-associated hematopoiesis, in which the floor hemangioblasts are progressively replaced by somite endothelial cells. Thus the aortic floor appears as a temporary structure that becomes spend out and replaced during the first 4 days of development. In addition, the somite contributes to smooth muscle cells of the aorta. Our in vivo lineage tracing experiments with nonreplicative retroviral vectors showed that endothelial cells do not give rise to smooth muscle cells. However when cultured in vitro, purified endothelial cells may acquire smooth muscle cells characteristics. Taken together these data point to the critical role of the somite in shaping the aorta and also give an explanation for the short life of aortic hematopoiesis. doi:10.1016/j.bcmd.2006.10.066
56 The protein methyltransferase PRMT5 links histone H4 methylation to Dnmt3a-dependent DNA methylation and transcriptional silencing Quan Zhao 1, Gerhard Rank 1, Loretta Cerruti 1, David J. Curtis 1, John M. Cunningham 2, Stephen M. Jane 1,3 1 Rotary Bone Marrow Research Laboratories, Melbourne Health Research Directorate, Royal Melbourne Hospital, Parkville, VIC 3050, Australia 2 Division of Experimental Hematology, St Jude Children’s Research Hospital, 332 Nth Lauderdale St, Memphis, TN 38101, USA 3 Department of Medicine, University of Melbourne, Parkville, VIC 3050, Australia Direct mechanistic links between DNA methylation and histone modifications are emerging in the context of gene
147
silencing. In mammals, and in plants, restrictive heterochromatin is associated with hypermethylation of DNA at CG sites, and with specific histone modifications, such as methylation of histone H3 at lysine 9. We now report a direct link between the repressive histone modification, symmetric methylation of Arg3 on histone H4 (H4R3me2s) induced by the methyltransferase PRMT5 with other repressive epigenetic modifications, including Dnmt3a-induced DNA methylation. Assembly of a repression complex containing PRMT5, Dnmt3a, casein kinase 2, Suv4-20h1/2, and components of the MBD2/NuRD complex on the human fetal (g) globin gene promoters resulted in transcriptional silencing associated with multiple repressive epigenetic modifications, including H4K20 trimethylation, phosphorylation of H4S1, and DNA methylation. Knock down of PRMT5 in an erythroid cell line resulted in complete loss of H4R3me2s coincident with loss of all repressive markers at the g-promoters, and enhanced g-gene expression. Our findings provide a novel link between histone methylation and the establishment of an array of repressive epigenetic markers including DNA methylation that culminate in gene silencing. doi:10.1016/j.bcmd.2006.10.067
57 Differential sensitivities of transcription factor target genes underlie cell type-specific gene expression profiles Kirby D. Johnson, Shin-Il Kim, Megan E. Boyer, Emery H. Bresnick University of Wisconsin Medical School, Department of Pharmacology, Madison, WI, USA The hematopoietic transcription factor GATA-1 is a critical regulator of erythroid differentiation via direct transcriptional control of numerous erythroid-specific genes, including the globin genes. GATA-1 levels are low in hematopoietic stem cells but rise during erythropoiesis, yet little is known regarding how changes in GATA-1 levels or activity affect the full ensemble of GATA-1 target genes. We have investigated the relationship between the activity of GATA1, chromatin occupancy, and target gene sensitivity. In GATA1-null G1E erythroid precursor cells, graded activation of GATA-1 fused to the estrogen receptor hormone binding domain (ER-GATA-1) revealed high, intermediate, and low sensitivity targets. GATA-1 activity requirements for transcription often correlated with ER-GATA-1 occupancy at regulatory regions within loci. The sensitivity of regulatory regions to occupancy by ER-GATA-1 could not be predicted based solely upon the number of GATA motifs. Having categorized GATA-1 targets based upon sensitivity to ERGATA-1 activity, we further investigated the mechanisms that determine the sensitivity of individual GATA-1 targets. In particular, we define a novel role for the GATA-1 cofactor FOG-1 in limiting GATA-1 occupancy at a low-sensitivity GATA-1 target gene. Thus, by either facilitating or opposing GATA-1 chromatin occupancy in a context-dependent man-