The monoclonal antibody TER-119 recognizes a molecule associated with glycophorin A and specifically marks the late stages of murine erythroid lineage

Role of Ras signaling in erythroid differentiation of mouse fetal liver cells: functional analysis by a flow cytometry-based novel culture system

Ras signaling plays an important role in erythropoiesis. Its function has been extensively studied in erythroid and nonerythroid cell lines as well as in primary erythroblasts, but inconclusive results using conventional erythroid colony-forming unit (CFU-E) assays have been obtained concerning the role of Ras signaling in erythroid differentiation. Here we describe a novel culture system that supports terminal fetal liver erythroblast proliferation and differentiation and that closely recapitulates erythroid development in vivo. Erythroid differentiation is monitored step by step and quantitatively by a flow cytometry analysis; this analysis distinguishes CD71 and TER119 double-stained erythroblasts into different stages of differentiation. To study the role of Ras signaling in erythroid differentiation, different H-ras proteins were expressed in CFU-E progenitors and early erythroblasts with the use of a bicistronic retroviral system, and their effects on CFU-E colony formation and erythroid differentiation were analyzed. Only oncogenic H-ras, not dominant-negative H-ras, reduced CFU-E colony formation. Analysis of infected erythroblasts in our newly developed system showed that oncogenic H-ras blocks terminal erythroid differentiation, but not through promoting apoptosis of terminally differentiated erythroid cells. Rather, oncogenic H-ras promotes abnormal proliferation of CFU-E progenitors and early erythroblasts and supports their erythropoietin (Epo)-independent growth.

A population of c-Kit(low)(CD45/TER119)- hepatic cell progenitors of 11-day postcoitus mouse embryo liver reconstitutes cell-depleted liver organoids

Embryo liver morphogenesis takes place after gastrulation and starts with a ventral foregut evagination that reacts to factor signaling from both cardiac mesoderm and septum transversum mesenchyme. Current knowledge of the progenitor stem cell populations involved in this early embryo liver development is scarce. We describe here a population of 11-day postcoitus c-Kit(low)(CD45/TER119)- liver progenitors that selectively expressed hepatospecific genes and proteins in vivo, was self-maintained in vitro by long-term proliferation, and simultaneously differentiated into functional hepatocytes and bile duct cells. Purified c-Kit(low)(CD45/TER119)- liver cells cocultured with cell-depleted fetal liver fragments engrafted and repopulated the hepatic cell compartments of the latter organoids, suggesting that they may include the embryonic stem cells responsible for liver development.

Identification and characterization of 2 types of erythroid progenitors that express GATA-1 at distinct levels

Transcription factor GATA-1 is essential for the development of the erythroid lineage. To ascertain whether strict control of GATA-1 expression level is necessary for achieving proper erythropoiesis, we established transgenic mouse lines expressing green fluorescent protein (GFP) under the control of the GATA-1 gene hematopoietic regulatory domain. We examined the GATA-1 expression level by exploiting the transgenic mice and found 2 GFP-positive hematopoietic progenitor fractions in the bone marrow. One is the GFPhigh fraction containing mainly CFU-E and proerythroblasts, which coexpress transferrin receptor, while the other is the GFPlow/transferrin receptor-negative fraction containing BFU-E. Since the intensity of green fluorescence correlates well with the expression level of GATA-1, these results indicate that GATA-1 is highly expressed in erythroid colony-forming unit (CFU-E) but low in erythroid burst-forming unit (BFU-E), suggesting that the incremental expression of GATA-1 is required for the formation of erythroid progenitors. We also examined GFP-positive fractions in the transgenic mouse spleen and fetal liver and identified fractions containing BFU-E and CFU-E, respectively. This study also presents an efficient method for enriching the CFU-E and BFU-E from mouse hematopoietic tissues.

Attenuated signaling by a phosphotyrosine-null Epo receptor form in primary erythroid progenitor cells

Signals provided by the erythropoieitin receptor (EpoR) are required for erythroid development beyond the erythroid colony-forming unit (CFU-e) stage and are propagated via the EpoR-tethered Janus kinase, JAK2. JAK2 functions, in part, to phosphorylate 8 conserved EpoR phosphotyrosine (PY) sites for the binding of a diverse set of signaling factors. However, recent studies in transgenic and knock-in mice have demonstrated substantial bioactivity for PY-null EpoR forms. Presently, the activities of a PY-null EpoR-HM form in primary progenitor cells from knock-in mice were further assessed using optimized Epo dose-dependent proliferation, survival, and differentiation assays. As compared with the wild-type (wt)-EpoR, EpoR-HM activity was compromised several-fold in each context when Epo was limited to physiologic concentrations. Possible compensatory increases in serum growth factor levels also were investigated, and as assayed using embryonic stem (ES) cell-derived erythroid G1E2 cells, activities in serum from EpoR-HM mice were substantially elevated. In addition, when challenged with phenylhydrazine-induced anemia, EpoR-HM mice failed to respond with efficient splenic stress erythropoiesis. Thus, the function of this JAK2-coupled but minimal PY-null EpoR-HM form appears to be attenuated in several contexts and to be assisted in vivo by compensatory mechanisms. Roles normally played by EpoR PY sites and distal domains therefore should receive continued attention.

Identification of Friend murine retrovirus-infected immune cells and studies of the effects of sex and steroid hormones in the early phase of infection

Male mice are more susceptible than female mice to the murine retrovirus FIS-2. We previously reported that sex-related factors influence early virus replication via mechanisms involving a glucocorticoid response element (GRE) in the long terminal repeat (LTR) enhancer region. In the present study, we investigated further the influence of sex and steroid hormones on early murine retrovirus dissemination and immune functions. In male mice we found a correlation between an early expansion of the CD8+ cell subset and rapid infection of lymphocytes, including CD8+ cells. Virus load in blood declined faster in females than in males, and the postpeak declines coincided with more rapidly generation of antibodies against virus-positive cells. Moreover, female-derived T-cells responded better to in vitro mitogen stimuli than male-derived T-cells. Physiological concentrations of progesterone and dexamethasone induced a dose-dependent inhibition of T-cell proliferation. Administration of progestin in vivo did not modify early FIS-2 production in female mice. Male castrated mice, who were notably less involved in aggressive behaviour and fighting compared to male control mice, had a significant delay of virus dissemination. We suggest that testosterone-dependent aggression, with successive enhanced stress hormone levels, may influence both FIS-2 replication and immune responses during infection.

