LCR/MEL: a versatile system for high-level expression of heterologous proteins in erythroid cells

Distinct and overlapping DNMT1 interactions with multiple transcription factors in erythroid cells: Evidence for co-repressor functions

DNMT1 is the maintenance DNA methyltransferase shown to be essential for embryonic development and cellular growth and differentiation in many somatic tissues in mammals. Increasing evidence has also suggested a role for DNMT1 in repressing gene expression through interactions with specific transcription factors. Previously, we identified DNMT1 as an interacting partner of the TR2/TR4 nuclear receptor heterodimer in erythroid cells, implicated in the developmental silencing of fetal β-type globin genes in the adult stage of human erythropoiesis. Here, we extended this work by using a biotinylation tagging approach to characterize DNMT1 protein complexes in mouse erythroleukemic cells. We identified novel DNMT1 interactions with several hematopoietic transcription factors with essential roles in erythroid differentiation, including GATA1, GFI-1b and FOG-1. We provide evidence for DNMT1 forming distinct protein subcomplexes with specific transcription factors and propose the existence of a "core" DNMT1 complex with the transcription factors ZBP-89 and ZNF143, which is also present in non-hematopoietic cells. Furthermore, we identified the short (17a.a.) PCNA Binding Domain (PBD) located near the N-terminus of DNMT1 as being necessary for mediating interactions with the transcription factors described herein. Lastly, we provide evidence for DNMT1 serving as a co-repressor of ZBP-89 and GATA1 acting through upstream regulatory elements of the PU.1 and GATA1 gene loci.

Mice expressing RHAG and RHD human blood group genes

Anti-RhD prophylaxis of haemolytic disease of the fetus and newborn (HDFN) is highly effective, but as the suppressive mechanism remains uncertain, a mouse model would be of interest. Here we have generated transgenic mice expressing human RhAG and RhD erythrocyte membrane proteins in the presence and, for human RhAG, in the absence, of mouse Rhag. Human RhAG associates with mouse Rh but not mouse Rhag on red blood cells. In Rhag knockout mice transgenic for human RHAG, the mouse Rh protein is “rescued” (re-expressed), and co-immunoprecipitates with human RhAG, indicating the presence of hetero-complexes which associate mouse and human proteins. RhD antigen was expressed from a human RHD gene on a BAC or from RHD cDNA under control of β-globin regulatory elements. RhD was never observed alone, strongly indicative that its expression absolutely depends on the presence of transgenic human RhAG. This first expression of RhD in mice is an important step in the creation of a mouse model of RhD allo-immunisation and HDFN, in conjunction with the Rh-Rhag knockout mice we have developed previously.

Five friends of methylated chromatin target of protein-arginine-methyltransferase[prmt]-1 (chtop), a complex linking arginine methylation to desumoylation

Chromatin target of Prmt1 (Chtop) is a vertebrate-specific chromatin-bound protein that plays an important role in transcriptional regulation. As its mechanism of action remains unclear, we identified Chtop-interacting proteins using a biotinylation-proteomics approach. Here we describe the identification and initial characterization of Five Friends of Methylated Chtop (5FMC). 5FMC is a nuclear complex that can only be recruited by Chtop when the latter is arginine-methylated by Prmt1. It consists of the co-activator Pelp1, the Sumo-specific protease Senp3, Wdr18, Tex10, and Las1L. Pelp1 functions as the core of 5FMC, as the other components become unstable in the absence of Pelp1. We show that recruitment of 5FMC to Zbp-89, a zinc-finger transcription factor, affects its sumoylation status and transactivation potential. Collectively, our data provide a mechanistic link between arginine methylation and (de)sumoylation in the control of transcriptional activity.

Nuclear receptors TR2 and TR4 recruit multiple epigenetic transcriptional corepressors that associate specifically with the embryonic β-type globin promoters in differentiated adult erythroid cells

Nuclear receptors TR2 and TR4 (TR2/TR4) were previously shown to bind in vitro to direct repeat elements in the mouse and human embryonic and fetal β-type globin gene promoters and to play critical roles in the silencing of these genes. By chromatin immunoprecipitation (ChIP) we show that, in adult erythroid cells, TR2/TR4 bind to the embryonic β-type globin promoters but not to the adult β-globin promoter. We purified protein complexes containing biotin-tagged TR2/TR4 from adult erythroid cells and identified DNMT1, NuRD, and LSD1/CoREST repressor complexes, as well as HDAC3 and TIF1β, all known to confer epigenetic gene silencing, as potential corepressors of TR2/TR4. Coimmunoprecipitation assays of endogenous abundance proteins indicated that TR2/TR4 complexes consist of at least four distinct molecular species. In ChIP assays we found that, in undifferentiated murine adult erythroid cells, many of these corepressors associate with both the embryonic and the adult β-type globin promoters but, upon terminal differentiation, they specifically dissociate only from the adult β-globin promoter concomitant with its activation but remain bound to the silenced embryonic globin gene promoters. These data suggest that TR2/TR4 recruit an array of transcriptional corepressors to elicit adult stage-specific silencing of the embryonic β-type globin genes through coordinated epigenetic chromatin modifications.

Friend of Prmt1, a novel chromatin target of protein arginine methyltransferases

We describe the isolation and characterization of Friend of Prmt1 (Fop), a novel chromatin target of protein arginine methyltransferases. Human Fop is encoded by C1orf77, a gene of previously unknown function. We show that Fop is tightly associated with chromatin, and that it is modified by both asymmetric and symmetric arginine methylation in vivo. Furthermore, Fop plays an important role in the ligand-dependent activation of estrogen receptor target genes, including TFF1 (pS2). Fop depletion results in an almost complete block of estradiol-induced promoter occupancy by the estrogen receptor. Our data indicate that Fop recruitment to the promoter is an early critical event in the activation of estradiol-dependent transcription.

