Chromatin condensation in terminally differentiating mouse erythroblasts does not involve special architectural proteins but depends on histone deacetylation

Terminal erythroid differentiation in vertebrates is characterized by progressive heterochromatin formation and chromatin condensation and, in mammals, culminates in nuclear extrusion. To date, although mechanisms regulating avian erythroid chromatin condensation have been identified, little is known regarding this process during mammalian erythropoiesis. To elucidate the molecular basis for mammalian erythroblast chromatin condensation, we used Friend virus-infected murine spleen erythroblasts that undergo terminal differentiation in vitro. Chromatin isolated from early and late-stage erythroblasts had similar levels of linker and core histones, only a slight difference in nucleosome repeats, and no significant accumulation of known developmentally regulated architectural chromatin proteins. However, histone H3(K9) dimethylation markedly increased while histone H4(K12) acetylation dramatically decreased and became segregated from the histone methylation as chromatin condensed. One histone deacetylase, HDAC5, was significantly upregulated during the terminal stages of Friend virus-infected erythroblast differentiation. Treatment with histone deacetylase inhibitor, trichostatin A, blocked both chromatin condensation and nuclear extrusion. Based on our data, we propose a model for a unique mechanism in which extensive histone deacetylation at pericentromeric heterochromatin mediates heterochromatin condensation in vertebrate erythroblasts that would otherwise be mediated by developmentally-regulated architectural proteins in nucleated blood cells.

Mitophagy in mammalian cells: the reticulocyte model

Mitochondria are the site of oxidative phosphorylation in animal cells and a primary target of reactive oxygen species-mediated damage. To prevent the accumulation of damaged mitochondria, mammalian cells have evolved strategies for their elimination. Autophagy is one means for the controlled elimination of mitochondria; however, although there has been considerable progress in defining the requirements for nonselective autophagy, relatively little is known about the genes that regulate selective autophagy of organelles. To improve our understanding of mitochondrial autophagy in mammals, we have undertaken a genetic analysis of mitochondrial clearance in murine reticulocytes. Reticulocytes provide an ideal model to study this process, because mitochondria are rapidly cleared from reticulocytes during normal development through an autophagy-related process. Here we describe several methods for monitoring mitochondrial clearance and autophagy in reticulocytes, and show that in reticulocytes these processes require genes involved in both nonselective and selective autophagy.

Short-form Ron receptor is required for normal IFN-gamma production in concanavalin A-induced acute liver injury

Abrogation of Ron receptor tyrosine kinase function results in defects in macrophage activation and dysregulated acute inflammatory responses in vivo. Several naturally occurring constitutively active alternative forms of Ron have been identified, including from primary human tumors and tumor cell lines. One of these alternative forms, short-form (SF) Ron, is generated from an alternative start site in intron 10 of the Ron gene that eliminates most of the extracellular portion of the receptor and is overexpressed in several human cancers. To test the physiological significance of SF-Ron in vivo, mice were generated that solely express the full-length form of Ron (FL-Ron). Our results show that elimination of the capacity to express SF-Ron in vivo leads to augmented production of IFN-gamma from splenocytes following stimulation ex vivo with either concanavalin A or anti-CD3/T cell receptor monoclonal antibody. Moreover, in a concanavalin A-induced murine model of acute liver injury, FL-Ron mice have increased production of serum INF-gamma and serum alanine aminotransferase levels and worsened liver histology and overall survival compared with wild-type control mice. Taken together, these results suggest for the first time that SF-Ron impacts the progression of inflammatory immune responses in vivo and further support a role for the Ron receptor and its various forms in liver pathophysiology.

Gene expression during terminal erythroid differentiation

PURPOSE OF REVIEW

Expression profiling is a powerful technique to sample cell state. This review shows how expression profiling is being applied to the study of erythroid differentiation.

RECENT FINDINGS

Expression-based studies of multipotential hematopoietic progenitor cells has shown that these cells express lineage-restricted genes from multiple lineages at low levels, and that they are in effect 'primed' to develop into all hematopoietic cell types. Expression profiling of oligopotent and committed progenitor cells has further shown that commitment to the erythroid lineage is associated with a progressive decline in the number of expressed genes. Lineage commitment is regulated by lineage-restricted transcription factors, and studies show that the erythroid transcription factor GATA1, in addition to activating a subset of genes, has global repressive effects on gene expression. Terminal erythroid differentiation is associated with further reduction in the number of expressed genes. The erythroid program is defined by those genes that are still expressed, and their high-level expression depends on specific epigenetic modifications, recruitment of transcription factors, and posttranscriptional effects.