Identification of the earliest prethymic T-cell progenitors in murine fetal blood

During murine fetal development, hemato-poietic progenitors start to colonize the thymic anlage at day 11 of gestation via blood stream. The present study aims at identifying the earliest prethymic progenitors in circulation. Here, we show that the interleukin-7 receptor-positive (IL-7R+) cells in Lin- c-kit+ population are circulating exclusively between days 11 and 14 of fetal age. Clonal analysis revealed that these IL-7R+ cells mostly contain T-cell lineage-restricted progenitors (p-Ts). The proportion of circulating p-Ts reaches 30% of the total p-Ts during these fetal ages, whereas virtually all B-cell lineage-restricted progenitors stay in the fetal liver, suggesting that the p-Ts are selectively released to the circulation. The circulating p-Ts retain the potential to generate natural killer cells and dendritic cells and exhibit extensive proliferation before the occurrence of T-cell receptor beta (TCRbeta) chain gene rearrangement. We propose that the wave of p-Ts in fetal blood disclosed by this study represents the ontogenically earliest thymic immigrants.

Glycophorin A requirement for expression of O-linked antigens on the erythrocyte membrane

BACKGROUND

Glycophorin A (GPA) has a large number of sialic acid-containing oligosaccharide chains. GPA is highly conserved among vertebrates, mice with a GPA deletion have not been reported and GPA's physiologic role remains uncertain.

RESULTS

GPA-/- homozygotes were obtained by intercrossing GPA+/- heterozygotes based on Mendelian genetics. The amount of O-linked oligosaccharide chains in the erythrocyte membrane of GPA-/- mice decreased to 60% compared to that of the wild-type mice. Flow cytometry and Western blot analysis revealed that the TER antigen that is associated with GPA on the erythrocyte membrane was totally abrogated from the cell surface in GPA-/- mice. Several glycoproteins that were detected with peanut agglutinin (PNA), a lectin that recognizes O-linked oligosaccharide chains, were absent from the GPA-/- erythrocyte membrane. Erythrocytes lacking GPA were more sensitive to hypo-osmotic stress than wild-type erythrocyte.

CONCLUSIONS

GPA-/- mice show apparently normal phenotypes at least during the early generations. The disappearance of many glycoproteins recognized by PNA lectin on the GPA-/- erythrocyte membrane proteins suggests that GPA has an essential role in the expression of O-linked antigens on the erythrocyte membrane protein. These interactions of GPA and other glycoproteins may contribute to maintaining the physical strength of the erythrocyte membrane.

In vitro expansion of human cord blood CD36+ erythroid progenitors: temporal changes in gene and protein expression

OBJECTIVE

Erythropoiesis involves proliferation and differentiation of committed erythroid progenitors to mature red blood cells. The objective of this study was to characterize growth characteristics of human CD36+ erythroid progenitors and to profile temporal expression of lineage-specific transcription factors, structural proteins, and growth factor receptors involved in erythropoiesis.

MATERIALS AND METHODS

Erythropoietin-induced differentiation of human cord blood CD36+ erythroid progenitors was profiled for GATA-1, GATA-2, NFE2, EKLF, SCL, PU.1, Id1, Evi-1, c-myb, Hox2.2, c-kit, EpoR, glycophorin A (GPA), CD71, beta- and gamma-globin, and protein 4.2 gene and/or protein expression and DNA content analysis on days 4, 7, and 15 of culture.

RESULTS

Real-time RT-PCR analysis revealed upregulation of GATA-1, Id1, glycophorin A, and protein 4.2 mRNA expression on day 7 when compared to day 4 and decreased expression on day 15. EKLF, GATA-2, Hox2.2, c-myb, Evi-1, c-kit, and PU.1 mRNA expression decreased on days 7 and 15. NFE2, CD71, SCL, and EPO-R mRNA expression remained similar on days 4 and 7 but decreased on day 15. Expression of globin genes beta- and gamma-globin increased on both day 7 and day 15 compared to day 4. Values from flow cytometric quantitation of glycophorin A, transferrin receptor (CD71), and hemoglobin A proteins correlated with gene expression results. DNA analysis demonstrated that most cells lacked DNA content by day 15, a finding consistent with enucleation and terminal erythroid differentiation.

CONCLUSION

These data indicate that in vitro liquid cultures of committed CD36+ erythroid progenitor cells retain, in part, many features of erythropoiesis at the cellular and molecular level and may provide a useful model for assessment of disease-related or drug-induced erythropoietic abnormalities.

Cross-presentation of disialoganglioside GD3 to natural killer T cells

GD3, a ganglioside expressed on human melanoma, can be recognized by the humoral immune system. In this paper, we demonstrate that immunizing mice with the human melanoma cell line SK-MEL-28 (GD3+ GM2- CD1-) or with syngeneic APCs loaded with GD3 can induce a GD3-reactive natural killer T (NKT) cell response. GD3-reactive NKT cells were detected among splenocytes of immunized mice at frequencies of approximately 1:2000 both by ELISPOT and GD3-loaded mouse CD1d tetramer analysis. GD3-reactive NKT cells did not react with GM2, a closely related ganglioside, and were not detectable in unimmunized mice. GD3-reactive NKT cells initially produced IL-4 and IFN-gamma followed by IL-10. They were CD1d restricted in that reactivity was abrogated when APCs were blocked with anti-CD1d monoclonal antibody before being loaded with GD3 or when APCs from CD1d knockout mice were used. Because SK-MEL-28 does not express any isoform of human CD1, GD3 must be cross-presented by murine APCs in vivo. This is the first analysis of a natural ligand for mouse NKT cells and the first definitive paper of cross-presentation to NKT cells. This could be a mechanism for NKT cell recognition of tumor gangliosides in CD1- tumors.

