Hemoglobin switching in chickens. Is the switch initiated post-transcriptionally?

Modelling human haemoglobin switching

Genetic lesions of the β-globin gene result in haemoglobinopathies such as β-thalassemia and sickle cell disease. To discover and test new molecular medicines for β-haemoglobinopathies, cell-based and animal models are now being widely utilised. However, multiple in vitro and in vivo models are required due to the complex structure and regulatory mechanisms of the human globin gene locus, subtle species-specific differences in blood cell development, and the influence of epigenetic factors. Advances in genome sequencing, gene editing, and precision medicine have enabled the first generation of molecular therapies aimed at reactivating, repairing, or replacing silenced or damaged globin genes. Here we compare and contrast current animal and cell-based models, highlighting their complementary strengths, reflecting on how they have informed the scope and direction of the field, and describing some of the novel molecular and precision medicines currently under development or in clinical trial.

Differentially expressed genes in a flock of Chinese local-breed chickens infected with a subgroup J avian leukosis virus using suppression subtractive hybridization

Avian leukosis virus subgroup J (ALV-J) is a new type of virus that mainly induces myeloid leukosis (ML) in chickens. To further elucidate the pathogenesis of ALV-J infection and tumor development, expression profiles from the bone marrow tissue of 15 infected and 18 non-infected birds from a local-breed poultry-farm under naturally infected conditions, were analyzed by suppression-subtractive hybridization. The birds were diagnosed as ML+ (or ML-) by specific ALV-J detection methods, involving serological tests for antigens and antibodies, and RT-PCR to detect viral RNA. A total of 59 partial gene sequences were revealed by differential screening of 496 forward and 384 reverse subtracted cDNA clones. Of these, 22 identified genes, including 8 up-regulated and 14 down-regulated, were related to immune functions, these genes being, MHC B-G antigen, translationally-controlled tumor protein (TPT1/TPTC), transferrin and ferritin, hemoglobin and Carbonic anhydrase. Four of the down-regulated genes were selected for further analysis, in view of their predicted roles in infection and immunity by real-time qRT-PCR, using RNA collected from the same birds as those used for SSH. The four genes were expressed at significantly lower levels (p < 0.001) in ALV-J infected birds than in non-infected ones.

In embryonic chicken erythrocytes actively transcribed alpha globin genes are not associated with the nuclear matrix

The spatial organization of a 250 Kb region of chicken chromosome 14, which includes the alpha globin gene cluster, was studied using in situ hybridization of a corresponding BAC probe with nuclear halos. It was found that in non-erythroid cells (DT40) and cultured erythroid cells of definite lineage (HD3) the genomic region under study was partially (DT40 cells) or fully (HD3 cells) associated with the nuclear matrix. In contrast, in embryonic red blood cells (10-day RBC) the same area was located in the crown of DNA loops surrounding the nuclear matrix, although both globin genes and surrounding house-keeping genes were actively transcribed in these cells. This spatial organization was associated with the virtual absence of RNA polymerase II in nuclear matrices prepared from 10-day RBC. In contrast, in HD3 cells a significant portion of RNA polymerase II was present in nuclear matrices. Taken together, these observations suggest that in embryonic erythroid cells transcription does not occur in association with the nuclear matrix.

Hypoxic adaptations of hemoglobin in Tibetan chick embryo: high oxygen-affinity mutation and selective expression

Tibetan chicks (Gallus gallus) survived with high hatchability (35.0%) and Recessive White Feather broilers (RWF) from low elevations survived rarely and with a low hatchability (3.0%) after simulated incubation under hypoxia of 13% O2. The functional mutation of Met-32D(B13)-Leu of alpha(D) globin chain was related with hypoxia based on allele distribution, homology model building and oxygen affinity assay. Whole embryos on days 3-8 and whole blood on days 9-18 were collected to investigate the stage expression profiles of all seven globins and HIF-1alpha by real-time PCR. Under hypoxia (12.0% O2) on days 3-8, HbE was overexpressed, HbA was expressed earlier and HbP expression was restricted, which completely overturned the expression profile under normoxia. The amount of hemoglobin expression in Tibetan chicks was remarkably higher than that of RWF. HIF-1alpha expression peaked early in both breeds, with. In conclusion, the special hypoxic expression profile on days 3-8 certainly is a common molecular mechanism of hypoxia tolerance in surviving Tibetan chick and RWF embryos; the mutation Met-32D(B13)-Leu and increasing hemoglobins are important mechanisms of hypoxia adaptation in Tibetan chick embryos, and we suggest that HIF-1alpha could be responsible for the hypoxic expression profile.

