Regulation of specific gene expression during embryonic development: Synthesis of globin messenger RNA during red cell formation in chick embryos

The role of the epigenetic signal, DNA methylation, in gene regulation during erythroid development

The sequence complexity of the known vertebrate genomes alone is insufficient to account for the diversity between individuals of a species. Although our knowledge of vertebrate biology has evolved substantially with the growing compilation of sequenced genomes, understanding the temporal and spatial regulation of genes remains fundamental to fully exploiting this information. The importance of epigenetic factors in gene regulation was first hypothesized decades ago when biologists posited that methylation of DNA could heritably alter gene expression [Holliday and Pugh, 1975. Science 187(4173), 226-232; Riggs, 1975. Cytogenet. and Cell Genet.14(1), 9-25; Scarano et al., 1967. Proc. Natl. Acad. Sci. USA 57(5), 1394-1400)]. It was subsequently shown that vertebrate DNA methylation, almost exclusively at the 5' position of cytosine in the dinucleotide CpG, played a role in a number of processes including embryonic development, genetic imprinting, cell differentiation, and tumorigenesis. At the time of this writing, a large and growing list of genes is known to exhibit DNA methylation-dependent regulation, and we understand in some detail the mechanisms employed by cells in using methylation as a regulatory modality. In this context, we revisit one of the original systems in which the role of DNA methylation in vertebrate gene regulation during development was described and studied: erythroid cells. We briefly review the recent advances in our understanding of DNA methylation and, in particular, its regulatory role in red blood cells during differentiation and development. We also address DNA methylation as a component of erythroid chromatin architecture, and the interdependence of CpG methylation and histone modification.

MBD2 is a critical component of a methyl cytosine-binding protein complex isolated from primary erythroid cells

The chicken embryonic beta-type globin gene, rho, is a member of a small group of vertebrate genes whose developmentally regulated expression is mediated by DNA methylation. Previously, we have shown that a methyl cytosine-binding complex binds to the methylated rho-globin gene in vitro. We have now chromatographically purified and characterized this complex from adult chicken primary erythroid cells. Four components of the MeCP1 transcriptional repression complex were identified: MBD2, RBAP48, HDAC2, and MTA1. These 4 proteins, as well as the zinc-finger protein p66 and the chromatin remodeling factor Mi2, were found to coelute by gel-filtration analysis and pull-down assays. We conclude that these 6 proteins are components of the MeCPC. In adult erythrocytes, significant enrichment for MBD2 is seen at the inactive rho-globin gene by chromatin immunoprecipitation assay, whereas no enrichment is observed at the active beta(A)-globin gene, demonstrating MBD2 binds to the methylated and transcriptionally silent rho-globin gene in vivo. Knock-down of MBD2 resulted in up-regulation of a methylated rho-gene construct in mouse erythroleukemic (MEL)-rho cells. These results represent the first purification of a MeCP1-like complex from a primary cell source and provide support for a role for MBD2 in developmental gene regulation.

Presence of globin gene transcripts in chicken oocytes and of a partially processed globin RNA in early embryos

RNA isolated from chicken oocytes and early embryos of various stages of development were probed with cloned cDNA of the alpha type (pi, alpha D and alpha A) and beta type (beta A, globin genes. Transcripts of all genes were present, although at a very low level, in the RNA of oocytes, and of embryos of the blastula and gastrula stages, prior to the onset of globin synthesis at about 30 h incubation. Interestingly, Northern blotting of electrophoretically fractionated embryonic RNA made it possible to observe, at all stages of development and for all genes tested, RNA molecules several hundred nucleotides longer than mature mRNA. PCR amplification of the pi globin transcripts indicates that these additional sequences are localized upstream of the CAP site. These higher-MW forms were found to be replaced by normal-size globin mRNA several hours after the onset of globin synthesis. The relevance of these data to comprehension of how globin gene expression is controlled during development is discussed.

Analysis of histone H5 and globin mRNAs in chick-embryo development

Transcripts from histone H5 and alpha and beta chicken globin genes were analysed in total RNA extracted from 2-6-day-old chick embryos. Denatured RNA, separated on gels, was transferred to either aminophenylthioether (APT) paper or to nitrocellulose and was detected with 32P-labelled recombinant DNA probes. Good resolution was achieved with methyl mercury hydroxide gels and aminophenylthioether transfer, allowing estimation of H5 and alpha and beta globin mRNA sizes. With glyoxal gels and nitrocellulose transfer, alpha and beta globin mRNAs were not resolved, but an increased sensitivity of detection of at least 10-fold was achieved. Levels of H5 and alpha globin mRNAs increased in parallel over 2-6 days. The appearance of definitive red cells in 6-day embryos was indicated by detection of adult beta globin transcripts at this stage.