The cytokine-inducible Scr homology domain-containing protein negatively regulates signaling by promoting apoptosis in erythroid progenitor cells

The small cytokine-inducible SH2 domain-containing protein (CIS) has been implicated in the negative regulation of signaling through cytokine receptors. CIS reduces growth of erythropoietin receptor (EpoR)-dependent cell lines, but its role in proliferation, differentiation, and survival of erythroid progenitor cells has not been resolved. To dissect the function of CIS in cell lines and erythroid progenitor cells, we generated green fluorescent protein (GFP)-tagged versions of wild type CIS, a mutant harboring an inactivated SH2 domain (CIS R107K), and a mutant with a deletion of the SOCS Box (CISDeltaBox). Retroviral expression of the GFP fusion proteins in BaF3-EpoR cells revealed that both Tyr-401 in the EpoR and an intact SH2 domain within CIS are prerequisites for receptor recruitment. As a consequence, both are essential for the growth inhibitory effect of CIS, whereas the CIS SOCS box is dispensable. Accordingly, the retroviral expression of GFP-CIS but not GFP-CIS R107K impaired proliferation of erythroid progenitor cells in colony assays. Erythroid differentiation was unaffected by either protein. Interestingly, apoptosis of erythroid progenitor cells was increased upon GFP-CIS expression and this required the presence both of an intact SH2 domain and the SOCS box. Thus, CIS negatively regulates signaling at two levels, apoptosis and proliferation, and thereby sets a threshold for signal transduction.

Liposomal clodronate as a novel agent for treating autoimmune hemolytic anemia in a mouse model

Autoimmune hemolytic anemia (AIHA) is a disease in which autoantibodies against red blood cells (RBCs) lead to their premature destruction. Most clinically significant autoantibodies are of the immunoglobulin G (IgG) type, which leads primarily to the uptake and destruction of RBCs by splenic and hepatic macrophages. Therapies such as corticosteroids and splenectomy are directed at interfering with this process. Liposomally encapsulated clodronate (dichloromethylene diphosphonate) has previously been found to be a potent antimacrophage agent. It selectively depletes animals of macrophages within 24 hours of administration by inducing apoptosis in these cells. Therefore, we hypothesized that liposomal clodronate would be a useful agent for treating AIHA. We tested this hypothesis in a mouse model of AIHA in which animals were given either anti-RBC antibodies or preopsonized RBCs. In either case, liposomal clodronate substantially decreased RBC destruction. This drug formulation was effective within hours by first blocking and then depleting phagocytic macrophages, and its action lasted for 1 to 2 weeks. Thus, in AIHA, liposomal clodronate therapy may act like a temporary, medicinal splenectomy. As such, it may prove useful in situations where rapid response to therapy is critical or other medical therapies are inadequate.

Flow cytometric-based isolation of nucleated erythroid cells during maturation: an approach to cell surface antigen studies

Nucleated red blood cells (NRBCs) are involved in normal physiologic processes, as well as in several malignancies. They are usually counted manually under the microscope. However, blood sample manipulation may be a source of variability and manual counting is imprecise, time-consuming, and subjective. To improve identification of CD45-negative cells, we used a flow cytometry technique that avoids the addition of lysing reagents and stains viable cell nuclei. We applied this method for counting and isolating NRBC subpopulations in whole blood samples, using DNA/RNA viable staining to discriminate nonnucleated erythroid cells and debris. NRBC counts gave 197.95 cells per mm(3) in mobilized peripheral blood samples (1.00%, n = 20), 3897.59 cells per mm(3) in leukapheresis products (3.08%, n = 20), and 765.21 cells per mm(3) in cord blood samples (6.09%, n = 20). Normal bone marrow counts were 5449.42 cells per mm(3) (11.76%, n = 20). Scatter profiles showed three distinct populations, from early to late-stage erythroblasts, consisting of erythroblasts, orthochromatic erythroblasts, and ejected nuclei, as confirmed by Wright-Giemsa staining. In addition, flow cytometry immunophenotyping showed that glycophorin A was expressed dimly on NRBCs during maturation. These findings point to the feasibility of live NRBCs studies, which offer great potential for a wide range of disciplines.

Early dissemination rates of Friend murine leukaemia virus variants correlate with late pathogenesis

FIS-2, a less oncogenic, immunosuppressive variant of the Friend murine leukaemia virus (F-MuLV), was used to explore whether the differences in biological features were related to early virus dissemination rates or sites of replication. We found that erythroblasts were the primary target cells for both F-MuLV and FIS-2, while B- and T-cells were infected later in the infection. Although FIS-2 replicated to similar titres as F-MuLV, we observed a delay in peak viraemia titre and in the number of virus-positive cells in bone marrow and spleen. Studies including the chimeric viruses RE3 (FIS-2LTR with a F-MuLV background) and RE4 (F-MuLV LTR with a FIS-2 background) indicated that the delay in dissemination was due to mutations in FIS-2 LTR. The kinetics for early virus replication correlated with previously reported mean latency time for virus-induced erythroleukaemia in mice inoculated as newborns and with the onset of immunosuppression in adult mice. In addition, F-MuLV-induced late immunosuppression coincided with signs of erythroleukaemia and persistent viraemia. FIS-2 induced a more moderate late immunosuppression without persistent viraemia or signs of erythroleukaemia. Overall, our findings indicated that early viral replication is a prognostic factor in murine retrovirus-induced pathogenesis.