Cryptic splicing sites are differentially utilized in vivo

It has long been considered that cryptic splice sites are ignored by the splicing machinery in the context of intact genuine splice sites. In the present study, it is shown that cryptic splice sites are utilized in all circumstances, when the authentic site is intact, partially functional or completely abolished. Their use would therefore contribute to a background lack of fidelity in the context of the wild-type sequence. We also found that a mutation at the 5' splice site of beta-globin intron 1 accommodates multiple cryptic splicing pathways, including three previously reported pathways. Focusing on the two major cryptic 5' splice sites within beta-globin exon 1, we show that cryptic splice site selection ex vivo varies depending upon: (a) the cell stage of development during terminal erythroid differentiation; (b) the nature of the mutation at the authentic 5' splice site; and (c) the nature of the promoter. Finally, we found that the two major cryptic 5' splice sites are utilized with differential efficiencies in two siblings sharing the same beta-globin chromosome haplotype in the homozygous state. Collectively, these data suggest that intrinsic, sequence specific factors and cell genetic background factors both contribute to promote a subtle differential use of cryptic splice sites in vivo.

Gata1 regulates dendritic-cell development and survival

Dendritic cells are key initiators and regulators of the immune response. Dendritic cell commitment and function require orchestrated regulation of transcription. Gata1 is a transcription factor expressed in several hematopoietic lineages. However, Gata1 function has not been explored in the monocytic or dendritic cell compartment. Here, we show that Gata1 is expressed in myeloid and plasmacytoid dendritic cells and that Gata1 ablation affects the survival of dendritic cells. Furthermore, lipopolysaccharide (LPS) stimulation of dendritic cells prompts Gata1 up-regulation, which is accompanied by increased levels of BclX and Ifng. Our findings show that Gata1 is a transcriptional regulator of dendritic cell differentiation and suggest that Gata1 is involved in the dendritic cell and macrophage lineage separation.

Dynamic regulation of Gata factor levels is more important than their identity

Three Gata transcription factors (Gata1, -2, and -3) are essential for hematopoiesis. These factors are thought to play distinct roles because they do not functionally replace each other. For instance, Gata2 messenger RNA (mRNA) expression is highly elevated in Gata1-null erythroid cells, yet this does not rescue the defect. Here, we test whether Gata2 and -3 transgenes rescue the erythroid defect of Gata1-null mice, if expressed in the appropriate spatiotemporal pattern. Gata1, -2, and -3 transgenes driven by beta-globin regulatory elements, directing expression to late stages of differentiation, fail to rescue erythropoiesis in Gata1-null mutants. In contrast, when controlled by Gata1 regulatory elements, directing expression to the early stages of differentiation, Gata1, -2, and -3 do rescue the Gata1-null phenotype. The dramatic increase of endogenous Gata2 mRNA in Gata1-null progenitors is not reflected in Gata2 protein levels, invoking translational regulation. Our data show that the dynamic spatiotemporal regulation of Gata factor levels is more important than their identity and provide a paradigm for developmental control mechanisms that are hard-wired in cis-regulatory elements.

Real-time monitoring of stress erythropoiesis in vivo using Gata1 and beta-globin LCR luciferase transgenic mice

Erythroid progenitors have the potential to proliferate rapidly in response to environmental stimuli. This process is referred to as stress erythropoiesis, with erythropoietin (EPO) playing central roles in its promotion. In this study, we wanted to elucidate the molecular mechanisms governing the regulation of stress erythropoiesis and the maintenance of red-cell homeostasis. This was achieved by our development of a noninvasive real-time monitoring system for erythropoiesis using transgenic mouse lines expressing luciferase under the control of the mouse Gata1 hematopoietic regulatory domain (G1-HRD-luc) or human beta-globin locus control region (Hbb-LCR-luc). Optical bioluminescence images revealed that the luciferase was specifically expressed in spleen and bone marrow and was induced rapidly in response to anemia and hypoxia stimuli. The G1-HRD-luc activity tracked the emergence and disappearance of proerythroblast-stage progenitors, whereas the Hbb-LCR-luc activity tracked erythroblasts and later stage erythroid cells. Increased plasma EPO concentration preceded an increase in G1-HRD-luc, supporting our contention that EPO acts as the key upstream signal in stress erythropoiesis. Hence, we conclude that G1-HRD-luc and Hbb-LCR-luc reporters are differentially activated during stress erythropoiesis and that the transgenic mouse lines used serve as an important means for understanding the homeostatic regulation of erythropoiesis.

The active spatial organization of the beta-globin locus requires the transcription factor EKLF

Three-dimensional organization of a gene locus is important for its regulation, as recently demonstrated for the beta-globin locus. When actively expressed, the cis-regulatory elements of the beta-globin locus are in proximity in the nuclear space, forming a compartment termed the Active Chromatin Hub (ACH). However, it is unknown which proteins are involved in ACH formation. Here, we show that EKLF, an erythroid transcription factor required for adult beta-globin gene transcription, is also required for ACH formation. We conclude that transcription factors can play an essential role in the three-dimensional organization of gene loci.

HS5 of the human beta-globin locus control region: a developmental stage-specific border in erythroid cells

Elements with insulator/border activity have been characterized most extensively in Drosophila melanogaster. In vertebrates, the first example of such an element was provided by a hypersensitive site of the chicken beta-globin locus, cHS4. It has been proposed that the homologous site in humans, HS5, functions as a border of the human beta-globin locus. Here, we have characterized HS5 of the human beta-globin locus control region. We have examined its tissue-specificity and assessed its insulating properties in transgenic mice using a lacZ reporter assay. Most importantly, we have tested its enhancer blocking activity in the context of the full beta-globin locus. Our results show that HS5 is erythroid-specific rather than ubiquitous in human tissues. Furthermore, HS5 does not fulfil the criteria of a general in vivo insulator in the transgene protection assay. Finally, a HS5 conditional deletion from the complete locus demonstrates that HS5 has no discernable activity in adult erythroid cells. Surprisingly, HS5 functions as an enhancer blocker in embryonic erythroid cells. We conclude that HS5 is a developmental stage-specific border in erythroid cells.