SUMMARY

Expression profiling provides the means to identify novel targets for the therapy of erythrocytes disorders, and to obtain insights into the mechanisms of cellular differentiation.

Global transcriptional coactivators CREB-binding protein and p300 are highly essential collectively but not individually in peripheral B cells

CREB-binding protein (CBP) and its para-log p300 are transcriptional coactivators that physically or functionally interact with over 320 mammalian and viral proteins, including 36 that are essential for B cells in mice. CBP and p300 are generally considered limiting for transcription, yet their roles in adult cell lineages are largely unknown since homozygous null mutations in either gene or compound heterozygosity cause early embryonic lethality in mice. We tested the hypotheses that CBP and p300 are limiting and that each has unique properties in B cells, by using mice with Cre/LoxP conditional knockout alleles for CBP (CBP(flox)) and p300 (p300(flox)), which carry CD19(Cre) that initiates floxed gene recombination at the pro-B-cell stage. CD19(Cre)-mediated loss of CBP or p300 led to surprisingly modest deficits in B-cell numbers, whereas inactivation of both genes was not tolerated by peripheral B cells. There was a moderate decrease in B-cell receptor (BCR)-responsive gene expression in CBP or p300 homozygous null B cells, suggesting that CBP and p300 are essential for this signaling pathway that is crucial for B-cell homeostasis. These results indicate that individually CBP and p300 are partially limiting beyond the pro-B-cell stage and that other coactivators in B cells cannot replace their combined loss.

Role of erythropoietin receptor signaling in Friend virus-induced erythroblastosis and polycythemia

Friend virus is an acutely oncogenic retrovirus that causes erythroblastosis and polycythemia in mice. Previous studies suggested that the Friend virus oncoprotein, gp55, constitutively activates the erythropoietin receptor (EPOR), causing uncontrolled erythroid proliferation. Those studies showed that gp55 confers growth factor independence on an interleukin-3 (IL-3)-dependent cell line (Ba/F3) when the EPOR is coexpressed. Subsequently, we showed that a truncated form of the stem-cell kinase receptor (sf-STK) is required for susceptibility to Friend disease. Given the requirement for sf-STK, we sought to establish the in vivo significance of gp55-mediated activation of the EPOR. We found that the cytoplasmic tyrosine residues of the EPOR, and signal transducer and activator of transcription-5 (STAT5), which acts through these sites, are not required for Friend virus-induced erythroblastosis. The EPOR itself was required for the development of erythroblastosis but not for gp55-mediated erythroid proliferation. Interestingly, the murine EPOR, which is required for gp55-mediated Ba/F3-cell proliferation, was dispensable for erythroblastosis in vivo. Finally, gp55-mediated activation of the EPOR and STAT5 are required for Friend virus-induced polycythemia. These results suggest that Friend virus activates both sf-STK and the EPOR to cause deregulated erythroid proliferation and differentiation.

Transgenic mice with pancellular enhanced green fluorescent protein expression in primitive hematopoietic cells and all blood cell progeny

Transgenic mice homogeneously expressing enhanced green fluorescence protein (EGFP) in primitive hematopoietic cells and all blood cell progeny, including erythrocytes and platelets, have not been reported. Given previous data indicating H2Kb promoter activity in murine hematopoietic stem cells (HSCs), bone marrow (BM), and lymphocytes, an H2Kb enhancer/promoter EGFP construct was used to generate transgenic mice. These mice demonstrated pancellular EGFP expression in both primitive BM Sca-1+Lin-Kit+ cells and side population (SP) cells. Additionally, all peripheral blood leukocytes subsets, erythrocytes, and platelets uniformly expressed EGFP strongly. Competitive BM transplantation assays established that transgenic H2Kb-EGFP HSCs had activity equivalent to wildtype HSCs in their ability to reconstitute hematopoiesis in lethally irradiated mice. In addition, immunohistochemistry revealed EGFP transgene expression in all tissues examined. This transgenic strain should be a useful reagent for both murine hematopoiesis studies and functional studies of specific cell types from particular tissues.