Identification, characterization, and expression pattern of the chicken EKLF gene

EKLF/KLF1 was the first of the Krüppel-like factors (KLFs) to be identified in mammals and plays an important role in primitive and definitive erythropoiesis. Here, we identify and characterize EKLF in the chicken (cEKLF). The predicted amino acid sequence of the zinc finger region of cEKLF is at least 87.7% similar to mammalian EKLF proteins and is 98.8% and 95% similar to the EKLF orthologues in Xenopus and zebrafish, respectively. During early embryonic development, cEKLF expression is seen in the posterior primitive streak, which gives rise to hematopoietic cells, and then in the blood islands and in circulating blood cells. cEKLF mRNA is expressed in blood cells but not in brain later in chicken embryonic development. cEKLF mRNA is increased in definitive compared with primitive erythropoiesis. The conserved sequence and expression pattern of cEKLF suggests that its function is similar to its orthologues in mammals, Xenopus, and zebrafish.

cAMP and in vivo hypoxia induce tob, ifr1, and fos expression in erythroid cells of the chick embryo

During avian embryonic development, terminal erythroid differentiation occurs in the circulation. Some of the key events, such as the induction of erythroid 2,3-bisphosphoglycerate (2,3-BPG), carbonic anhydrase (CAII), and pyrimidine 5'-nucleotidase (P5N) synthesis are oxygen dependent (Baumann R, Haller EA, Schöning U, and Weber M, Dev Biol 116: 548-551, 1986; Dragon S and Baumann R, Am J Physiol Regulatory Integrative Comp Physiol 280: R870-R878, 2001; Dragon S, Carey C, Martin K, and Baumann R, J Exp Biol 202: 2787-2795, 1999; Dragon S, Glombitza S, Götz R, and Baumann R, Am J Physiol Regulatory Integrative Comp Physiol 271: R982-R989, 1996; Dragon S, Hille R, Götz R, and Baumann R, Blood 91: 3052-3058, 1998; Million D, Zillner P, and Baumann R, Am J Physiol Regulatory Integrative Comp Physiol 261: R1188-R1196, 1991) in an indirect way: hypoxia stimulates the release of norepinephrine (NE)/adenosine into the circulation (Dragon et al., J Exp Biol 202: 2787-2795, 1999; Dragon et al., Am J Physiol Regulatory Integrative Comp Physiol 271: R982-R989, 1996). This leads via erythroid beta-adrenergic/adenosine A(2) receptor activation to a cAMP signal inducing several proteins in a transcription-dependent manner (Dragon et al., Am J Physiol Regulatory Integrative Comp Physiol 271: R982-R989, 1996; Dragon et al., Blood 91: 3052-3058, 1998; Glombitza S, Dragon S, Berghammer M, Pannermayr M, and Baumann R, Am J Physiol Regulatory Integrative Comp Physiol 271: R973-R981, 1996). To understand how the cAMP-dependent processes are initiated, we screened an erythroid cDNA library for cAMP-regulated genes. We detected three genes that were strongly upregulated (>5-fold) by cAMP in definitive and primitive red blood cells. They are homologous to the mammalian Tob, Ifr1, and Fos proteins. In addition, the genes are induced in the intact embryo during short-term hypoxia. Because the genes are regulators of proliferation and differentiation in other cell types, we suggest that cAMP might promote general differentiating processes in erythroid cells, thereby allowing adaptive modulation of the latest steps of erythroid differentiation during developmental hypoxia.

Erythroid carbonic anhydrase and hsp70 expression in chick embryonic development: role of cAMP and hypoxia

In the second half of avian embryonic development cAMP affects major aspects of red blood cell (RBC) function. At day 13/14, progressive developmental hypoxia causes the release of norepinephrine and erythroid beta-adrenergic receptor stimulation initiates the coordinate induction of adaptive key events of erythroid differentiation like carbonic anhydrase (CAII) and 2,3-biphosphoglycerate synthesis. Although cAMP-dependent regulation of CAII protein synthesis has been described in detail, no data exist about the transcriptional regulation in embryonic RBC. Here we report that after day 12 of embryonic development, the caII mRNA is accumulating. Hypoxic incubation at day 10 as well as in vitro incubation of isolated RBC with cAMP-elevating agonists strongly induces erythroid caII expression. The induction of caII occurs fast and does not require new protein synthesis. By screening several late erythroid genes, we could identify hsp70 as another cAMP-induced gene in definitive RBC. Because caII (but not hsp70) is also induced by cAMP in primitive RBC, the signal may regulate key events of late primitive and definitive erythropoiesis.