Efficient biotinylation and single-step purification of tagged transcription factors in mammalian cells and transgenic mice

Proteomic approaches require simple and efficient protein purification methodologies that are amenable to high throughput. Biotinylation is an attractive approach for protein complex purification due to the very high affinity of avidin/streptavidin for biotinylated templates. Here, we describe an approach for the single-step purification of transcription factor complex(es) based on specific in vivo biotinylation. We expressed the bacterial BirA biotin ligase in mammalian cells and demonstrated very efficient biotinylation of a hematopoietic transcription factor bearing a small (23-aa) artificial peptide tag. Biotinylation of the tagged transcription factor altered neither the factor's protein interactions or DNA binding properties in vivo nor its subnuclear distribution. Using this approach, we isolated the biotin-tagged transcription factor and at least one other known interacting protein from crude nuclear extracts by direct binding to streptavidin beads. Finally, this method works efficiently in transgenic mice, thus raising the prospect of using biotinylation tagging in protein complex purification directly from animal tissues. Therefore, BirA-mediated biotinylation of tagged proteins provides the basis for the single-step purification of proteins from mammalian cells.

Stability of protein production from recombinant mammalian cells

One of the most important criteria for successful generation of a therapeutic protein from a recombinant cell is to obtain a cell line that maintains stability of production. If this is not achieved it can generate problems for process yields, effective use of time and money, and for regulatory approval of products. However, selection of a cell line that sustains stability of production over the required time period may be difficult to achieve during development of a therapeutic protein. There are several studies in the literature that have reported on the instability of protein production from recombinant cell lines. The causes of instability of production are varied and, in many cases, the exact molecular mechanisms are unknown. The production of proteins by cells is modulated by molecular events at levels ranging from transcription, posttranscriptional processing, translation, posttranslational processing, to secretion. There is potential for regulation of stability of protein production at many or all of these stages. In this study we review published information on stability of protein production for three industrially important cell lines: hybridoma, Chinese hamster ovary (CHO), and nonsecreting (NS0) myeloma cell lines. We highlight the most likely molecular loci at which instability may be engendered and indicate other areas of protein production that may affect stability from mammalian cells. We also outline approaches that could help to overcome the problems associated with unpredictable expression levels and maximized production, and indicate the consequences these might have for stability of production.

Differences in conductance of M2 proton channels of two influenza viruses at low and high pH

The M2 protein of influenza A viruses forms a proton channel involved in modifying virion and trans Golgi pH during infection. Previous studies of the proton current using whole-cell patch clamp of mouse erythroleukaemia (MEL) cells expressing the M2 protein of the "Weybridge" strain provided evidence for two protonation sites, one involved in permeation, the other in activation by acid pH. The present report compares the M2 channels of two different strains of influenza virus, "Weybridge" (WM2) and "Rostock" (RM2). Whereas with external acid pH the current-voltage relations showed similar small degrees of inward rectification, a similar apparent K(d) of approximately 10 microM for proton permeation and a high selectivity for protons over Na(+), the two M2 proteins differed in whole-cell conductance at low and high pH. The proton conductance of unit membrane area was on average 7-fold greater in RM2- than WM2-expressing MEL cells. At high external pH WM2 was shown previously to have small conductance for outward current at positive driving potential. In contrast, RM2 shows high conductance for outward current with high external pH, but shows small conductance for inward current with high internal pH, conditions in which WM2 shows high conductance for inward current. The different properties of the conductances due to the two channels at high pH were determined by three amino acids in their transmembrane domains. All intermediate mutants possessed one or other property and transformation of the WM2 phenotype into that of RM2 required substitution in all three residues V27I, F38L and D44N; single substitutions in RM2 effected the opposite phenotypic change. The significance of this difference for virus replication is not clear and it may be that the higher proton flux associated with RM2 is the main factor determining its increased ability to dissipate pH gradients. It is apparent, however, from the specific differences in the sidedness of the pH-induced changes in voltage dependence of the whole-cell current that this is an intrinsic property of the virus proton channel which may have parallels with regulation of other proton channels.

Expression and purification of biologically active IGF-binding proteins using the LCR/Mel expression system

The anabolic effects and bioavailability of insulin-like growth factors I and II (IGF-I, IGF-II) are regulated in part by a family of IGF-binding proteins (IGFBPs). There are six known members of the IGFBP family, which share distinct structural characteristics and functional activities. To study the binding properties of these proteins, we have expressed recombinant IGFBP-3 and IGFBP-4 using the LCR/Mel expression system. Using this system, we found that recombinant IGFBP-3 was secreted by Mel cells and had a glycosylation pattern similar to that of native IGFBP-3. Recombinant IGFBP-4 secreted from Mel cells had a molecular size identical to that of non-glycosylated native IGFBP-4. The binding kinetics of recombinant IGFBPs was measured using a solid-phase ligand-binding assay, an in vitro solution-binding assay, and a cellular proliferation assay. IGF-I bound with high affinity to recombinant IGFBP-3 and IGFBP-4 with K(D)s of <0.25 nmol. As reported for native IGFBPs, IGF-II bound with affinity higher than IGF-I to recombinant IGFBP-3 and IGFBP-4 (K(D) of <0.05 nmol). Recombinant IGFBP-3 and IGFBP-4 were found to inhibit the IGF-induced proliferation of an NIH3T3 cell line engineered to overexpress the IGF-I receptor. We have compared the binding kinetics of Mel cell-expressed IGFBPs with that of recombinant protein expressed in Escherichia coli and found them to be equivalent. Here, we show that the LCR/Mel expression system represents an effective route for expression of biologically active IGFBPs.

