A novel in vivo transcription assay demonstrates the presence of globin-inducing trans-acting factors in uninduced murine erythroleukemia cells

Hemoglobin switching in Rana/Xenopus erythroid heterokaryons: factors mediating the metamorphic hemoglobin switch are conserved

Hemoglobin switching, which occurs in all classes of vertebrates as well as in certain invertebrates, is due to developmental regulation of different globin genes which are typically arranged in clustered families. By fusing erythroid cells of different developmental programs, trans-acting factors that regulate this switch in gene expression have been detected [Ramseyer et al. (1989): Dev Biol 133:262-271]. Adult erythroid cells of one anuran species, Xenopus laevis, were fused with tadpole erythroid cells of another frog, Rana catesbeiana, creating developmental erythroid heterokaryons that synthesize adult Rana globin mRNA and hemoglobins. The results show that factors from adult Xenopus erythroid cells are capable of inducing adult Rana globin gene expression in the Rana tadpole erythroid cell nucleus. We have used the cross-induction of adult Rana hemoglobin synthesis in these adult Xenopus/Rana tadpole erythroid heterokaryons to address two practical questions, answers to which may be helpful in isolating developmental stage-specific globin gene regulatory proteins: 1) Are erythroblasts which are actively expressing globin mRNAs and hemoglobins richer in specific globin-inducing activities than other stages of erythroid cellular differentiation? 2) Do mature, circulating erythrocytes still have the activities necessary to mediate the cross-induction of Hb synthesis? The results reported here show that the answers to both questions are affirmative and show that quiescent, fully differentiated adult erythroid cells are still capable of expressing the trans-activator(s). These findings show that factors which mediate the metamorphic hemoglobin switch are conserved between these two genera of frogs.

Reversibility of the differentiated state in somatic cells

Analysis of de novo gene activation in multinucleated heterokaryons has shown that the differentiated state, although stable, is not irreversible, and can be reprogrammed in the presence of appropriate combinations of trans-acting regulatory molecules. These properties have been exploited to design strategies for identifying novel regulators of cellular differentiation.

Role of activator protein-1 and methylation function in 12-O-tetradecanoylphorbol-13-acetate--mediated inhibition of differentiation of Friend erythroleukemia cells

Friend erythroleukemia cells (FELCs) differentiate after hexamethylene-bis-acetamide treatment. This differentiation is characterized by an increase in beta-globin gene expression that is followed by appearance of the hemoglobin. Phorbol-ester tumor promoters, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), inhibit differentiation of TPA-sensitive cells but not TPA-resistant cells. We have shown that the increase in beta-globin expression is inhibited by TPA in a TPA-sensitive clone but not in a TPA-resistant clone. To study the molecular mechanisms of regulation of gene expression by TPA, we examined the possible involvement of gene methylation and the TPA-responsive element (TRE). Both clones showed similar patterns of methylation around the beta-globin gene. Moreover, TPA-induced TRE binding and TRE enhancer activity were similar in both variants. These results suggest that the TPA inhibition of induced differentiation may not be explained by regulation of the methylation state. The activator protein-1 also does not play a crucial role in the sensitivity of FELCs to TPA.

Expression of the human carbonic anhydrase I gene is activated late in fetal erythroid development and regulated by stage-specific trans-acting factors

Using flow cytometric analysis of red cells from different stages of ontogeny with anti-CAI antibody, it was shown that the human carbonic anhydrase I (HCAI) gene product appears in a developmental stage-specific manner. Virtually no CAI protein was detectable in fetal red cells prior to birth. However, at about the time of normal delivery (40 weeks gestation) CAI production is switched on. The proportion of cells containing CAI reaches adult levels during the second half of the first year of life. Northern analysis suggests that the appearance of CAI protein results directly from the activation of the gene and the production of new mRNA. A transient heterokaryon system was set up by fusing the erythroleukaemic cell lines MEL C88 (a mouse cell line in which CAI is expressed) and K562 SAI (a human cell line with an embryonic/fetal phenotype, not expressing CAI). SP6 RNAase mapping of RNA from the fused cells showed activation of the human CAI gene. This shows the developmental stage-specific expression of HCAI to be regulated by trans-acting factors.

Evolution and transcription of old world monkey globin genes

Although apparently inactive in the whole animal, the delta globin genes from three species of Old World monkey (rhesus, baboon and green monkey) are all functional in an in vitro transcription assay. Their activities in vitro are similar to that of the functional human delta gene. A fourth monkey gene, from the colobus monkey, is transcribed approximately fivefold less efficiently than the others. This reduced in vitro activity results from a 20 base-pair deletion, which removes the normal site of mRNA initiation. When the deletion is repaired by site-directed mutagenesis, transcriptional activity increases to the level observed for the other delta genes. We also report the complete nucleotide sequences of the colobus beta, colobus delta, and rhesus delta genes. Sequence comparisons show that the delta and beta genes in the same species have not exchanged genetic information since the divergence of the human and monkey lineages. Phylogenetic analysis of these sequences affirms that the Old World monkey delta genes are evolving more rapidly than their functional counterparts. Moreover, the rate of replacement substitutions has risen to equal that of non-coding DNA, as expected for genes no longer under selective constraint.

Intracellular signals for developmental hemoglobin switching

We have detected trans-acting factors that regulate developmental hemoglobin switching by fusing erythroid cells of different developmental programs. Adult erythroid cells of one anuran species, Xenopus laevis, were fused with tadpole erythroid cells of another frog, Rana catesbeiana. In a second set of experiments, dimethyl sulfoxide-induced murine erythroleukemia cells, which express only adult mouse globins, were fused with Rana tadpole erythroid cells, which express only embryonic and fetal-like globins. Adult Rana globin gene expression was detected in both sets of transient heterokaryons at 6 hr after fusion. Dot blots and Northern blots of total RNA from the heterokaryons contained material that reacted with an adult Rana alpha-globin probe; newly synthesized adult Rana hemoglobin tetramers were detected with native polyacrylamide gel electrophoresis. These results show that developmental stage-specific transacting factors for globin genes can function across vertebrate classes (mammalia to amphibia) and suggest that the mechanisms that regulate developmental hemoglobin switching are highly conserved.

The beta-globin dominant control region activates homologous and heterologous promoters in a tissue-specific manner

We have introduced a human beta-globin minilocus, containing the recently described dominant control region (DCR), the beta-globin or Thy-1 gene, and a thymidine kinase (tk)-neoR gene into erythroid and non-erythroid cells. Analysis of the transcription levels of the genes shows that the DCR directs high levels of human beta-globin, Thy-1 and tk-neo expression independent of integration sites in an erythroid-specific manner. The presence of the DNAasel hypersensitive sites at the 5' end of the locus is required for this effect on the homologous and heterologous gene. An analysis of the DCR chromatin in transfected mouse erythroleukemic cells suggests that the formation of the hypersensitive sites in this region precedes beta-globin gene expression.