The developmental switch in embryonic rho-globin expression is correlated with erythroid lineage-specific differences in transcription factor levels

During chicken embryogenesis, the rho-globin gene is expressed only in the early developmental stages. We have examined the mechanisms that are responsible for this behavior. The transcription of the rho-globin gene is strongly correlated with the presence during development of primitive erythroid lineage cells, consistent with the idea that the expression of the rho-globin gene is restricted to that lineage. The “switching off” of rho-globin during development thus reflects the change from primitive to definitive cell lineages which occurs during erythropoiesis in chicken. We use transient expression assays in primary erythroid and other cells to show that the information for lineage- and tissue-specific expression of the rho-globin gene is contained in a 456 bp region upstream of the gene's translational start site. DNA-binding studies, coupled with analysis of the effect on expression of deletions and binding site mutations, were used to identify important control elements within this 456 bp region. We find that binding sites for the ubiquitous transcription factor Sp1, and the specific hematopoietic factor GATA-1, are crucial for expression of the gene in primitive erythroid cells. Quantitative analysis shows that nuclei of the primitive erythroid lineage contain 10-fold more of these factors than do the nuclei of definitive cells. We show that in principle these differences in factor concentration are sufficient to explain the lineage-specific behavior that we observe in our assays. We suggest that this may be an important part of the mechanism for lineage-restricted rho-globin expression during chicken erythroid development. Similar mechanisms may be involved in regulation of other (but not all) members of the globin family.

Nucleosome spacing is compressed in active chromatin domains of chick erythroid cells

We have cleaved the chromatin of embryonic and adult chicken erythroid cells using a novel nuclease that is capable of resolving clearly the nucleosomes of active chromatin. We found that in active chromatin, nucleosomes are spaced up to 40 base pairs closer together than in inactive chromatin. This was true for both "housekeeping" and "luxury" genes and was observed whether the digestion was carried out on isolated nuclei in vitro or by activating the endogenous nuclease in vivo. The close spacing extended several kilobases into flanking chromatin, indicating that this is a domain property of active chromatin, not just a characteristic of regions disrupted by transcription. A simple interpretation of our results is that the nucleosomes of active chromatin are mobile in vivo and, not being constrained by linker histones, freely move closer together.

Decline in histone H5 phosphorylation during erythroid senescence in chick embryos

Previous studies have implicated histone H5 dephosphorylation as a causal factor in genetic inactivation and chromatin condensation during erythroid senescence in adult chickens. We show that histone H5 phosphorylation declines in two stages as various cohorts of erythroid cells senesce in chick embryos. The first decline occurs between 5 and 6 days and coincides with the senescence of primitive erythrocytes. The second decline in H5 phosphorylation occurs between 17 and 19 days of chicken development, when the definitive erythrocytes undergo senescence and chromatin condensation. These results point to a role for histone dephosphorylation during the programmed senescence of erythroid cells.

Enhancer dependent expression of the chicken beta-hatching globin gene during erythroid differentiation

The activity of the chicken beta H globin gene promoter has been analysed in functional assays in both chicken and murine erythroleukaemia cells. Sequences between -251 and -146 bp, in the presence or absence of the chicken beta globin locus enhancer, strongly repress transcription in erythroid cells before and after the induction of terminal differentiation. A 50 bp sequence (-98 to -146 bp), which contains adjacent cGATA-1 and NF1 protein binding sites in vitro, and which is bound by non-histone protein in vivo, is essential for full promoter activity. Mutagenesis studies indicate that both protein binding sites are required. During terminal differentiation, both the absence of repressor and the presence of the erythroid enhancer are required for maximal promoter activity, suggesting that the beta A, beta epsilon and beta H globin gene promoters compete for the enhancer during development.