The Common Myelolymphoid Progenitor: A Key Intermediate Stage in Hemopoiesis Generating T and B Cells

We have previously shown that the common progenitors for myeloid, T, and B cell lineages are enriched in the earliest population of murine fetal liver. However, it remained unclear whether such multipotent progenitors represent the pluripotent progenitors capable of generating all hemopoietic cells or they also comprise progenitors restricted to myeloid, T, and B cell lineages. To address this issue, we have developed a new clonal assay covering myeloid, erythroid, T, and B cell lineages, and using this assay the developmental potential of individual cells in subpopulations of lineage marker-negative (Lin(-)) c-kit(+) murine fetal liver cells was investigated. We identified the progenitor generating myeloid, T, and B cells, but not erythroid cells in the Sca-1(high) subpopulation of Lin(-)c-kit(+) cells that can thus be designated as the common myelolymphoid progenitor (CMLP). Common myeloerythroid progenitors were also detected. These findings strongly suggest that the first branching point in fetal hemopoiesis is between the CMLP and common myeloerythroid progenitors. T and B cell progenitors may be derived from the CMLP through the previously identified myeloid/T and myeloid/B bipotent stages, respectively.

Long-term reversal of chronic anemia using a hypoxia-regulated erythropoietin gene therapy

Anemia is a common clinical problem, and there is much interest in its role in promoting left ventricular hypertrophy through increasing cardiac workload. Normally, red blood cell production is adjusted through the regulation of erythropoietin (Epo) production by the kidney. One important cause of anemia is relative deficiency of Epo, which occurs in most types of renal disease. Clinically, this can be corrected by supplementation with recombinant Epo. Here we describe an oxygen-regulated gene therapy approach to treating homozygous erythropoietin-SV40 T antigen (Epo-TAg(h)) mice with relative erythropoietin deficiency. We used vectors in which murine Epo expression was directed by an Oxford Biomedica hypoxia response element (OBHRE) or a constitutive cytomegalovirus (CMV) promoter. Both corrected anemia, but CMV-Epo-treated mice acquired fatal polycythemia. In contrast, OBHRE-Epo corrected the hematocrit level in anemic mice to a normal physiologic level that stabilized without resulting in polycythemia. Importantly, the OBHRE-Epo vector had no significant effect on the hematocrit of control mice. Homozygous Epo-TAg(h) mice display cardiac hypertrophy, a common adaptive response in patients with chronic anemia. In the OBHRE-Epo-treated Epo-TAg(h) mice, we observed a significant reversal of cardiac hypertrophy. We conclude that the OBHRE promoter gives rise to physiologically regulated Epo secretion such that the hematocrit level is corrected to healthy in anemic Epo-TAg(h) mice. This establishes that a hypoxia regulatory mechanism similar to the natural mechanism can be achieved, and it makes EPO gene therapy more attractive and safer in clinical settings. We envisage that this control system will allow regulated delivery of therapeutic gene products in other ischemic settings.

Erythropoietin modulates calcium influx through TRPC2

Mammalian isoforms of calcium-permeable Drosophila transient receptor potential channels (TRPC) are involved in the sustained phase of calcium entry in nonexcitable cells. Erythropoietin (Epo) stimulates a rise in intracellular calcium ([Ca](i)) via activation of voltage-independent calcium channel(s) in erythroid cells. Here, involvement of murine orthologs of classical TRPC in the Epo-modulated increase in [Ca](i) was examined. RT-PCR of TRPC 1-6 revealed high expression of only TRPC2 in Epo-dependent cell lines HCD-57 and Ba/F3 Epo-R, in which Epo stimulates a rise in [Ca](i). Using RT-PCR, Western blotting, and immunolocalization, expression of the longest isoform of mTRPC2, clone 14, was demonstrated in HCD-57 cells, Ba/F3 Epo-R cells, and primary murine erythroblasts. To determine whether erythropoietin is capable of modulating calcium influx through TRPC2, CHO cells were cotransfected with Epo-R subcloned into pTracer-CMV and either murine TRPC2 clone 14 or TRPC6, a negative control, into pQBI50. Successful transfection of Epo-R was verified in single cells by detection of green fluorescent protein from pTracer-CMV using digital video imaging, and successful transfection of TRPC was confirmed by detection of blue fluorescent protein fused through a flexible linker to TRPC. [Ca](i) changes were simultaneously monitored in cells loaded with Rhod-2 or Fura Red. Epo stimulation of CHO cells cotransfected with Epo-R and TRPC2 resulted in a rise in [Ca](i) above base line (372 +/- 71%), which was significantly greater (p < or = 0.0007) than that seen in cells transfected with TRPC6 or empty pQBI50 vector. This rise in [Ca](i) required Epo and extracellular calcium. These results identify a calcium-permeable channel, TRPC2, in erythroid cells and demonstrate modulation of calcium influx through this channel by erythropoietin.

Absence of erythrogenesis and vasculogenesis in Plcg1-deficient mice

Mice nullizygous for Plcg1 cease growing at early to mid-gestation. An examination of carefully preserved wild-type embryos shows clear evidence of erythropoiesis, but erythropoiesis is not evident in Plcg1 nullizygous embryos at the same stage. The analyses of embryonic materials demonstrate that in the absence of Plcg1, erythroid progenitors cannot be detected in the yolk sac or embryo body by three different assays, burst-forming units, colony-forming units, and analysis for the developmental marker Ter119. However, non-erythroid granulocyte/macrophage colonies are produced by Plcg1 null embryos. Further analysis of these embryos demonstrates significantly diminished vasculogenesis in Plcg1 nullizygous embryos based on the lack of expression of the endothelial marker platelet endothelial cell adhesion molecule-1. In addition, Plcg1 nullizygous embryos express a greatly reduced level of vascular endothelial growth factor receptor-2/Flk-1, consistent with significantly impaired vasculogenesis and erythropoiesis. Interestingly, these early embryos do express phospholipase C-gamma2, however, it is unable to substitute for the absence of phospholipase C-gamma1, which can be detected in its tyrosine-phosphorylated state.

