Molecular cloning and expression of the murine interleukin-5 receptor

Murine interleukin-5 (IL-5) is known to play an essential role in Ig production of B cells and proliferation and differentiation of eosinophils. Here, we have isolated cDNA clones encoding a murine IL-5 receptor by expression screening of a library prepared from a murine IL-5 dependent early B cell line. A cDNA library was expressed in COS7 cells and screened by panning with the use of anti-IL-5 receptor monoclonal antibodies. The deduced amino acid sequence analysis demonstrates that the receptor is a glycoprotein of 415 amino acids (Mr 45,284), including an N-terminal hydrophobic region (17 amino acids), a glycosylated extracellular domain (322 amino acids), a single transmembrane segment (22 amino acids) and a cytoplasmic tail (54 amino acids). COS7 cells transfected with the cDNA expressed a 60 kd protein that bound IL-5 with a single class of affinity (KD = 2-10 nM). FDC-P1 cells transfected with the cDNA for murine IL-5 receptor showed the expression of IL-5 binding sites with both low (KD = 6 nM) and high affinity (KD = 30 pM) and acquired responsiveness to IL-5 for proliferation, although parental FDC-P1 cells did not show any detectable IL-5 binding. In addition, several cDNA clones encoding soluble forms of the IL-5 receptor were isolated. Northern blot analysis showed that two species of mRNAs (5.0 kb and 5.8 kb) were detected in cell lines that display binding sites for murine IL-5. Homology search for the amino acid sequence of the IL-5 receptor reveals that the IL-5 receptor contains a common motif of a cytokine receptor family that is recently identified.     

A cytosolic domain of the erythropoietin receptor contributes to endoplasmic reticulum-associated degradation.

The erythropoietin receptor (EPO-R) is the cellular target for erythropoietin (EPO), the primary hormone that mediates the proliferation of immature erythroblasts and their differentiation into mature erythrocytes. Unusual features of the EPO-R are its short half-life (t(1/2) 1-2 h), its degradation via multiple pathways and the fact that less than 1% of total cellular EPO-R molecules are found on the cell surface. The contribution of EPO-R structural determinants to the regulation of its intracellular metabolism is still unclear. The epidermal growth factor receptor (EGF-R), unlike the EPO-R, is efficiently transported to the cell surface and displays a much longer metabolic half-life. To determine which EPO-R cytosolic domains are involved in intracellular degradation, we studied chimeric receptor molecules constructed of EGF-R extracellular and transmembrane parts, linked to the full length or truncated cytosolic part of the EPO-R. The chimeras were expressed in transiently transfected COS 7 cells and stably expressed in Ba/F3 cells. Our experiments indicate that the cytosolic part of the EPO-R contains determinants that mark it for rapid degradation, in association with the endoplasmic reticulum (ER). This degradation was insensitive to brefeldin A and was inhibited by specific proteasomal inhibitors. A truncated EGF-R/EPO-R chimera containing only 50 amino acids of the EPO-R membrane-proximal cytosolic part was also rapidly degraded suggesting that these 50 amino acids are involved in receptor degradation.     

Expression cloning of a receptor for human granulocyte-macrophage colony-stimulating factor.

Two cDNA clones encoding a receptor for human granulocyte-macrophage colony-stimulating factor (hGM-CSF-R) were isolated by expression screening of a library made from human placental mRNA. Pools of recombinant plasmid DNA were electroporated into COS cells which were then screened for their capacity to bind radioiodinated hGM-CSF using a sensitive microscopic autoradiographic approach. The cloned GM-CSF-R precursor is a 400 amino acid polypeptide (Mr 45,000) with a single transmembrane domain, a glycosylated extracellular domain and a short (54 amino acids) intracytoplasmic tail. It does not contain a tyrosine kinase domain nor show homology with members of the immunoglobulin super gene family, but does show some significant sequence homologies with receptors for several other haemopoietic growth factors, including those for interleukin-6, erythropoietin and interleukin-2 (beta-chain) and also to the prolactin receptor. When transfected into COS cells the cloned cDNA directed the expression of a GM-CSF-R showing a single class of affinity (KD = 2(-8) nM) and specificity for human GM-CSF but not interleukin-3. Messenger RNA coding for this receptor was detected in a variety of haemopoietic cells known to display hGM-CSF binding, and cross-linking experiments revealed a similar size for the glycosylated receptors in transfected COS and haemopoietic cells.     