TGGCA protein is present in erythroid nuclei and binds within the nuclease-hypersensitive sites 5' of the chicken beta H- and beta A-globin genes

The developmentally regulated 5'-flanking DNase-I-hypersensitive site of the chicken beta H-globin gene in nuclei contains a subregion which is resistant to DNase I and which disappears when nuclei are extracted with 0.3 M NaCl, suggesting that there are salt-extractable proteins bound to sequences within this region. The 0.3 M NaCl extract contains two proteins which bind in vitro to these sequences. One of the binding sequences has an inverted repeat very similar to that bound by TGGCA protein. Partially purified TGGCA protein from chicken liver binds to this sequence in vitro giving exactly the same footprint as that obtained with erythroid nuclear proteins. Similarly TGGCA protein binds to an inverted repeat with the beta A-globin 5'-hypersensitive site giving a footprint identical to that obtained with erythroid nuclear protein extracts. From competition footprinting experiments and the electrophoretic mobility of the protein-DNA complex, it is concluded that the erythroid proteins previously described as binding to the beta H- and beta A-globin inverted repeats within the 5'-flanking hypersensitive sites both belong to the TGGCA protein family.

An erythrocyte-specific protein that binds to the poly(dG) region of the chicken beta-globin gene promoter

The promoter region of the chicken adult beta-globin gene contains a sequence of 16 deoxyguanosine residues located at a nucleosome boundary in tissues where the gene is inactive. In definitive erythrocytes that express the beta-globin gene, the nucleosome is displaced, the G-string and adjacent sequences are occupied by sequence-specific DNA-binding proteins, and a nuclease hypersensitive domain is generated in this region. To gain insight into the role of the G-string in this series of events, we have examined the proteins that bind to it. Using the gel mobility shift assay and a monoclonal antibody that blocks specific binding to the G-string, we have identified a specific protein, BGP1, that is found only in chicken erythroid cells and appears at the same time, or shortly before, the changes in chromatin structure. The antibody interacts strongly with BGP1 and cross-reacts weakly with Sp1. Although both BGP1 and Sp1 require Zn2+ for their DNA-binding activity, these proteins differ in their binding-site specificities, chromatographic properties, and molecular weights. In contrast to Sp1, which is found in a wide variety of cell types, BGP1 is restricted to erythrocytes and is most abundant in definitive erythrocytes. Thus, its presence corresponds to the tissue- and stage-specific occupancy of the G-string in vivo.

Evidences that hemoglobin switch in the chick embryo depends on erythroid cell line substitution

Chemical identifications of various hemoglobin types were performed on unfractionated erythroid cells derived from chicken embryos at 5 and 7 days of development and on purified primitive and definitive cells. Proteins were pulse-labelled in primitive erythroid cells at various times of culture to identify those actually synthesized. The data show that primitive cells contain and synthesize only embryonic hemoglobins at all stages of maturation and definitive cells contain adult and minor embryonic hemoglobins, but no major embryonic hemoglobins, not even in trace amounts. These results support a model for hemoglobin switch in the chicken embryo based on cell line substitution.

Fructose 2,6-bisphosphate and glucose 1,6-bisphosphate in erythrocytes during chicken development

In contrast to mammalian erythrocytes, chicken erythrocytes contain fructose 2,6-bisphosphate at levels (0.5 nmol/10(9) cells) similar to those of 2,3-bisphosphoglycerate (1.2 nmol/10(9) cells) and slightly lower than those of glucose 1,6-bisphosphate (5.2 nmol/10(9) cells). In chick embryo erythrocytes the levels of both fructose 2,6-bisphosphate and glucose 1,6-bisphosphate are much lower. They begin to increase at hatching and reach the levels in chicken in a few days.

Demonstration of two erythrocyte populations in young chickens by counter-current distribution of 59Fe-labelled cells in dextran-poly(ethylene glycol) two-phase systems

The technique of fractionating cells by a counter-current distribution procedure in dextran-poly(ethylene glycol) biphasic systems was modified by increasing the settling time and decreasing the top/bottom two-phase volume ratio. Two sub-populations of 59Fe-labelled erythrocytes, namely definitive medullar cells and primitive embryonic cells, were present in the blood of young chicks until about day 8 of age. The primitive cells were progressively replaced by definitive red cells as age increased.

Torsional stress promotes the DNAase I sensitivity of active genes

Active genes are known to have an altered chromatin structure that is preferentially sensitive to digestion with DNAase I. We find that when chicken red blood cells are incubated in media containing the topoisomerase II inhibitor novobiocin, the preferential DNAase I sensitivity of the active beta-globin genes is reversed in vivo with as little as 20 min of drug treatment. Control experiments suggest that inhibition of a topoisomerase II is responsible for this alteration in active gene conformation. Reversal of DNAase I sensitivity can also be induced in vitro by partial cleavage of the nuclear DNA with staphylococcal nuclease. We propose that the altered structure around active genes is maintained by continuous DNA supercoiling and that in the absence of this superhelical tension active chromatin reverts to a less DNAase I-sensitive ground state.