Disruption of hematopoiesis and thymopoiesis in the early premalignant stages of infection with SL3-3 murine leukemia virus

A time course analysis of SL3-3 murine leukemia virus (SL3) infection in thymus and bone marrow of NIH/Swiss mice was performed to assess changes that occur during the early stages of progression to lymphoma. Virus was detectable in thymocytes, bone marrow, and spleen as early as 1 to 2 weeks postinoculation (p.i.). In bone marrow, virus infection was detected predominantly in immature myeloid or granulocytic cells. Flow cytometry revealed significant reductions of the Ter-119(+) and Mac-1(+) populations, and significant expansions of the Gr-1(+) and CD34(+) populations, between 2 and 4 weeks p.i. Analysis of colony-forming potential confirmed these findings. In the thymus, SL3 replication was associated with significant disruption in thymocyte subpopulation distribution between 4 and 7 weeks p.i. A significant thymic regression was observed just prior to the clonal outgrowth of tumor cells. Proviral long terminal repeats (LTRs) with increasing numbers of enhancer repeats were observed to accumulate exclusively in the thymus during the first 8 weeks p.i. Observations were compared to the early stages of infection with a virtually nonpathogenic SL3 mutant, termed SL3DeltaMyb5, which was shown by real-time PCR to be replication competent. Comparison of SL3 with SL3DeltaMyb5 implicated certain premalignant changes in tumorigenesis, including (i) increased proportions of Gr-1(+) and CD34(+) bone marrow progenitors, (ii) a significant increase in the proportion of CD4(-) CD8(-) thymocytes, (iii) thymic regression prior to tumor outgrowth, and (iv) accumulation of LTR enhancer variants. A model in which disrupted bone marrow hematopoiesis and thymopoiesis contribute to the development of lymphoma in the SL3-infected animal is discussed.

Role of JunB in erythroid differentiation

The role of junB as a regulator of erythroid cell survival, proliferation, and differentiation was tested by controlled expression of JunB in the erythropoietin (EPO)-dependent erythroleukemia cell line HCD57. JunB induced erythroid differentiation as evidenced by increased expression of the erythroid-specific proteins beta-globin, spectrin-alpha, and TER-119. Expression of JunB for at least 48 h was required for the differentiated phenotype to emerge. Differentiation was accompanied by a slower rate of proliferation and an increase in the expression of the cell cycle inhibitory protein p27. p27 protein expression increased due to reduced turnover without changes in transcription, indicating global changes in cell physiology following JunB induction. JunB expression was also studied in mouse and human primary erythroid cells. JunB expression increased immediately in both primary mouse cells and HCD57 cells treated with EPO and quickly returned to base-line levels, followed by a secondary rise in JunB in primary erythroid cells, but not in HCD57 cells, 36-48 h later. This result suggested that the initial EPO-dependent JunB induction was not sufficient to induce differentiation, but that the late EPO-independent JunB expression in primary erythroid cells was necessary for differentiation. This study suggests that JunB is an important regulator of erythroid differentiation.

Erythropoietin receptor-dependent erythroid colony-forming unit development: capacities of Y343 and phosphotyrosine-null receptor forms

Red cell development depends on the binding of erythropoietin (EPO) to receptors expressed by erythroid colony-forming units (CFUe) and the subsequent activation of receptor-bound Janus kinase (Jak2). Jak2 then mediates the phosphorylation of receptor tyrosine sites and the recruitment of 25 or more Src homology 2 domain-encoding proteins and associated factors. Previous studies have shown that an EPO receptor form containing Jak2-binding domains plus a single phosphotyrosine343 (PY343)–STAT5-binding site provides all signals needed for erythroid cell development. However, roles for PY343 and STAT5 remain controversial, and findings regarding PY-null receptor activities and erythropoiesis in STAT5-deficient mice are disparate. To study activities of a PY-null EPO receptor in primary cells while avoiding compensatory mechanisms, a form retaining domains for Jak2 binding and activation, but lacking all cytoplasmic tyrosine sites, was expressed in transgenic mice from aGATA1 gene-derived vector as a human epidermal growth factor receptor- murine EPO receptor chimera (EE-T-Y343F). The bio-signaling capacities of this receptor form were investigated in CFUe from thiamphenicol-treated mice. Interestingly, this PY-null EPO receptor form supported CFUe development (in the absence of detectable STAT5 activation) at efficiencies within 3-fold of those levels mediated by either an EE-T-Y343 form or the endogenous EPO receptor. However, EE-T-Y343F–dependent Ter119+ erythroblast maturation was attenuated. In tests of cosignaling with c-Kit, EE-T-Y343F nonetheless retained full capacity to synergize with c-Kit in promoting erythroid progenitor cell proliferation. Thus, EPO receptor PY-dependent events can assist late erythropoiesis but may be nonessential for EPO receptor–c-Kit synergy.

Selective increase of autoimmune epitope expression on aged erythrocytes in mice: implications in anti-erythrocyte autoimmune responses

We investigated the impact of changes occurring during red blood cell (RBC) ageing on the RBC-binding activity of pathogenic anti-erythrocyte monoclonal antibodies derived from autoimmune-prone New Zealand black (NZB) mice. As assessed by flow cytometric analysis on in vivo biotinylated RBCs, all five NZB-derived anti-RBC mAb exhibited more efficient binding to aged RBCs than to young RBCs, and resulted in a selective elimination of more aged RBCs from the circulating blood. In addition, treatment of RBCs with proteases markedly enhanced the binding of all five anti-RBC mAb, raising the possibility that increased exposure of autoimmune epitopes on aged RBCs may be in part, a result of contacts with proteolytic enzymes during the lifetime of circulating RBCs. In marked contrast, the binding activity of mAb raised in non-autoimmune animals against antigens expressed on RBCs, such as CD44, CD47, CD147 and TER-119, was either decreased or unchanged with RBC ageing, and these epitopes, except for that recognized by anti-CD47 mAb, were highly sensitive to mild treatment with proteases. Our data unravel the unique molecular feature of RBC epitopes involved in autoimmune haemolytic anaemia, suggesting that membrane alterations in aged RBCs might play a significant role in the development of the autoantibody response to RBCs.