The cytoplasmic region of the erythropoietin receptor contains nonoverlapping positive and negative growth-regulatory domains.

The erythropoietin (EPO) receptor (EPO-R), a member of a large cytokine receptor superfamily, has a 236-amino-acid cytoplasmic region which contains no obvious tyrosine kinase or other catalytic domain. In order to delineate the linear functional domains of the cytoplasmic tail, we generated truncated mutant cDNAs which were transfected into a murine interleukin-3-dependent cell line, Ba/F3, and the EPO-dependent growth characteristics of the stable transfectants were assayed. We identified two unique domains of the cytoplasmic tail. A membrane-proximal positive signal transduction domain of less than or equal to 103 amino acids, in a region highly similar to the interleukin-2 receptor beta chain, was sufficient for EPO-mediated signal transduction. A carboxy-terminal negative-control domain, a serine-rich region of approximately 40 amino acids, increased the EPO requirement for the Ba/F3 transfectants without altering EPO-R cell surface expression, affinity for EPO, receptor oligosaccharide processing, or receptor endocytosis. Truncation of this negative-control domain allowed the Ba/F3 transfectants to grow maximally in only 1 pM EPO, 1/10 the concentration required for growth of cells expressing the wild-type EPO-R. All truncated EPO-R mutants which retained the transmembrane region of the EPO-R polypeptide bound to the gp55 envelope protein of Friend spleen focus-forming virus. Only the functional EPO-R mutants were activated by the gp55, however, suggesting that gp55- and EPO-mediated signaling occur via a similar mechanism.     

Isolation of a full-length mouse cDNA clone coding for an immunologically distinct p53 molecule.

Transfection of a cloned p53 gene into a p53 nonproducer Abelson murine leukemia virus-transformed cell line, L12, reconstituted p53 expression. The protein expressed in these cells was indistinguishable from that naturally expressed in p53 producer tumor cells. Conversely, p53 protein expressed in L12-derived clones that were established by transfection with a full-length p53 cDNA clone (pM8) exhibited a discrete immunological form. Immunoprecipitation of p53 with a panel of monoclonal anti-p53 antibodies showed that L12-derived clones that were transfected with the genomic p53 clone contained the same antigenic determinants as those found in the p53 protein expressed in tumor cells. These p53 proteins bound all monoclonal antibody types as well as the polyclonal anti-p53 tested. However, L12-derived clones established by transfection of the p53 cDNA clone (pM8) expressed a p53 protein that bound the RA3-2C2 and PAb200.47 anti-p53 monoclonal antibodies as well as polyclonal anti-p53 serum but totally lacked the antigenic receptor for the PAb122 and PAb421 monoclonal antibodies. The p53 proteins expressed by either genomic or cDNA p53 clones exhibited the same apparent molecular sizes and identical partial peptide maps. We suggest that transfection of the p53 gene induced expression of the entire group of the possible mRNA species, whereas cloned p53 cDNA (pM8) represented a single mRNA molecule that codes for a discrete species of p53 protein.     

Isolation of cDNAs for two distinct human Fc receptors by ligand affinity cloning.