Increased expression of the Abcg2 transporter during erythroid maturation plays a role in decreasing cellular protoporphyrin IX levels

ABCG2/BCRP is a member of the adenosine triphosphate-binding cassette (ABC) transporter family and is expressed in intestine, kidney, and liver, where it modulates the absorption and excretion of xenobiotic compounds. ABCG2 is also expressed in hematopoietic stem cells and erythroid cells; however, little is known regarding its role in hematopoiesis. Abcg2 null mice have increased levels of protoporphyrin IX (PPIX) in erythroid cells, yet the mechanism for this remains uncertain. We have found that Abcg2 mRNA expression was up-regulated in differentiating erythroid cells, coinciding with increased expression of other erythroid-specific genes. This expression pattern was associated with significant amounts of ABCG2 protein on the membrane of mature peripheral blood erythrocytes. Erythroid cells engineered to express ABCG2 had significantly lower intracellular levels of PPIX, suggesting the modulation of PPIX level by ABCG2. This modulating activity was abrogated by treatment with a specific ABCG2 inhibitor, Ko143, implying that PPIX may be a direct substrate for the transporter. Taken together, our results demonstrate that ABCG2 plays a role in regulating PPIX levels during erythroid differentiation and suggest a potential role for ABCG2 as a genetic determinant in erythropoietic protoporphyria.

Role of AP1/NFE2 binding sites in endogenous alpha-globin gene transcription

High-level alpha-globin expression depends on cis-acting regulatory sequences located far upstream of the alpha-globin cluster. Sequences that contain the alpha-globin positive regulatory element (PRE) activate alpha-globin expression in transgenic mice. The alpha-globin PRE contains a pair of composite binding sites for the transcription factors activating protein 1 and nuclear factor erythroid 2 (AP1/NFE2). To determine the role of these binding sites in alpha-globin gene transcription, we mutated the AP1/NFE2 sites in the alpha-globin PRE in mice. We replaced the AP1/NFE2 sites with a neomycin resistance gene (neo) that is flanked by LoxP sites (floxed). Mice with this mutation exhibited increased embryonic death and alpha-thalassemia intermedia. Next, we removed the neo gene by Cre-mediated recombination, leaving a single LoxP site in place of the AP1/NFE2 sites. These mice were phenotypically normal. However, alpha-globin expression, measured by allele-specific RNA polymerase chain reaction (PCR), was decreased 25%. We examined the role of the hematopoietic-restricted transcription factor p45Nfe2 in activating expression through these sites and found that it is not required. Thus, we have demonstrated that AP1/NFE2 binding sites in the murine alpha-globin PRE contribute to long-range alpha-globin gene activation. The proteins that mediate this effect remain to be determined.

A transcription-factor-binding surface of coactivator p300 is required for haematopoiesis

The coactivators CBP (Cre-element binding protein (CREB)-binding protein) and its paralogue p300 are thought to supply adaptor molecule and protein acetyltransferase functions to many transcription factors that regulate gene expression. Normal development requires CBP and p300, and mutations in these genes are found in haematopoietic and epithelial tumours. It is unclear, however, which functions of CBP and p300 are essential in vivo. Here we show that the protein-binding KIX domains of CBP and p300 have nonredundant functions in mice. In mice homozygous for point mutations in the KIX domain of p300 designed to disrupt the binding surface for the transcription factors c-Myb and CREB, multilineage defects occur in haematopoiesis, including anaemia, B-cell deficiency, thymic hypoplasia, megakaryocytosis and thrombocytosis. By contrast, age-matched mice homozygous for identical mutations in the KIX domain of CBP are essentially normal. There is a synergistic genetic interaction between mutations in c-Myb and mutations in the KIX domain of p300, which suggests that the binding of c-Myb to this domain of p300 is crucial for the development and function of megakaryocytes. Thus, conserved domains in two highly related coactivators have contrasting roles in haematopoiesis.