Erythropoietin receptor-dependent erythroid colony-forming unit development: capacities of Y343 and phosphotyrosine-null receptor forms

Red cell development depends on the binding of erythropoietin (EPO) to receptors expressed by erythroid colony-forming units (CFUe) and the subsequent activation of receptor-bound Janus kinase (Jak2). Jak2 then mediates the phosphorylation of receptor tyrosine sites and the recruitment of 25 or more Src homology 2 domain-encoding proteins and associated factors. Previous studies have shown that an EPO receptor form containing Jak2-binding domains plus a single phosphotyrosine(343) (PY(343))-STAT5-binding site provides all signals needed for erythroid cell development. However, roles for PY(343) and STAT5 remain controversial, and findings regarding PY-null receptor activities and erythropoiesis in STAT5-deficient mice are disparate. To study activities of a PY-null EPO receptor in primary cells while avoiding compensatory mechanisms, a form retaining domains for Jak2 binding and activation, but lacking all cytoplasmic tyrosine sites, was expressed in transgenic mice from a GATA1 gene-derived vector as a human epidermal growth factor receptor- murine EPO receptor chimera (EE-T-Y343F). The bio-signaling capacities of this receptor form were investigated in CFUe from thiamphenicol-treated mice. Interestingly, this PY-null EPO receptor form supported CFUe development (in the absence of detectable STAT5 activation) at efficiencies within 3-fold of those levels mediated by either an EE-T-Y343 form or the endogenous EPO receptor. However, EE-T-Y343F-dependent Ter119(+) erythroblast maturation was attenuated. In tests of cosignaling with c-Kit, EE-T-Y343F nonetheless retained full capacity to synergize with c-Kit in promoting erythroid progenitor cell proliferation. Thus, EPO receptor PY-dependent events can assist late erythropoiesis but may be nonessential for EPO receptor-c-Kit synergy.

Heme-regulated eIF2alpha kinase (HRI) is required for translational regulation and survival of erythroid precursors in iron deficiency

Although the physiological role of tissue-specific translational control of gene expression in mammals has long been suspected on the basis of biochemical studies, direct evidence has been lacking. Here, we report on the targeted disruption of the gene encoding the heme-regulated eIF2alpha kinase (HRI) in mice. We establish that HRI, which is expressed predominantly in erythroid cells, regulates the synthesis of both alpha- and beta-globins in red blood cell (RBC) precursors by inhibiting the general translation initiation factor eIF2. This inhibition occurs when the intracellular concentration of heme declines, thereby preventing the synthesis of globin peptides in excess of heme. In iron-deficient HRI(-/-) mice, globins devoid of heme aggregated within the RBC and its precursors, resulting in a hyperchromic, normocytic anemia with decreased RBC counts, compensatory erythroid hyperplasia and accelerated apoptosis in bone marrow and spleen. Thus, HRI is a physiological regulator of gene expression and cell survival in the erythroid lineage.

Ineffective erythropoiesis in Stat5a(-/-)5b(-/-) mice due to decreased survival of early erythroblasts

Erythropoietin (Epo) controls red cell production in the basal state and during stress. Epo binding to its receptor, EpoR, on erythroid progenitors leads to rapid activation of the transcription factor Stat5. Previously, fetal anemia and increased apoptosis of fetal liver erythroid progenitors were found in Stat5a(-/-)5b(-/-) mice. However, the role of Stat5 in adult erythropoiesis was not clear. The present study shows that some adult Stat5a(-/-)5b(-/-) mice have a near-normal hematocrit but are deficient in generating high erythropoietic rates in response to stress. Further, many adult Stat5a(-/-)5b(-/-) mice have persistent anemia despite a marked compensatory expansion in their erythropoietic tissue. Analysis of erythroblast maturation in Stat5a(-/-)5b(-/-) hematopoietic tissue shows a dramatic increase in early erythroblast numbers, but these fail to progress in differentiation. Decreased expression of bcl-x(L) and increased apoptosis in Stat5a(-/-)5b(-/-) early erythroblasts correlate with the degree of anemia. Hence, Stat5 controls a rate-determining step regulating early erythroblast survival.

Selective rescue of early haematopoietic progenitors in Scl–/– mice by expressing Scl under the control of a stem cell enhancer

The stem cell leukaemia gene (Scl) encodes a basic helix-loop-helix transcription factor with a pivotal role in both haematopoiesis and endothelial development. During mouse development, Scl is first expressed in extra-embryonic mesoderm, and is required for the generation of all haematopoietic lineages and normal yolk sac angiogenesis. Ectopic expression of Scl during zebrafish development specifies haemangioblast formation from early mesoderm. These results suggest that SCL is essential for establishing the transcriptional programme responsible for the formation of haematopoietic stem cells and have focused attention on the transcriptional regulation of Scl itself. Previous studies have identified a panel of Scl enhancers each of which directed expression to a subdomain of the normal Scl expression pattern. Among them, a 3′ enhancer directed expression during development to vascular endothelium and haematopoietic progenitors but not to Ter119+ erythroid cells. The expression in haematopoietic stem cells, however, remained undetermined. We demonstrate that this 3′ enhancer directs lacZ expression in transgenic mice to most foetal and adult long-term repopulating haematopoietic stem cells, and therefore functions as a stem cell enhancer. Consistent with these results, expression in Scl–/– embryos of exogenous Scl driven by the stem cell enhancer rescued the formation of early haematopoietic progenitors and also resulted in normal yolk sac angiogenesis. By contrast, erythropoiesis remained markedly deficient in rescued embryos. This observation is consistent with the inactivity of the stem cell enhancer in erythroid cells and reveals an essential role for SCL during erythroid differentiation in vivo.