Two cDNA clones encoding different but related receptors for immunoglobulin G constant domains were isolated from cDNA expression libraries by a ligand-mediated selection procedure ('affinity cloning'). Because both of the receptors encoded by the cDNAs react with CDw32 monoclonal antibodies, and both show the appropriate IgG binding affinity, both appear to be forms of the receptor formerly thought to be a single species called FcRII. The extracellular domains encoded by the isolated clones are closely related to the murine IgG2b/1 beta receptor extracellular domains, but the intracellular domains are unrelated. The receptors expressed in COS cells show a preference for IgG1 among IgG subtypes and no affinity for IgM, IgA or IgE. Abundant expression of the RNAs was detected in myeloid cell lines and placenta.     

Active human erythropoietin expressed in insect cells using a baculovirus vector: a role for N-linked oligosaccharide

Biologically active recombinant human erythropoietin has been expressed at high levels in an insect cell background. Expression involved the preparation of a human erythropoietin cDNA, the transfer of this cDNA to the Autographa californica nuclear polyhedrosis virus (AcNPV) genome under the polyhedrin gene promoter, and the subsequent infection of Spodoptera frugiperda cells with recombinant AcNPV. Erythropoietin cDNA was prepared through the expression of the human erythropoietin gene in COS cells using pSV2 and the construction of a COS cell cDNA library in bacteriophage Lambda GT10. Prior to transfer to the AcNPV genome, erythropoietin cDNA isolated from this library was modified at the 3'-terminus in order to replace genomic erythropoietin for SV40 cDNA derived from pSV2. Transfer of this cDNA to AcNPV and the infection of S. frugiperda cells with cloned recombinant virus led to the secretion of erythropoietin: based on bioassay, rates of hormone secretion (over 40 U/ml per h) were 50-fold greater than observed for COS cells. The purified recombinant product possessed full biological activity (at least 200,000 U/mg), but was of lower Mr (23,000) than human erythropoietin produced in COS cells (30,000) or purified from urine (30,000 to 38,000). This difference was attributed to the glycosylation of erythropoietin in S. frugiperda cells with oligosaccharides of only limited size. Further removal of N-linked oligosaccharides from this Mr 23,000 hormone using N-Glycanase yielded an apo-erythropoietin (Mr 18,000) which possessed substantially reduced biological activity. These results indicate that glycosylation, but not the normal processing of oligosaccharides to complex types, is required for the full hormonal activity of human erythropoietin during red cell development.     

A constitutively activated erythropoietin receptor stimulates proliferation and contributes to transformation of multipotent, committed nonerythroid and erythroid progenitor cells.

If the env gene of spleen focus-forming virus (SFFV) is replaced by a cDNA encoding a constitutively active form of the erythropoietin receptor, EPO-R(R129C), the resultant recombinant virus, SFFVcEPO-R, induces transient thrombocytosis and erythrocytosis in infected mice. Clonogenic progenitor cell assays of cells from the bone marrow and spleens of these infected mice suggest that EPO-R(R129C) can stimulate proliferation of committed megakaryocytic and erythroid progenitors as well as nonerythroid multipotent progenitors. From the spleens of SFFVcEPO-R-infected mice, eight multiphenotypic immortal cell lines were isolated and characterized. These included primitive erythroid, lymphoid, and monocytic cells. Some expressed proteins characteristic of more than one lineage. All cell lines resulting from SFFVcEPO-R infection contained a mutant form of the p53 gene. However, in contrast to infection by SFFV, activation of PU.1 gene expression, by retroviral integration, was not observed. One cell line had integrated a provirus upstream of the fli-1 gene, in a location typically seen in erythroleukemic cells generated by Friend murine leukemia virus infection. This event led to increased expression of fli-1 in this cell line. Thus, infection by SFFVcEPO-R can induce proliferation and lead to transformation of nonerythroid as well as very immature erythroid progenitor cells. The sites of proviral integration in clonal cell lines are distinct from those in SFFV-derived lines.     