Sf-Stk kinase activity and the Grb2 binding site are required for Epo-independent growth of primary erythroblasts infected with Friend virus

During the initial stage of Friend virus-induced erythroleukemia in mice, interaction of the viral protein gp55 with the erythropoietin receptor, and other host factors, drives the expansion of erythroid precursor cells. Recently, we demonstrated that the Friend virus susceptibility locus, Fv2, which is required for the expansion of infected cells, encodes a naturally occurring, N-terminally truncated form of the Stk receptor tyrosine kinase (Sf-Stk). Here we show that in vitro expression of Sf-Stk confers Friend virus sensitivity to erythroid progenitor cells from Fv2(rr) mice. Furthermore, our data reveal that Sf-Stk kinase activity and Y436, but not Y429, are required for Epo-independent colony formation following Friend virus infection. Introduction of a mutation that results in failure to bind Grb2 abrogates the ability of Sf-Stk to induce colonies in response to Friend virus, while the Grb2 binding site from EGFR restores this response. Consistent with the ability of Grb2 to recruit SOS and Gab1, the Ras/MAPK and PI3K pathways are activated by Sf-Stk, and both of these pathways are required for gp55-mediated erythroblast proliferation. These data clearly demonstrate a requirement for signaling through Sf-Stk in the Epo-independent expansion of Friend virus-infected cells, and suggest a pivotal role for Grb2 in this response.

The distal region and receptor tyrosines of the Epo receptor are non-essential for in vivo erythropoiesis

The erythropoietin receptor (EpoR) is required for the proliferation and survival of committed erythroid lineage cells. Previous studies have utilized receptor mutations to show the requirement for the distal half of the cytoplasmic domain of the EpoR and receptor tyrosines for activation of signaling pathways potentially critical to Epo function. To extend these studies to in vivo erythropoiesis, we have created two mutant strains of mice. One strain (H) contains a truncation of the distal half of the cytoplasmic domain, while the second strain (HM) contains the same truncation as well as the mutation of the residual tyrosine (Y(343)) to a phenylalanine. Strikingly, both strains of mice are viable, with only slight alterations in constitutive erythropoiesis or in in vitro assays of red cell lineage function. Challenging H mutant mice with continuous injections of Epo results in an erythrocytosis that is not seen in HM mice. The results demonstrate that neither the distal region nor receptor tyrosines are essential for in vivo EpoR function, but contribute to receptor function in a subtle manner.

p45(NFE2) is a negative regulator of erythroid proliferation which contributes to the progression of Friend virus-induced erythroleukemias

In previous studies, we identified a common site of retroviral integration designated Fli-2 in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia cell lines. Insertion of F-MuLV at the Fli-2 locus, which was associated with the loss of the second allele, resulted in the inactivation of the erythroid cell- and megakaryocyte-specific gene p45(NFE2). Frequent disruption of p45(NFE2) due to proviral insertion suggests a role for this transcription factor in the progression of Friend virus-induced erythroleukemias. To assess this possibility, erythroleukemia was induced by F-MuLV in p45(NFE2) mutant mice. Since p45(NFE2) homozygous mice mostly die at birth, erythroleukemia was induced in +/- and +/+ mice. We demonstrate that +/- mice succumb to the disease moderately but significantly faster than +/+ mice. In addition, the spleens of +/- mice were significantly larger than those of +/+ mice. Of the 37 tumors generated from the +/- and +/+ mice, 10 gave rise to cell lines, all of which were derived from +/- mice. Establishment in culture was associated with the loss of the remaining wild-type p45(NFE2) allele in 9 of 10 of these cell lines. The loss of a functional p45(NFE2) in these cell lines was associated with a marked reduction in globin gene expression. Expression of wild-type p45(NFE2) in the nonproducer erythroleukemic cells resulted in reduced cell growth and restored the expression of globin genes. Similarly, the expression of p45(NFE2) in these cells also slows tumor growth in vivo. These results indicate that p45(NFE2) functions as an inhibitor of erythroid cell growth and that perturbation of its expression contributes to the progression of Friend erythroleukemia.

Fv2 encodes a truncated form of the Stk receptor tyrosine kinase

The Friend virus susceptibility 2 (Fv2) locus encodes a dominant host factor that confers susceptibility to Friend virus-induced erythroleukaemia in mice. We mapped Fv2 to a 1.0-Mb interval that also contained the gene (Ron) encoding the stem cell kinase receptor (Stk). A truncated form of Stk (Sf-stk), which was the most abundant form of Stk in Fv2-sensitive (Fv2ss) erythroid cells, was not expressed in Fv2 resistant (Fv2rr) cells. Enforced expression of Sf-stk conferred susceptibility to Friend disease, whereas targeted disruption of Ron caused resistance. We conclude that the Fv2 locus encodes Ron, and that a naturally expressed, truncated form of Stk confers susceptibility to Friend virus-induced erythroleukaemia.