Synthesis and levels of organic phosphates in erythrocytes during avian development: specific formation of BPG and IP5 in two distinct populations from young chicks

Two generations of red cells (embryonic and definitive), different types of haemoglobins, and special organic phosphates involved in the control of haemoglobin oxygenation (2:3-bisphosphoglycerate, BPG, and inositol-5-phosphate, IP5), have been found progressively during development of the chick. Levels of both organic phosphates, as well as activities of the enzymes involved in BPG synthesis (2:3-bisphosphoglycerate synthase, BPGM) and IP5 formation (phytase), were studied in the erythrocyte populations from embryo, young and adult chickens. Measurement of specific activities of BPGM and phytase in the two subpopulations present in young chickens showed that these phosphates could be specifically and predominantly formed in these two red cell populations.

Developmental regulation of globin and nonglobin messenger RNAs in avian erythroid cells

During embryonic development in the chicken two morphologically distinct erythroid cell populations sequentially appear. Coincidentally with the change in cell populations that begins on the sixth day of embryonic life, the hemoglobins of the early embryo are gradually replaced by a new set of hemoglobins, which are almost identical to those of the adult chicken. We have used recombinant DNAs to investigate the molecular mechanisms underlying these developmental changes. With respect to the eight nonglobin species of messenger RNA that we have studied, seven are present at approximately equal concentrations in erythroid cells from 5-day embryos and from anemic adults. This suggests that the replacement of erythroid cell populations is not accompanied by a general reorganization of gene expression. With respect to globin gene expression, however, we find that all but one of the globin genes studied (alpha D-globin) undergo dramatic developmental regulation. We have also shown that the expression of the gene for the embryo specific alpha-like globin, pi'-globin, is principally regulated at the level of transcription.

Changes in hepatic levels of tyrosine aminotransferase messenger RNA during chemical hepatocarcinogenesis

The activity of tyrosine aminotransferase (TAT) decreased biphasically in livers of rats fed 3'-methyl-4-dimethylaminoazobenzene (3'-MDAB). TAT activity decreased to an extremely low level at later stages of hepatocarcinogenesis. The activity of TAT is negatively correlated with alpha-fetoprotein (AFP) levels. The level of TAT enzyme activity in precancerous liver and hepatoma is a reflection of the amount of TAT mRNA. Dexamethasone increased the TAT enzyme activity and TAT mRNA concentration in rat livers during chemical carcinogenesis.

The enhancement of specific gene transcription in isolated nuclei by added HeLa whole cell extract

Transcription was carried out in isolated rat liver nuclei by endogenous RNA polymerase with the addition of HeLa whole cell extract (HWCE) in the presence of nucleoside 5'-[gamma-S]triphosphate. The resulting 5'-gamma-thiophosphate on the synthesized RNA allows separation of in vitro initiated RNA from bulk RNA by mercury-agarose chromatography. HWCE not only increased initiation of new RNA chains greater than 3 times but also had no effect on the RNA synthesis by RNA polymerase II. The initiation of transcription of albumin and alpha-fetoprotein genes in an isolated nuclei system was selectively enhanced by HWCE. Using this system, we studied the effect of glucocorticoid on albumin and alpha-fetoprotein gene expression in vitro.

Induced differentiation of murine erythroleukemia cells: cellular and molecular mechanisms

Study of inducer-mediated differentiation of murine erythroleukemia cells provides insights into the cellular and molecular mechanisms implicated in cell differentiation. The loss of proliferative capacity is revealed to be a complex multistep process during which the cells progress through a series of stages, including a precommitment "initiation" stage, a stage suggestive of the accumulation of commitment-related factors, and, finally, a stage of expression of the characteristics of the differentiated state. Cell cycle arrest in G1 phase of the cell cycle may, in part at least, be related to down-regulation of protein p53 synthesis. Expression of induced differentiation is accompanied by an acceleration of transcription at the globin loci, and possibly by posttranscriptional modulation of globin mRNA accumulation, as well. Cells at the stage of erythroid cell development represented by the transformed, differentiation-arrested MELC, have acquired a unique DNA structure and chromatin configuration around the globin genes which distinguish them from other, nonerythroid cells; additional complex changes in chromatin configuration accompany, and probably precede, inducer-mediated acceleration of globin gene transcription during terminal differentiation. Passage through G1 and early S phase of the cell cycle, in the presence of inducer, is critical for subsequent globin gene expression and may be important in establishing the chromatin reconfiguration required for gene expression.