The tomato RNA-directed RNA polymerase has no effect on gene silencing by RNA interference in transgenic mice

Double-stranded RNA (dsRNA) has been shown to interfere with the function of specific genes in various invertebrate species. The application of dsRNA interference (RNAi) in vertebrates (zebrafish and mouse) is still limited to embryos and it is not clear whether the method is generally applicable. Using a transgenic mouse model we investigated whether a stably inherited dsRNA introduced as a transgene can interfere with the expression of a specific target gene in erythroid tissue during development. In our globin gene system we do not observe any specific RNA interference. We, therefore, also introduced another gene that may be involved in a mechanism of post transcriptional gene silencing (PTGS), namely RNA-dependent RNA polymerase (RdRP) that was proposed to be involved in producing RNAs that trigger PTGS in plants. However, even though the tomato RdRP is catalytically active in erythroid tissue, no RNAi was observed.

pH-Dependent channel activity of heterologously-expressed main intrinsic protein (MIP) from rat lens

Wild-type rat lens main intrinsic protein (MIP) was heterologously expressed in the membrane of Spodoptera frugiperda (Sf21) cells using the baculovirus expression system and in mouse erythroid leukaemia cells (MEL C88). Both MEL and Sf21 cell lines expressing wild-type MIP were investigated for the conductance of ions using a whole cell patch clamp technique. An increase in conductance was seen in both expression systems, particularly on lowering the pH to 6.3. In Sf21 cells, addition of antibodies to the NPA1 box resulted in a reduction of current flow. These results suggest that MIP has pH-dependent ion channel activity, which involves the NPA1 box domain.

Heterologous expression and topography of the main intrinsic protein (MIP) from rat lens

Wild type rat lens main intrinsic protein (MIP) and MIP mutated (F73I, F75L) to resemble the glycerol facilitator of Escherichia coli in the region of the NPA1 box were used to investigate the topology of MIP in the membrane of Spodoptera frugiperda (Sf21) cells using the baculovirus expression system and expression in mouse erythroid leukaemia cells (MEL C88). Differential fixation for staining was used, with paraformaldehyde for externally exposed antigenic sites, and acetone for both externally and internally exposed protein antigenic sites. Immunofluorescence using antibodies to synthetic MIP peptides showed that wild type MIP had a six transmembrane topography. The N- and C-termini were intracellular in both expression systems, and both NPA boxes were found to be extracellular. These results show that residues around the NPA1 box can influence the folding of the MIP in the membrane, and provide structural evidence for the poor water transport properties of MIP, as the NPA boxes lie outside the plane of the membrane.

Functional expression of a locust tyramine receptor in murine erythroleukaemia cells

The LCR/MEL system (Locus Control Region/Murine Erythroleukaemia cells) was employed to express and characterize the Locusta migratoria tyramine receptor (TyrLoc), an insect G protein-coupled receptor. Functional agonist-dependent responses were recorded in stable, tyramine receptor expressing cell clones (MEL-TyrLoc). Tyramine elicited a dose-dependent increase of cytosolic Ca2+-ions and an attenuation of forskolin-induced cyclic adenosine monophosphate (AMP) production. Octopamine was shown to be a weak agonist for both responses. In addition, yohimbine proved to be a potent tyramine receptor antagonist. This study reports the first application of the LCR/MEL expression system in functional assays for G protein-coupled receptors and therefore expands the capabilities of this system by exploiting the functionality of the signal transduction pathways.

Androgen-induced up-regulation of tubulin isoforms in neuroblastoma cells

Androgens regulate the physiology of motor neurones both during development and in adult life. In particular, androgens increase the rate of axonal regeneration after axotomy, an effect correlated with the up-regulation of tubulin. In order to determine whether this was the result of a direct hormone action on neurones, we examined the effect of testosterone on microtubular proteins in human neuroblastoma SH-SY5Y cells. Treatment of proliferating SH-SY5Y cells with testosterone resulted in an up-regulation of alpha- and beta-tubulin. By contrast, no change in tubulin was observed either in cells differentiated into a neuronal phenotype by retinoic acid or in adrenal SW13 cells. We also show that an up-regulation of the ubiquitous beta(II)-tubulin and of the neurone-specific beta(III)-tubulin isoforms contributes to the overall increase in tubulin in response to androgen treatment. The increase in tubulin levels following testosterone treatment was abolished by co-incubation with antiandrogens, indicating that this effect is mediated through a classical mechanism of steroid action. The two microtubule-associated proteins, tau and MAP2b, remained unchanged following testosterone exposure. Thus, these results demonstrate that tubulin is a direct neuronal target of androgen regulation and suggest that dysregulation of tubulin expression may contribute to the pathogenesis of some motor neuronopathies.

Erythroid overexpression of C/EBPgamma in transgenic mice affects gamma-globin expression and fetal liver erythropoiesis

The CCAAT boxes of the beta-like globin genes interact with three proteins: NF-Y, GATA-1 and NFE-6. We demonstrate that NFE-6 contains C/EBPgamma, and address its role in globin gene regulation by erythroid overexpression of C/EBPgamma, and a dominant-negative form C/EBPgammaDeltaB, in mice. Elevated levels of C/EBPgamma, but not C/EBPgammaDeltaB, increase expression of the (fetal) gamma-globin relative to the (adult) beta-globin gene. Interestingly, fetal liver erythropoiesis is ablated when the C/EBPgamma and C/EBPgammaDeltaB levels are further increased in homozygous transgenics. We suggest that targeted expression of dominant-negative leucine zipper proteins is a generally applicable approach to ablate specific tissues in mice.