Is there specific transcription from isolated chromatin?

Hg-UMP-containing transcripts made from chick erythroid chromatins with E. coli RNA polymerase hybridize to chick globin cDNA. Contamination with endogenous globin RNA has been largely removed by purification on SH-agarose columns at 55 degrees C. Some endogenous globin mRNA sequences remain, probably as hybrids with "anti-sense" Hg-transcripts produced by RNA-dependent RNA synthesis. Heating to 115 degrees C before SH-agarose chromatography eliminates these contaminants. Hg-transcripts from adult and embryonic erythroid chromatins purified by this method are hybridized to globin cDNA; they contain a 4- to 6-fold higher proportion of globin-specific sequences (10-13 PPM) than do transcripts from brain chromatin. Dissociation of erythroid chromatins in salt and urea, followed by reconstitution using standard methods, destroys even this low degree of specificity.

RNA aggregation during sulfhydryl-agarose chromatography of mercurated RNA

Isolation of newly synthesized mercurated RNA transcripts by chromatography on sulfhydryl-agarose has recently been used to reduce contamination by endogenous RNA derived from the chromatin template. We show that substantial RNA aggregation occurs during standard isolation procedures, causing significant retention of endogenous (unmercurated) RNA on sulfhydryl-agarose. We describe methods to reduce substantially this problem and discuss the implications of our findings for interpretation of previous hybridization and transcription experiments.

Esterase isozyme patterns in developing embryos of Brachydanio rerio (zebra danio), Brachydanio abolineatus (pearl danio), and their hybrids

The ontogeny of esterase isozymes in Brachydanio rerio (zebra danio), Brachydanio albolineatus (pearl danio), and hybrids formed by their reciprocal crosses was investigated using polyacrylamide disc electrophoresis. Seven esterase isozymes were identified in each species from the unfertilized egg stage to nine days posthatch. Electrophoretic analysis of qualitative changes in enzyme pattern indicated that some esterases were present at all stages of development while other esterases abruptly appeared at a specific stage of morphological differentiation. The esterases of both species were classified on the basis differential substrate and inhibitor specificities. In developing hybrids formed by B. rerio eggs inseminated with B. albolineatus sperm, the maternal isozyme pattern persisted until Stage 17 (gastrulation). Embryonic extracts from Stage 17 onward showed a slow-moving, DFP-sensitive carboxylesterase of paternal origin. In developing hybrids formed by B. albolineatus eggs inseminated with B. rerio sperm, a paternal contribution to the esterase pattern was probably present by the end of gastrulation; esterase activity of distinctively paternal origin was present by Stage 22 (retinal pigmentation) The maternal contribution to the total esterase profile appeared to remain high through hatching. Additional evidence for gene activity at gastrulation was obtained in experiments utilizing actinomycin-D and cycloheximide. Results of exposing embryos of B. rerio to 15 mug/ml of actinomycin-D indicated that transcription of the template RNA coding for cholinesterase occurred during gastrulation or some 20-30 hours prior to the appearance of the isozyme at Stage 22. This template RNA was translated sometime during that 10-hour interval immediately preceding Stage 22.

The initiation of globin synthesis in differentiating rabbit-bone-marrow erythroid cells

At a particular stage of erythropoiesis, the immature erythroid cell commences synthesis of the messenger RNA for globin. Experiments were performed to determine whether this mRNA is immediately translated into globin or whether there is any substantial delay and accumulation of mRNA awaiting a further stage of differentiation. Rabbit bone marrow erythroid cells were separated according to their degree of maturity by velocity sedimentation on a sucrose gradient. Their relative abilities to synthesise globin were measured by radioactive labelling in culture and the relative proportions of globin mRNA in cellular RNA were determined by hybridisation to DNA complementary to globin mRNA. The increasing ability to synthesise globin closely parallelled the concentration of globin mRNA at all stages of differentiation, thus suggesting that there is no significant delay between transcription and translation.

The regulation of haemoglobin synthesis in cultured chick blastoderms by steroids related to 5beta-androstane

1. After 24h of incubation, the blastoderm may be dissected from the early developing chick embryo and successfully maintained under conditions of organ culture in vitro. 2. Low concentrations of steroids related to 5beta-androstane stimulate the synthesis of foetal haemoglobins, types E and P, in a highly steroid- and tissue-specific manner.