Erythropoiesis in the absence of janus-kinase 2: BCR-ABL induces red cell formation in JAK2(-/-) hematopoietic progenitors

The receptor-associated protein tyrosine kinase janus-kinase 2 (JAK2) is essential for normal red cell development and for erythropoietin receptor (EpoR) signaling. JAK2(-/-) embryos are severely deficient in erythropoiesis and die at an early stage of development from fetal anemia. The binding of erythropoietin (Epo) to the EpoR triggers the activation of JAK2, the phosphorylation of the EpoR, and the initiation of the EpoR signaling cascade. In addition to Epo binding to its receptor, signaling pathways downstream of the EpoR can also be stimulated by the BCR-ABL oncoprotein. This study explored whether JAK2 is required for BCR-ABL-mediated stimulation of erythropoiesis. Here, it is shown that JAK2 is constitutively tyrosine phosphorylated in cultured and primary erythroid cells expressing BCR-ABL. However, BCR-ABL effectively supports normal erythroid proliferation, differentiation, and maturation in JAK2-deficient fetal liver cells. Using mutants of BCR-ABL, this study shows that certain signaling pathways activated by BCR-ABL segments distinct from its tyrosine kinase domain are essential for rescue of erythropoiesis in JAK2(-/-) progenitors. The consequences of these multiple signaling pathways for normal erythroid development are discussed.

An optimized system for studies of EPO-dependent murine pro-erythroblast development

OBJECTIVE

Objectives were to develop new means to isolate useful numbers of primary progenitor cells and to quantitatively assay the stepwise maturation of erythroblasts.

METHODS

Approaches involved dosing mice with thiamphenicol (TAP) to yield staged cohorts of pro-erythroid cells; optimizing conditions for their EPO-dependent in vitro growth and survival; developing assays for CFU-E maturation; analyzing stage-specific transcript expression; and expressing a heterologous, erythroid-specific tag (EE372) in transgenic mice.

RESULTS

Per TAP-treated mouse, 3 x 10(7) highly EPO-responsive erythroid progenitor cells were generated that represented up to 30% of total splenocytes and showed strict dependence on EPO for survival, growth, and immediate response gene expression. In this developing cohort, a tightly programmed sequence of gene expression was observed, and maximal expression of c-kit, EPO receptor, and beta-globin transcripts occurred at 72, 96, and 120 hours post-TAP withdrawal, respectively. Also, the newly discovered erythroid-specific dual-specificity kinase, DYRK3, was revealed to be expressed at a late CFU-E stage. In vitro, these progenitor cells matured stepwise from high FALS Ter119- cells (24-hour culture) to high FALS Ter119+ cells (24-36 hours) to low FALS Ter119+ maturing erythroblasts (40-48 hours) and sharp differences in their morphologies were observed. Finally, a MACS-based procedure for the purification of erythroid progenitor cells from TAP-treated EE372 transgenic mice also was developed.

CONCLUSIONS

A comprehensive new system for isolating large numbers of primary murine erythroid progenitor cells and quantitatively monitoring their development is established that should serve well in investigations of endogenous and pharmacological regulators of red blood cell development.

GATA1-Cre mediates Piga gene inactivation in the erythroid/megakaryocytic lineage and leads to circulating red cells with a partial deficiency in glycosyl phosphatidylinositol-linked proteins (paroxysmal nocturnal hemoglobinuria type II cells)

Patients with paroxysmal nocturnal hemoglobinuria (PNH) have blood cells deficient in glycosyl phosphatidylinositol (GPI)-linked proteins owing to a somatic mutation in the X-linked PIGA gene. To target Piga recombination to the erythroid/megakaryocytic lineage in mice, the Cre/loxP system was used, and Cre was expressed under the transcriptional regulatory sequences of GATA-1. Breeding of GATA1-cre (G) transgenic mice with mice carrying a floxed Piga (L) allele was associated with high embryonic lethality. However, double-transgenic (GL) mice that escaped early recombination looked healthy and were observed for 16 months. Flow cytometric analysis of peripheral blood cells showed that GL mice had up to 100% of red cells deficient in GPI-linked proteins. The loss of GPI-linked proteins on the cell surface occurred late in erythroid differentiation, causing a proportion of red cells to express low residual levels of GPI-linked proteins. Red cells with residual expression of GPI-linked proteins showed an intermediate sensitivity toward complement and thus resemble PNH type II cells in patients with PNH. Recombination of the floxed Piga allele was also detected in cultured megakaryocytes, mast cells, and eosinophils, but not in neutrophils, lymphocytes, or nonhematopoietic tissues. In summary, GATA1-Cre causes high-efficiency Piga gene inactivation in a GATA-1-specific pattern. For the first time, mice were generated that have almost 100% of red cells deficient in GPI-linked proteins. These animals will be valuable to further investigate the consequences of GPI-anchor deficiency on erythroid/megakaryocytic cells.

Anuria, omphalocele, and perinatal lethality in mice lacking the CD34-related protein podocalyxin

Podocalyxin is a CD34-related sialomucin that is expressed at high levels by podocytes, and also by mesothelial cells, vascular endothelia, platelets, and hematopoietic stem cells. To elucidate the function of podocalyxin, we generated podocalyxin-deficient (podxl(-/)-) mice by homologous recombination. Null mice exhibit profound defects in kidney development and die within 24 hours of birth with anuric renal failure. Although podocytes are present in the glomeruli of the podxl(-/)- mice, they fail to form foot processes and slit diaphragms and instead exhibit cell--cell junctional complexes (tight and adherens junctions). The corresponding reduction in permeable, glomerular filtration surface area presumably leads to the observed block in urine production. In addition, podxl(-/)- mice frequently display herniation of the gut (omphalocele), suggesting that podocalyxin may be required for retraction of the gut from the umbilical cord during development. Hematopoietic and vascular endothelial cells develop normally in the podocalyxin-deficient mice, possibly through functional compensation by other sialomucins (such as CD34). Our results provide the first example of an essential role for a sialomucin in development and suggest that defects in podocalyxin could play a role in podocyte dysfunction in renal failure and omphalocele in humans.