Erythropoietin receptor Y479 couples to ERK1/2 activation via recruitment of phospholipase Cgamma

Red blood cell development is primarily controlled by erythropoietin (EPO). Several studies have revealed the importance of EPO-R Y343 and Y479 for erythroid cell growth, differentiation, and survival. In order to isolate critical signaling proteins that bind to EPO-R, we initiated a Cloning of Ligand Target (COLT) screen using a murine embryonic day 16 phage library and a biotinylated EPO-R Y343 phosphopeptide. One of the clones isolated encodes Phospholipase C (PLC)gamma1. PLCgamma1 is rapidly tyrosine phosphorylated upon EPO stimulation and associates with EPO-R in an SH2-domain-dependent manner. Although PLCgamma1 bound EPO-R Y343, Y401, Y429, Y431, and Y479 in the COLT screen, PLCgamma1 required Y479 for association with EPO-R in Ba/F3-EPO-R cells. Studies have identified EPO-R Y479 as important for ERK activation. Since PI3-kinase binds EPO-R Y479, one group has suggested that ERK activation downstream of PI3-kinase accounts for the importance of this residue in EPO signaling. However, we show that inhibition of PI3-kinase does not abolish ERK activation. Furthermore, we demonstrate interaction of PLCgamma1 with Grb2 and SOS2. Hence, we have identified a novel adapter function for PLCgamma1 in EPO signaling in which recruitment of PLCgamma1 to EPO-R may lead to activation of the ERK pathway.     

The erythropoietin receptor transmembrane region is necessary for activation by the Friend spleen focus-forming virus gp55 glycoprotein.

The erythropoietin receptor (EPO-R), a member of the cytokine receptor superfamily, can be activated by binding either erythropoietin (EPO) or gp55, the Friend spleen focus-forming virus glycoprotein. The highly specific interaction between gp55 and EPO-R triggers cell proliferation and thereby causes the first stage of Friend virus-induced erythroleukemia. We have generated functional chimeric receptors containing regions of the EPO-R and the interleukin-3 receptor (AIC2A polypeptide), a related cytokine receptor which does not interact with gp55. All chimeric receptors were expressed at similar levels, had similar binding affinities for EPO, and conferred EPO-dependent cell growth. Only those chimeric receptors which contained the EPO-R transmembrane region were activated by gp55. These results demonstrate that the transmembrane region of the EPO-R is critical for activation by gp55. In addition, analysis of a soluble, secreted EPO-R and cysteine point mutants of the EPO-R show that the extracytoplasmic region of the EPO-R specifically interacts with gp55.     

Molecular cloning, sequencing, and expression of mouse ferrochelatase

The cDNA encoding mouse ferrochelatase (protoheme ferrolyase, EC 4.99.1.1) was isolated from a mouse erythroleukemia (MEL) cell cDNA library in lambda gt11 expression vector, by immunoscreening with a polyclonal antibody. Two full-length clones containing cDNA inserts of 2.2 and 2.90 kilobases were obtained. These clones have the same entire enzyme coding region, but alternative putative polyadenylation sites in the 3'-noncoding regions. From the deduced primary structure, a putative leader sequence of 53 amino acid residues resulted in a precursor protein of 420 amino acid residues (Mr 47,130) and a mature protein of 367 residues (Mr 41,692). The cDNA allows for the expression of active ferrochelatase by transfected culture cells. RNA blot analysis showed two species of ferrochelatase mRNA consistent with findings of two polyadenylation sites. Both the mRNAs increased by treatment of the MEL cells with dimethyl sulfoxide. The band pattern of the RNA of the mouse liver was the same as that of the MEL cells. Based on these results, we deduce that ferrochelatase in erythroid and hepatic cells can be only of one type.     