Physical and functional interactions between the transactivation domain of the hematopoietic transcription factor NF-E2 and WW domains

Tandem binding sites for the hematopoietic transcription factor NF-E2 in the beta-globin locus control region activate high-level beta-globin gene expression in transgenic mice. NF-E2 is a heterodimer consisting of a hematopoietic subunit p45 and a ubiquitous subunit p18. Gavva et al. [Gavva, N. R., Gavva, R., Ermekova, K., Sudol, M., and Shen, J. C. (1997) J. Biol. Chem. 272, 24105-24108] reported that human p45 contains a PPXY motif that binds WW domains. We show that murine NF-E2, which contains two PPXY motifs (PPXY-1 and -2) within its transactivation domain, differentially interacted with nine GST-WW domain fusion proteins. Quantitative analysis revealed high-affinity binding (KD = 5.7 nM) of p45 to a WW domain from a novel human ubiquitin ligase homologue (WWP1) expressed in hematopoietic tissues. The amino-terminal WW domain of WWP1 formed a multimeric complex with DNA-bound NF-E2. A WWP1 ligand peptide, isolated by phage display, and a peptide spanning PPXY-1 inhibited p45 binding, whereas an SH3 domain-interacting peptide and a peptide spanning PPXY-2 did not. Mutation of PPXY-1, but not PPXY-2, inhibited the transactivation function of NF-E2, providing support for the hypothesis that WW domain interactions are important for NF-E2-mediated transactivation.

Erythroid maturation and globin gene expression in mice with combined deficiency of NF-E2 and nrf-2

NF-E2 binding sites, located in distant regulatory sequences, may be important for high level alpha- and beta-globin gene expression. Surprisingly, targeted disruption of each subunit of NF-E2 has either little or no effect on erythroid maturation in mice. For p18 NF-E2, this lack of effect is due, at least in part, to the presence of redundant proteins. For p45 NF-E2, one possibility is that NF-E2-related factors, Nrf-1 or Nrf-2, activate globin gene expression in the absence of NF-E2. To test this hypothesis for Nrf-2, we disrupted the Nrf-2 gene by homologous recombination. Nrf-2-deficient mice had no detectable hematopoietic defect. In addition, no evidence was found for reciprocal upregulation of NF-E2 or Nrf-2 protein in fetal liver cells deficient for either factor. Fetal liver cells deficient for both NF-E2 and Nrf-2 expressed normal levels of alpha- and beta-globin. Mature mice with combined deficiency of NF-E2 and Nrf-2 did not exhibit a defect in erythroid maturation beyond that seen with loss of NF-E2 alone. Thus, the presence of a mild erythroid defect in NF-E2-deficient mice is not the result of compensation by Nrf-2.

Multiple regions of p45 NF-E2 are required for beta-globin gene expression in erythroid cells

Regulated expression of genes in the beta-globin cluster depends upon sequences located between 5 and 20 kb upstream of the epsilon gene, known as the locus control region (LCR). beta-Globin expression in murine erythroleukemia (MEL) cells depends on NF-E2, a transcription factor which binds to enhancer sequences in the LCR. To gain insight into the mechanism of globin gene activation by NF-E2, an NF-E2 null MEL cell line was used to map regions of NF-E2 required for beta-globin expression. Within the transactivation domain, two discrete proline-rich regions were required for rescue of beta-globin expression. The first was located at the N-terminus of NF-E2, while the second was located N-terminal of the cap 'n collar (CNC) domain. Other proline-rich sequences were dispensable, indicating that proline content per se does not determine NF-E2 activity. Mutations within the conserved CNC domain markedly diminished rescue of beta-globin expression. This domain was required, in addition to the basic leucine zipper domain, for DNA binding activity. The requirement for discrete proline-rich sequences within the transactivation domain suggests that globin gene expression in MEL cells depends on specific interactions between NF-E2 and downstream effector molecules.