Mutually exclusive expression of beta(III)-tubulin and vimentin in adrenal cortex carcinoma SW13 cells

During embryogenesis, the maturation of neuroblasts into neurones is accompanied by the down-regulation of vimentin and by the expression of neuronal microtubular proteins. Here, we show that human adrenal cortex SW13 cells express beta(III)-tubulin, MAP2b and tau. Analysis of vimentin-positive and -negative subclones of SW13 cells revealed that, under defined cultured conditions, beta(III)-tubulin and MAP2b were present only in vimentin-deficient cells and that beta(III)-tubulin repression occurred at the transcriptional level in vimentin-positive cells. These results suggest that vimentin repression and beta(III)-tubulin expression are co-ordinated by an upstream mechanism relevant to the control of cytoskeletal protein expression during neuronal development.

High-level, stable expression of blood group antigens in a heterologous system

The detection and identification of blood group antibodies in patients is crucial for successful allogeneic blood transfusions. Current methods are highly subjective and rely on red blood cells (RBCs), which simultaneously express many blood group antigens, have a short shelf-life, and carry potential biohazard risks. To overcome these problems, we have used the approach of expressing individual blood group antigen-bearing proteins in a heterologous system. We report here the high-level surface expression of type I (Knops), type II (Kell), and type III/multi-pass (Duffy) membrane proteins that carry blood group antigens in mouse erythroleukaemic (MEL) cells using a vector containing the beta-globin locus control region. Importantly, the antigens expressed were detected specifically by a panel of patients' sera containing alloantibodies at sensitivities that are comparable to antigen-positive RBCs. Furthermore, in contrast to other mammalian expression systems, antigen expression was stable following freezing and thawing of the cell lines. Thus, this system has the potential both to replace the current use of RBCs by providing a one step method to detect and identify blood group antibodies and to allow the automation of antibody identification for the clinical laboratory.

Rapid determination of transgene copy number in stably-transfected mammalian cells by competitive PCR

We describe here an application of the competitive PCR technique to the analysis of copy number of recombinant rat parathyroid hormone-related protein (rPTHrP) gene in stably-transfected murine erythroleukemia (MEL) cell lines. A single-copy reference gene (endogenous mouse PTHrP gene or mPTHrP) is used as an internal control. This control gene, present in the genome of MEL cells, shares the same primer binding sites as the rPTHrP cDNA but contains an internal PvuII site, which allows resolution of the amplified products after restriction enzyme digestion by polyacrylamide gel electrophoresis (PAGE). The transgene copy number is determined by the ratio of band intensity of the rPTHrP product to that of the mPTHrP product. Using this method, we have determined the copy number of the rPTHrP transgene from isolated genomic DNA, and compared the results with those obtained from Southern blot analysis. In addition, we have demonstrated that the procedure can be applied very simply to whole MEL cells without DNA extractions and that as few as 10(4) cells are required for the analysis.

Gp96/GRP94 is a putative high density lipoprotein-binding protein in liver

We have previously shown that three high density lipoproteins (HDL)-binding proteins in liver, of 90, 110 and 180 kDa, are structurally related. In this study, these proteins are identified as gp96/GRP94. This protein is known to occur as a homodimer and has a dual subcellular localization: it is both an endoplasmic reticulum resident protein, where it is supposed to act as a chaperonin, and a plasma membrane protein, whose significance is unknown. In ultrastructural studies the plasma membrane localization of the homodimeric form was verified. The 90-kDa protein was abundantly present at the membranes of the endosomal/lysosomal vesicles as well as at the apical hepatocyte membranes, comprising the bile canaliculi. The monomeric protein is scarcely present at the basolateral membrane of the hepatocytes, but could be demonstrated in coated pits, suggesting involvement in receptor-mediated endocytosis. Labeling of the endoplasmic reticulum was virtually absent. Gp96/GRP94 was transiently expressed in COS-1 cells. However, the expressed protein was exclusively localized in the endoplasmic reticulum. Transfection with constructs in which the C-terminal KDEL sequence had been deleted, resulted in plasma membrane localized expression of protein, but only in an extremely low percentage of cells. In order to evaluate the HDL-binding capacities of this protein, stably transfected cells were generated, using several cell types. It appeared to be difficult to obtain a prolonged high level expression of gp96. In these cases, however, a marked increase of HDL-binding activity compared with the control cells could be observed.

Altered DNA-binding specificity mutants of EKLF and Sp1 show that EKLF is an activator of the beta-globin locus control region in vivo

The locus control region of the beta-globin cluster contains five DNase I hypersensitive sites (5'HS1-5) required for locus activation. 5'HS3 contains six G-rich motifs that are essential for its activity. Members of a protein family, characterized by three zinc fingers highly homologous to those found in transcription factor Sp1, interact with these motifs. Because point mutagenesis cannot distinguish between family members, it is not known which protein activates 5'HS3. We show that the function of such closely related proteins can be distinguished in vivo by matching point mutations in 5'HS3 with amino acid changes in the zinc fingers of Sp1 and EKLF. Testing their activity in transgenic mice shows that EKLF is a direct activator of 5'HS3.