Active human erythropoietin expressed in insect cells using a baculovirus vector: A role for N-linked oligosaccharide

Biologically active recombinant human erythropoietin has been expressed at high levels in an insect cell background. Expression involved the preparation of a human erythropoietin cDNA, the transfer of this cDNA to the Autographa californica nuclear polyhedrosis virus (AcNPV) genome under the polyhedrin gene promoter, and the subsequent infection of Spodoptera frugiperda cells with recombinant AcNPV. Erythropoietin cDNA was prepared through the expression of the human erythropoietin gene in COS cells using pSV2 and the construction of a COS cell cDNA library in bacteriophage Lambda GT10. Prior to transfer to the AcNPV genome, erythropoietin cDNA isolated from this library was modified at the 3′-terminus in order to replace genomic erythropoietin for SV40 cDNA derived from pSV2. Transfer of this cDNA to AcNPV and the infection of S. frugiperda cells with cloned recombinant virus led to the secretion of erythropoietin: based on bioassay, rates of hormone secretion (over 40 U/ml per h) were 50-fold greater than observed for COS cells. The purified recombinant product possessed full biological activity (at least 200000 U/mg), but was of lower M_r (23000) than human erythropoietin produced in COS cells (30000) or purified from urine (30000 to 38000). This difference was attributed to the glycosylation of erythropoietin in S. frugiperda cells with oligosaccharides of only limited size. Further removal of N-linked oligosac-charides from this M_r 23000 hormone using N-Glycanase yielded an apo-erythropoietin (M_r 18000) which possessed substantially reduced biological activity. These results indicate that glycosylation, but not the normal processing of oligosaccharides to complex types, is required for the full hormonal activity of human erythropoietin during red cell development.     

Nucleotide sequence of mouse IL-2 receptor cDNA and its comparison with the human IL-2 receptor sequence.

We have cloned cDNA encoding the mouse interleukin-2 (IL-2) receptor from a murine T cell line, CTLL using human IL-2 receptor cDNA as probe. COS 7 cells transfected with the cDNA expressed the antigen recognized by the monoclonal antibody against the murine IL-2 receptor. The cDNA identified 4 species of mRNA (4.5, 3.5, 2.2 and 1.5 kb) of the mouse IL-2 receptor in CTLL cells. Difference in the length of mRNA seems to be ascribed to the variable length of the 3' untranslated sequence. Total nucleotide sequence (approximately 1400 bp) of this cDNA was determined and compared with the human receptor. The nucleotide and amino acid sequences of the IL-2 receptor are 70% and 60%, respectively, homologous in average between the two species. The comparison has revealed several conserved regions localized to particular exons such as transmembrane and cytoplasmic portions, suggesting that these regions are important for receptor function and its regulation.     

A dominant negative erythropoietin (EPO) receptor inhibits EPO-dependent growth and blocks F-gp55-dependent transformation.

The erythropoietin receptor (EPO-R), a member of the cytokine receptor superfamily, can be activated to signal cell growth by binding either EPO or F-gp55, the Friend spleen focus-forming virus glycoprotein. Activation by F-gp55 results in constitutive EPO-R signalling and the first stage of Friend virus-induced erythroleukemia. We have generated a truncated form of the EPO-R polypeptide [EPO-R(T)] which lacks the critical cytoplasmic signal-transducing domain of the EPO-R required for EPO- or F-gp55-induced cell growth. EPO-R(T) specifically inhibited the EPO-dependent growth of EPO-R-expressing Ba/F3 cells without changing the interleukin-3-dependent growth of these cells. In addition, Ba/F3 cells that coexpressed wild-type EPO-R and EPO-R(T) were resistant to transformation by F-gp55 despite efficient expression of the F-gp55 transforming oncoprotein in infected cells. EPO-R(T) inhibited the EPO-dependent tyrosine phosphorylation of wild-type EPO-R, the tyrosine kinase (JAK2), and the SH2 adaptor protein (Shc). In conclusion, the EPO-R(T) polypeptide is a dominant negative polypeptide which specifically interferes with the early stages of EPO-R-mediated signal transduction and which prevents Friend virus transformation of erythroblasts.     