Unavailability of CD147 leads to selective erythrocyte trapping in the spleen

Adhesive interactions with stromal cells and the extracellular matrix are essential for the differentiation and migration of hematopoietic progenitors. In the erythrocytic lineage, a number of adhesion molecules are expressed in the developing erythrocytes and are thought to play a role in the homing and maturation of erythrocytic progenitors. However, many of these molecules are lost during the final developmental stages leading to mature erythrocytes. One of the adhesion molecules that remains expressed in mature, circulating erythrocytes is CD147. This study shows that blockade of this molecule on the cell surface by treatment with F(ab')(2) fragments of anti-CD147 monoclonal antibody disrupts the circulation of erythrocytes, leading to their selective trapping in the spleen. Consequently, mice develop an anemia, and de novo, erythropoietin-mediated erythropoiesis in the spleen. In contrast, these changes were not seen in mice similarly treated with another antierythrocyte monoclonal antibody with a different specificity. These results suggest that the CD147 expressed on erythrocytes likely plays a critical role in the recirculation of mature erythrocytes from the spleen into the general circulation. (Blood. 2001;97:3984-3988)

Transcripts of Fliz1, a nuclear zinc finger protein, are expressed in discrete foci of the murine fetal liver

The origin and expansion of hematopoietic progenitor and stem cells during fetal development and their differentiation into mature effector cells are thought to be driven by the activation of developmental stage- and cell-lineage-specific genes. To gain further insight into the molecular mechanisms regulating the expansion and differentiation of fetal hematopoietic progenitor and stem cells, we performed differential display RT-PCR analysis on fractionated murine E12 fetal liver cells. We identified a novel transcript predicted to encode a protein of 305 amino acids with a calculated molecular mass of 35 kDa, containing a charged domain and three putative C(3)H-type zinc fingers. The fetal liver zinc-finger protein 1 (Fliz1) transcript is approximately 1.8 kb and is variably expressed both during embryogenesis and in adult tissues. Fliz1 expression was detected in discrete cell foci in the fetal liver and in LIN(-)/ckit(+) cells. Nuclear localization studies revealed that Fliz1 is targeted to the nucleus. Thus, Fliz1 is a newly identified nuclear protein expressed in hematopoietic progenitor cells of the developing fetal liver.

A developmental transition in definitive erythropoiesis: erythropoietin expression is sequentially regulated by retinoic acid receptors and HNF4

The cytokine erythropoietin (Epo) promotes erythropoietic progenitor cell proliferation and is required for erythropoietic differentiation. We have found that the Epo gene is a direct transcriptional target gene of retinoic acid signaling during early erythropoiesis (prior to embryonic day E12.5) in the fetal liver. Mouse embryos lacking the retinoic acid receptor gene RXR alpha have a morphological and histological phenotype that is comparable with embryos in which the Epo gene itself has been mutated, and flow cytometric analysis indicates that RXR alpha-deficient embryos are deficient in erythroid differentiation. Epo mRNA levels are reduced substantially in the fetal livers of RXR alpha(-/-) embryos at E10.25 and E11.25, and genetic analysis shows that the RXR alpha and Epo genes are coupled in the same pathway. We furthermore show that the Epo gene is retinoic acid inducible in embryos, and that the Epo gene enhancer contains a DR2 sequence that represents a retinoic acid receptor-binding site and a retinoic acid receptor transcriptional response element. However, unlike Epo-deficient embryos that die from anemia, the erythropoietic deficiency in RXR alpha(-/-) embryos is transient; Epo mRNA is expressed at normal levels by E12.5, and erythropoiesis and liver morphology are normal by E14.5. We show that HNF4, like RXR alpha a member of the nuclear receptor family, is abundantly expressed in fetal liver hepatocytes, and is competitive with retinoic acid receptors for occupancy of the Epo gene enhancer DR2 element. We propose that Epo expression is regulated during the E9.5--E11.5 phase of fetal liver erythropoiesis by RXR alpha and retinoic acid, and that expression then becomes dominated by HNF4 activity from E11.5 onward. This transition may be responsible for switching regulation of Epo expression from retinoic acid control to hypoxic control, as is found throughout the remainder of life.

Role of natural killer cells in resistance against friend retrovirus-induced leukemia

We have previously shown that immunization with a synthetic peptide that contains a single CD4(+) T-cell epitope protects mice against immunosuppressive Friend retrovirus infection. Cells producing infectious Friend virus were rapidly eliminated from the spleens of mice that had been immunized with the single-epitope peptide. However, actual effector mechanisms induced through T-helper-cell responses after Friend virus inoculation were unknown. When cytotoxic effector cells detected in the early phase of Friend retrovirus infection were separated based on their expression of cell surface markers, those lacking CD4 and CD8 but expressing natural killer cell markers were found to constitute the majority of effector cells that lysed Friend virus-induced leukemia cells. Depletion of natural killer cells by injecting anti-asialo-ganglio-N-tetraosylceramide antibody did not affect the number of CD4(+) or CD8(+) T cells in the spleen, virus antigen-specific proliferative responses of CD4(+) T cells, or cytotoxic activity against Friend virus-induced leukemia cells exerted by CD8(+) effector cells. However, the same treatment markedly reduced the killing activity of CD4(-) CD8(-) effector cells and completely abolished the effect of peptide immunization. Although the above enhancement of natural killer cell activity in the early stage of Friend virus infection was also observed in mice given no peptide, these results have demonstrated the importance and requirement of natural killer cells in vaccine-induced resistance against the retroviral infection.

Immature multipotent hemopoietic progenitors lacking long-term bone marrow-reconstituting activity in the aorta-gonad-mesonephros region of murine day 10 fetuses

Previous studies indicated that multipotent progenitors exist in early fetuses that do not contain long-term reconstituting (LTR) activity. However, it remained unclear whether these multipotent progenitors are committed to the hemopoietic lineage or are immature mesodermal cells or hemangioblasts. In this study, we have succeeded in enriching the multipotent progenitors that are capable of generating myeloid, T, and B cells in the LFA-1(-) subpopulation of TER-119(-)c-kit(+)CD45(+) cells from the aorta-gonad-mesonephros (AGM) region of day 10 fetuses. We found that these day 10 AGM LFA-1(-) cells do not show the LTR activity, whereas day 11 AGM LFA-1(-) cells do have such an activity. These results strongly suggest that multipotent progenitors lacking LTR activity emerge as CD45(+) hemopoietic progenitor cells in the AGM region on the 10th day of gestation, and such p-Multi mature into hemopoietic stem cells by acquiring LTR activity.