Role of ferritin in the control of the labile iron pool in murine erythroleukemia cells

In vitro studies have shown that ferritin iron incorporation is mediated by a ferroxidase activity associated with ferritin H subunits (H-Ft) and a nucleation center associated with ferritin L subunits (L-Ft). To assess the role played by the ferritin subunits in regulating intracellular iron distribution, we transfected mouse erythroleukemia cells with the H-Ft subunit gene mutated in the iron-responsive element. Stable transfectants displayed high H-Ft levels and reduced endogenous L-Ft levels, resulting in a marked change in the H:L subunit ratio from 1:1 in control cells to as high as 20:1 in some transfected clones. The effects of H-Ft overexpression on the labile iron pool were determined in intact cells by a novel method based on the fluorescent metallosensor calcein. H-Ft overexpression resulted in a significant reduction in the iron pool, from 1.3 microM in control cells to 0.56 microM in H-Ft transfectants, and in higher buffering capacity following iron loads. A fraction of the H-Ft-associated iron was labile, available to cell-permeant, but not cell-impermeant, chelators. The results of this study provide the first in vivo direct demonstration of the capacity of H-Ft to sequester cell iron and to regulate the levels of the labile iron pool.

A mouse model for the basal transcription/DNA repair syndrome trichothiodystrophy

The sun-sensitive form of the severe neurodevelopmental, brittle hair disorder trichothiodystrophy (TTD) is caused by point mutations in the essential XPB and XPD helicase subunits of the dual functional DNA repair/basal transcription factor TFIIH. The phenotype is hypothesized to be in part derived from a nucleotide excision repair defect and in part from a subtle basal transcription deficiency accounting for the nonrepair TTD features. Using a novel gene-targeting strategy, we have mimicked the causative XPD point mutation of a TTD patient in the mouse. TTD mice reflect to a remarkable extent the human disorder, including brittle hair, developmental abnormalities, reduced life span, UV sensitivity, and skin abnormalities. The cutaneous symptoms are associated with reduced transcription of a skin-specific gene strongly supporting the concept of TTD as a human disease due to inborn defects in basal transcription and DNA repair.

Erythroid Krüppel-like factor (EKLF) is active in primitive and definitive erythroid cells and is required for the function of 5'HS3 of the beta-globin locus control region

Disruption of the gene for transcription factor EKLF (erythroid Krüppel-like factor) results in fatal anaemia caused by severely reduced expression of the adult beta-globin gene, while other erythroid-specific genes, including the embryonic epsilon- and fetal gamma-globin genes, are expressed normally. Thus, EKLF is thought to be a stage-specific factor acting through the CACC box in the beta-gene promoter, even though it is already present in embryonic red cells. Here, we show that a beta-globin gene linked directly to the locus control region (LCR) is expressed at embryonic stages, and that this is only modestly reduced in EKLF-/- embryos. Thus, embryonic beta-globin expression is not intrinsically dependent on EKLF. To investigate whether EKLF functions in the locus control region, we analysed the expression of LCR-driven lacZ reporters. This shows that EKLF is not required for reporter activation by the complete LCR. However, embryonic expression of reporters driven by 5'HS3 of the LCR requires EKLF. This suggests that EKLF interacts directly with the CACC motifs in 5'HS3 and demonstrates that EKLF is also a transcriptional activator in embryonic erythropoiesis. Finally, we show that overexpression of EKLF results in an earlier switch from gamma- to beta-globin expression. Adult mice with the EKLF transgene have reduced platelet counts, suggesting that EKLF levels affect the balance between the megakaryocytic and erythroid lineages. Interestingly, the EKLF transgene rescues the lethal phenotype of EKLF null mice, setting the stage for future studies aimed at the analysis of the EKLF protein and its role in beta-globin gene activation.

New non-peptide endothelin-A receptor antagonists: synthesis, biological properties, and structure-activity relationships of 5-(dimethylamino)-N-pyridyl-,-N-pyrimidinyl-,-N-pyridazinyl-, and -N-pyrazinyl-1-naphthalenesulfonamides

Use of automated synthesis led to the discovery of several 6-membered nitrogen heterocycles as replacements for the N-isoxazolyl substituent present in the 1-naphthalenesulfonamides endothelin-A (ETA) antagonist 5-(dimethylamino)-N-(3,4-dimethyl-5-isoxazolyl)-1-naphthalenesu lfo namides (BMS 182874). In each of these heterocycles, a small substituent such as halogen para to the position of attachment to the sulfonamide nitrogen atom was found to be advantageous for ETA receptor affinity. Of these heterocycles, 2-pyrazines offered the greatest scope for improving receptor affinity. Optimization of the substituents at the 3- and 5-positions in the pyrazine ring led to potent, ETA-selective compounds such as 5-(dimethylamino)-N-(5-chloro-3-methoxy-2-pyrazinyl)-1- naphthalenesulfonamides (7m, ETA pIC50 8.1). When dosed orally at 10 mg/kg to conscious, normotensive rats infused with big ET-1, compounds such as 7m showed significant inhibition of the pressor response with a duration of effect lasting for the 5-h course of the experiment.

The use of the rat LAP LCR and promoter for the high-level constitutive expression of K+ channel cDNAs in a rat liver cell line

Locus control regions (LCRs) are cis-acting elements that confer position-independent and copy-number-dependent expression upon associated genes in transgenic mice. Here we show the second example of the use of an LCR (the rat LAP LCR) in a stable expression vector system, used here in conjunction with the rat liver (NRLM) cell line. Non-transfected NRLM cells are electrically silent and highly suitable for patch clamp electrophysiology. We report reliable constitutive expression from two different K+ channel cDNAs; the voltage-gated rat clone Kv3.4 and the inward rectifier mouse clone Kir2.1. We further show that constitutive expression levels are stable for at least 8 weeks from initial recording.