Microcytic anemia in mk/mk mice is not corrected by retroviral-mediated gene transfer of wild-type p45 NF-E2

Mice homozygous for the mk mutation have a severe hypochromic, microcytic anemia that is characterized by a decreased mean-corpuscular hemoglobin concentration and balanced alpha- and beta-globin-chain synthesis. Transplantation studies have shown that the defect in homozygous mk/mk mice is intrinsic to both the hematopoietic system and the gut. The gene for the hematopoietic-specific transcription factor, p45 NF-E2, has been found to cosegregate with the mk phenotype and contain a point mutation in mk/mk mice that results in an amino acid substitution (173V-->A). In order to test the hypothesis that this amino acid substitution is responsible for the mk phenotype, we have used recombinant retroviruses to introduce wild-type p45 NF-E2 into the bone marrow of mk/mk mice. Despite gene transfer and expression of p45 NF-E2 in erythroid cells, we found no evidence for correction of the phenotype in mk/mk mice. These results indicate that the mk mutation cannot be corrected by enforced expression of wild-type p45 NF-E2 and suggest that the 173V-->A mutation of the p45 NF-E2 gene is not the cause of anemia in mk/mk mice.

The regulatory element 3' to the A gamma-globin gene binds to the nuclear matrix and interacts with special A-T-rich binding protein 1 (SATB1), an SAR/MAR-associating region DNA binding protein

A cis-acting DNA regulatory element 3' to the A gamma-globin gene contains eight distinct regions of DNA-protein interaction distributed over 750 bp of DNA. The sequences of two foot-printed regions (sites I and IV) are A-T rich and generate a highly retarded complex on gel shift analysis with nuclear extract from human erythroleukemia (K562) cells. We have purified a 98-kD protein that reproduces this gel shift. Tryptic cleavage and peptide sequence analysis demonstrated that the 98-kD protein is identical to a recently cloned protein, special A-T-rich binding protein 1 (SATB1), that binds selectively to nuclear matrix/scaffold-associated regions of DNA (MARs/SARs). We have shown by functional analysis that the 3' A gamma regulatory element associates with the nuclear matrix. SATB1 mRNA was identified in K562 cells, and reverse transcriptase-polymerase chain reaction (RT-PCR) demonstrated its transcript in several other hematopoietic lines. Antisera to SATB1 caused ablation of the gel shift complex generated by both the crude nuclear extract and the purified 98-kD protein with the site I oligonucleotide. Furthermore, oligonucleotides that bind SATB1 inhibited formation of the site I gel shift complex when added as excess unlabeled competitor. An immunoblot analysis of the site I gel shift complex documented the presence of SATB1. Binding of SATB1 to two sites within the 3' A gamma regulatory element and its MAR/SAR activity suggests that this element may influence gene expression through interaction with the nuclear matrix.

The ubiquitous subunit of erythroid transcription factor NF-E2 is a small basic-leucine zipper protein related to the v-maf oncogene

Erythroid transcription factor NF-E2 is a tissue-restricted heterodimeric protein which recognizes an extended AP-1 motif [(T/C)TGCTGA(C/G)TCA(T/C)] found in the upstream locus control regions of the alpha- and beta-globin gene clusters. A cDNA clone encoding a cell-type-specific subunit of NF-E2, designated p45 NF-E2, has previously been characterized and shown to encode a basic-leucine zipper DNA-binding protein. Here we describe protein purification and cloning of cDNA that encodes the second basic-leucine zipper subunit of the native NF-E2 heterodimer. This polypeptide, designated p18, is widely expressed. It displays extensive homology to the v-maf oncogene product and a human retinal-specific protein, NRL. Unusual features in the basic region shared by v-Maf, NRL, and p18 place them in a distinct subfamily of AP-1-like proteins.

Single-copy transduction and expression of human gamma-globin in K562 erythroleukemia cells using recombinant adeno-associated virus vectors: the effect of mutations in NF-E2 and GATA-1 binding motifs within the hypersensitivity site 2 enhancer

The use of recombinant adeno-associated virus (rAAV) vectors provides a new strategy to investigate the role of specific regulatory elements and trans-acting factors in globin gene expression. We linked hypersensitivity site 2 (HS2) from the locus control region (LCR) to a A gamma-globin gene (A gamma*) mutationally marked to allow its transcript to be distinguished from endogenous gamma-globin mRNA. The vector also contains the phosphotransferase gene that confers resistance to neomycin (NeoR). HS2 region mutations within the NF-E2 motifs prevented NF-E2 binding while preserving AP-1 binding. Another set in the GATA-1 motif prevented binding of the factor. Several NeoR K562 clones containing a single unrearranged RAAV genome with the A gamma* gene linked to the native HS2 core fragment (WT), mutant NF-E2 HS2 (mut-NFE2), mutant GATA-1 HS2 (mut-GATA1), or no HS [(-)HS] were identified. In uninduced K562 cells, mut-NFE2 clones expressed A gamma* mRNA at the same level as the WT clones, compared with a lack of A gamma* signal in the (-)HS2 clones. However, hemin induction of mut-NFE2 clones did not result in an increase in the A gamma* signal above the level seen in uninduced cells. Mut-GATA1 clones expressed the A gamma* mRNA at the same level as WT clones in both uninduced and induced cells. Thus, GATA-1 binding to this site does not appear to be required for the enhancing function of HS2 in this context. This single-copy rAAV transduction model is useful for evaluating the effects of specific mutations in regulatory elements on the transcription of linked genes.