Expression cloning of a receptor for C5a anaphylatoxin on differentiated HL-60 cells

A cDNA clone encoding the human C5a anaphylatoxin receptor has been isolated by expression cloning from a CDM8 expression library prepared from mRNA of human myeloid HL-60 cells differentiated to the granulocyte phenotype with dibutyryladenosine cyclic monophosphate. The cDNA clone was able to transfer to COS-7 cells the capacity to specifically bind iodinated human recombinant C5a. The cDNA was 2.3 kb long, with an open reading frame encoding a 350-residue polypeptide. Cross-linking of iodinated C5a to the plasma membrane of transfected COS cells revealed a complex with an apparent molecular mass of 52-55 kDa, similar to that observed for the constitutively expressed receptor in differentiated HL-60 cells or human neutrophils. Although differentiated HL-60 cells display a single class of binding sites, with a dissociation constant of approximately 800-900 pM, the C5a-R cDNA, expressed in COS cells, generates both high-affinity (1.7 nM) and low-affinity (20-25 nM) receptors. Sequence comparison established that the degree of sequence identity between the C5a receptor and the N-formylpeptide receptor is 34%.     

Fusion of the erythropoietin receptor and the Friend spleen focus-forming virus gp55 glycoprotein transforms a factor-dependent hematopoietic cell line.

The Friend spleen focus-forming virus (SFFV) gp55 glycoprotein binds to the erythropoietin receptor (EPO-R), causing constitutive receptor signaling and the first stage of Friend erythroleukemia. We have used three independent strategies to further define this transforming molecular interaction. First, using a retroviral selection strategy, we have isolated the cDNAs encoding three fusion polypeptides containing regions of both EPO-R and gp55. These fusion proteins, like full-length gp55, transformed the Ba/F3 factor-dependent hematopoietic cell line and localized the transforming activity of gp55 to its transmembrane domain. Second, we have isolated a mutant of gp55 (F-gp55-M1) which binds, but fails to activate, EPO-R. We have compared the transforming activity of this gp55 mutant with the EPO-R-gp55 fusion proteins and with other variants of gp55, including wild-type polycythemia Friend gp55 and Rauscher gp55. All of the fusion polypeptides and mutant gp55 polypeptides were expressed at comparable levels, and all coimmunoprecipitated with wild-type EPO-R, but only the Friend gp55 and the EPO-R-gp55 fusion proteins constitutively activated wild-type EPO-R. Third, we have examined the specificity of the EPO-R-gp55 interaction by comparing the differential activation of murine and human EPO-R by gp55. Wild-type gp55 had a highly specific interaction with murine EPO-R; gp55 bound, but did not activate, human EPO-R.     

Identification of JAK protein tyrosine kinases as signaling molecules for prolactin. Functional analysis of prolactin receptor and prolactin-erythropoietin receptor chimera expressed in lymphoid cells.

The mechanism of action of prolactin (PRL) was studied in murine lymphoid BAF-3 cells transfected with either the long form of the PRL receptor (PRL-R), or a chimeric receptor consisting of the extracellular domain of the PRL-R and the transmembrane and intracellular domain of the erythropoietin receptor (PRL/EPO-R). PRL sustained normal and long-term proliferation of BAF-3 cells expressing either the PRL-R or the hybrid PRL/EPO-R. Upon [125I]PRL cross-linking, both types of BAF-3 transfectants were shown to express two [125I]PRL cross-linked species differing in size by 20 kDa. These cross-linked complexes, after denaturation, were recognized by antibody against the PRL-R, indicating that they contain the transfected receptor. PRL induced rapid and transient tyrosine phosphorylation of both the PRL-R and the PRL/EPO-R in BAF-3 transfectants. Furthermore, PRL induced rapid tyrosine phosphorylation of the Janus kinase 2 (JAK2) which was already physically associated with the PRL-R or the PRL/EPO-R in the absence of ligand. JAK1 was also associated with PRL-R and PRL/EPO-R in the absence of ligand. However, only in BAF-3 cells expressing the PRL-R does PRL induce rapid and transient tyrosine phosphorylation of JAK1. These results demonstrate that JAK protein tyrosine kinases couple PRL binding to tyrosine phosphorylation and proliferation.