Selective proton permeability and pH regulation of the influenza virus M2 channel expressed in mouse erythroleukaemia cells

1. The M2 protein of influenza A virus is implicated in transmembrane pH regulation during infection. Whole-cell patch clamp of mouse erythroleukaemia cells expressing the M2 protein in the surface membrane showed a conductance due to M2 which was specifically blocked by the anti-influenza drug rimantadine. 2. The ion selectivity of the rimantadine-sensitive current through M2 was determined. Reversal potentials were close to equilibrium potentials for transmembrane pH gradients and not to those for Na+, K+ or Cl- concentration gradients. M2 permeability to Na+ relative to H+ was estimated to be less than 6 x 10(-7). 3. The M2 conductance increased as external pH decreased below 8.5 and approached saturation at an external pH of 4, effects attributable to increased permeability due to increased driving potential and to activation by low external pH. Both activation and permeation could be described by interaction of protons with sites on M2, with apparent dissociation constants of approximately 0.1 microM and 1 microM, respectively, under physiological conditions. 4. The M2 protein can transfer protons selectively across membranes with the H+ electrochemical gradient, properties consistent with its role in modifying virion and trans-Golgi pH during virus infection.

GATA transcription factors associate with a novel class of nuclear bodies in erythroblasts and megakaryocytes

The nuclear distribution of GATA transcription factors in murine haemopoietic cells was examined by indirect immunofluorescence. Specific bright foci of GATA-1 fluorescence were observed in erythroleukaemia cells and primary murine erythroblasts and megakaryocytes, in addition to diffuse nucleoplasmic localization. These foci, which were preferentially found adjacent to nucleoli or at the nuclear periphery, did not represent sites of active transcription or binding of GATA-1 to consensus sites in the beta-globin loci. Immunoelectron microscopy demonstrated the presence of intensely labelled structures likely to represent the GATA-1 foci seen by immunofluorescence. The GATA-1 nuclear bodies differed from previously described nuclear structures and there was no co-localization with nuclear antigens involved in RNA processing or other ubiquitous (Spl, c-Jun and TBP) or haemopoietic (NF-E2) transcription factors. Interestingly, GATA-2 and GATA-3 proteins also localized to the same nuclear bodies in cell lines co-expressing GATA-1 and -2 or GATA-1 and -3 gene products. This pattern of distribution is, thus far, unique to the GATA transcription factors and suggests a protein-protein interaction with other components of the nuclear bodies via the GATA zinc finger domain.

Mouse model for the lysosomal disorder galactosialidosis and correction of the phenotype with overexpressing erythroid precursor cells

The lysosomal storage disorder galactosialidosis results from a primary deficiency of the protective protein/cathepsin A (PPCA), which in turn affects the activities of beta-galactosidase and neuraminidase. Mice homozygous for a null mutation at the PPCA locus present with signs of the disease shortly after birth and develop a phenotype closely resembling human patients with galactosialidosis. Most of their tissues show characteristic vacuolation of specific cells, attributable to lysosomal storage. Excessive excretion of sialyloligosaccharides in urine is diagnostic of the disease. Affected mice progressively deteriorate as a consequence of severe organ dysfunction, especially of the kidney. The deficient phenotype can be corrected by transplanting null mutants with bone marrow from a transgenic line overexpressing human PPCA in erythroid precursor cells. The transgenic bone marrow gives a more efficient and complete correction of the visceral organs than normal bone marrow. Our data demonstrate the usefulness of this animal model, very similar to the human disease, for experimenting therapeutic strategies aimed to deliver the functional protein or gene to affected organs. Furthermore, they suggest the feasibility of gene therapy for galactosialidosis and other disorders, using bone marrow cells engineered to overexpress and secrete the correcting lysosomal protein.

Decreased immunoreactive androgen receptor levels are not the cause of isolated hypospadias

OBJECTIVE

To elucidate whether diminished levels of androgen receptor (AR) are the underlying cause for the development of hypospadias by determining AR levels in the foreskins of boys with hypospadias.

PATIENTS AND METHODS

The content of AR was determined by a sensitive immunoblotting technique in extracts from preputial tissue obtained from 15 patients (mean age 2.5, SD +/- 1.5 years) with hypospadias and from seven controls (mean age 2.5, SD +/- 1.5 years). Immunoreactivity of the protein was measured by densitometry.

RESULTS

No significant difference in mean AR content was found between those boys with hypospadias (2.1 +/- 0.9 fmol/mg protein, +/- SD) and the age-matched control group (2.2 +/- 0.3 fmol/mg protein). Stratification by the severity of hypospadias (distal and proximal on the shaft, and penoscrotal hypospadias) showed that the severity was not related to tissue AR levels.

CONCLUSION

It is unlikely that hypospadias is caused by a decreased expression of AR.

Heterologous expression as a tool for gene identification and analysis

These days, genome research mainly concerns the accumulation of sequence data and their theoretical interpretation based on analogies to known genes, proteins and structures. However, a final identification of gene function can only be verified by experimental data. One step in this process is the expression of the isolated gene in pro- and eukaryotes. In this article we will review some of the basic features of expression in Escherichia coli and mammalian cells that are relevant to the design of expression experiments. Emphasis is put on the first instance of attaining a high enough level of expression in order to be able to detect the cellular effects or to isolate the product of the transferred gene.

The application of the human beta-globin gene locus control region and murine erythroleukemia cell system to the expression and pharmacological characterization of human endothelin receptor subtypes

The cDNAs encoding both A and B subtypes of the human endothelin receptor have been inserted into mammalian cell expression vectors that utilize the human globin gene, locus control region. These constructs have been introduced into murine erythroleukemia cells and inducible high level expression of the receptors has been achieved (approximately 1.5-pM/mg membrane protein and approximately 13,500 binding sites/cell for both receptor subtypes). Cell lines expressing these receptors were obtained on a rapid time scale (3-4 weeks), facilitated by the need for the analysis of only small numbers of cell clones/receptor (approximately 6). Competitive binding assays with endothelin-1 gave IC50s of 130 +/- 30 pM for endothelin-A receptor and 160 +/- 30 pM for endothelin-B receptor. Similar studies with the different isoforms of endothelin, sarafatoxin-S6b and -S6c, BQ123 and BQ3020, all gave the expected selectivity profiles. The IC50s for all compounds were in close agreement with those reported for native receptors. Thus, this expression system, which has several advantages over other described expression systems, is capable of rapidly providing large quantities of receptor for detailed pharmacological analyses or drug screening. In addition, the expressed receptors display the expected pharmacological profiles in the absence of any complicating, competing interactions from other subtypes or binding sites.