Purification of the human NF-E2 complex: cDNA cloning of the hematopoietic cell-specific subunit and evidence for an associated partner

The human globin locus control region-binding protein, NF-E2, was purified by DNA affinity chromatography. Its tissue-specific component, p45 NF-E2, was cloned by use of a low-stringency library screen with murine p45 NF-E2 cDNA (N. C. Andrews, H. Erdjument-Bromage, M. B. Davidson, P. Tempst, and S. H. Orkin, Nature [London] 362:722-728, 1993). The human p45 NF-E2 gene was localized to chromosome 12q13 by fluorescent in situ hybridization. Human p45 NF-E2 and murine p45 NF-E2 are highly homologous basic region-leucine zipper (bZIP) proteins with identical DNA-binding domains. Immunoprecipitation experiments demonstrated that p45 NF-E2 is associated in vivo with an 18-kDa protein (p18). Because bZIP proteins bind DNA as dimers, we infer that native NF-E2 must be a heterodimer of 45- and 18-kDa subunits. Although AP-1 and CREB copurified with NF-E2, no evidence was found for heterodimer formation between p45 NF-E2 and proteins other than p18. Thus, p18 appears to be the sole specific partner of p45 NF-E2 in erythroid cells. Cloning of human p45 NF-E2 should permit studies of the role of NF-E2 in globin gene regulation and erythroid differentiation.

Methylation-enhanced binding of Sp1 to the stage selector element of the human gamma-globin gene promoter may regulate development specificity of expression

The human gamma-globin gene promoter contains a stage selector element (SSE) responsible for preferential interaction of the promoter with a powerful erythroid-specific enhancer in the fetal developmental stage (S.M. Jane, P.A. Ney, E.F. Vanin, D.L. Gumucio, and A.W. Nienhuis. EMBO J. 11:2691-2699, 1992). The element binds two proteins, the ubiquitous activator Sp1 and a protein previously known as -50 gamma and now named the stage selector protein (SSP). Binding of the second protein correlates with SSE activity in transient-transfection assays. We now report that a de novo binding site for the SSP is created by the -202(C-->G) mutation that causes hereditary persistence of fetal hemoglobin (HPFH). This site functions in an analogous manner to the SSE in hybrid beta-promoter/reporter gene constructs transfected into K562 cells. In contrast, the wild-type -202 sequence, which fails to bind the SSP, is incapable of activating the beta-gene promoter. Both the -50 and -202 HPFH sites for SSP binding overlap a consensus sequence for the transcriptional regulator Sp1. In addition, both sites contain CpG dinucleotides that are contact bases for SSP. Since the gamma promoter is known to be hypomethylated in fetal cells but fully methylated at CpG residues in adult erythroid cells, we examined the effects of this DNA modification on protein binding to the two regions. Gel mobility shift assays with nuclear extract from K562 cells (which contain both Sp1 and SSP) demonstrate preferential binding of SSP to the SSE and HPFH sites under conditions in which probe was limiting. Methylation of the CpG residues reverses this preference only in the SSE site, with a marked increase in the binding of Sp1 at the expense of the SSP. Purified Sp1 binds with 10-fold higher affinity to the methylated than to the nonmethylated -50 probe but with the same affinity to the -202 HPFH probe. The methylation-induced preferential binding of Sp1 to the SSE at the expense of SSP may be part of the mechanism by which the gamma genes are repressed in normal adult erythroid cells. In cells containing the -202 HPFH mutation, the inability of Sp1 to displace SSP in the methylated state may explain the persistence of gamma-promoter activity and gamma-gene expression observed in adults with this mutation.