Androgen-controlled specific gene expression in neuroblastoma cells

In Kennedy's syndrome, a mutation in the androgen receptor (AR) gene leads to sensory and lower motor neurone degeneration, therefore genes that are regulated by androgens in neurones may be important in the process of motor neurone cell death. The aim of this study was to identify androgen-inducible genes in the human neuroblastoma cell line SH-SY5Y. We have shown that SH-SY5Y cells expressed the AR by Northern blot analysis using a 32P-labelled probe derived from a human AR cDNA. Differential display of mRNAs has been used to identify and clone putative genes that are regulated by androgens in SH-SY5Y cells. Nine cDNA fragments, ranging in size from 180 bp to 480 bp, corresponding to mRNA species which appear to be differentially expressed in response to 5 alpha dihydrotestosterone (DHT), have been successfully cloned. These are now being sequenced and used as hybridisation probes for Northern blot analysis in order to confirm their induction by DHT. Characterisation of these genes may provide clues to the mechanisms of motor neurone degeneration.

Expression of a bovine GABAA receptor alpha1-subunit cDNA in murine erythroleukaemia cells

A plasmid vector has been constructed by insertion of the cDNA encoding the alpha1 subunit of the bovine GABAA receptor into the LCR/MEL expression vector pNV1 downstream of the human globin locus control region between the promoter and the second intron of the beta-globin gene to produce pNVGABAalpha. This plasmid was transfected into murine erythroleukemia (MEL) cells using electroporation to obtain recombinant cells. Parental and recombinant cells were tested by both RNA dot blot and electrophysiological analysis for the presence of bovine GABAA receptor alpha1 subunit mRNA. Parental MEL cells did not express GABA-gated chloride channels but recombinant cells were sensitive to pressure-applied GABA. The GABA responses reversed at the equilibrium potential predicted for chloride ions. These results show that the alpha1 subunit of the bovine GABAA receptor inserts in the plasma membrane of the MEL cells and forms homo-oligomeric chloride channels that are gated by GABA. Our studies suggest, therefore, that the LCR/MEL system can be used for the expression of neurotransmitter receptor genes.

Isolation and characterization of neurokinin A receptor cDNAs from guinea-pig lung and rabbit pulmonary artery

cDNA clones for NK-2 receptors (NK-2R) were isolated from guinea-pig lung (GPl) and rabbit pulmonary artery (Rpa) using a polymerase chain reaction based methodology. The GPl NK-2R consists of 402 amino acids and encodes a protein with a relative molecular mass of 45,097. The Rpa NK-2R consists of 384 amino acids and encodes a protein with a relative molecular mass of 43,169. The GPl and Rpa NK-2Rs share significant amino acid sequence homology amongst themselves (90.1%), as well as with human, bovine, hamster and rat NK-2 receptors. The two receptors were stably transfected into mouse erythroleukemia cells, high-speed membranes were prepared from induced cells and their pharmacological properties examined utilizing [3H]-NKA in a receptor-binding assay. [3H]NKA bound to both NK-2Rs with high affinity (KD = 2-7 nM) and saturable (Bmax = 633-9000 fmol/mg protein) manner which was inhibited by GTP analogs. Competition experiments with agonists demonstrated identical order of potency in both NK-2Rs; NKA > [Nle10]NKA(4-10) > [beta-Ala8]NKA(4-10) > > Substance P > > > Senktide. Similarly, an identical profile for both receptors was observed with selective NK-2 antagonists: SR48,968 > MEN10,376 > > R396. The rank order of antagonist affinity is consistent with that in cloned human NK-2R and the observations of NK-2 receptor pharmacology in native human, guinea pig and rabbit tissues.

Features in the 5' non-coding sequences of rabbit alpha and beta-globin mRNAs that affect translational efficiency

The 5' non-coding sequence of rabbit beta-globin mRNA was mutagenized in an attempt to identify structural features that might contribute to the ability to support translation in an homologous rabbit reticulocyte lysate. Translational efficiency was not reduced by substitutions introduced in nearly every position of the beta-globin leader sequence, suggesting that the 5' non-coding domain of this highly efficient mRNA contains no special effector motifs. Instead, efficient translation appears to require only a moderately long leader sequence devoid of secondary structure, especially near the 5' end. Consistent with that interpretation, substitutions in several positions actually improved translation relative to the wild-type beta-globin leader sequence; experimental assessment of the secondary structure of these derivatives revealed a perfect inverse correlation between secondary structure content and translational efficiency. Other experiments probed the structural basis for the long-noted difference in translational efficiency between rabbit alpha and beta-globin mRNAs, a difference that was reproduced here using only the 5' non-coding domains of those mRNAs. The possibility that translation of ribosomal protein mRNAs might be modulated by a mechanism similar to that of alpha-globin mRNA is discussed. Because the beta-globin leader sequence has been incorporated into some popular expression vectors, and because globin genes are targets for gene therapy, this analysis of how globin mRNA leader sequences function in translation and how they can be improved may have practical applications.

Inducible vectors for expression in mammalian cells

The most recent developments in mammalian cell inducible expression systems have involved the use of bacterial gene control elements and viral transactivator proteins. The combination of hybrid viral transactivator and bacterial repressor proteins, and simple chemical inducers can provide induction ratios of over 1000-fold. These developments will have applications in both cell-based research and the generation of transgenic animals.