Improving the soluble expression and purification of recombinant human stem cell factor (SCF) in endotoxin-free Escherichia coli by disulfide shuffling with persulfide

We here present a new method for the expression and purification of recombinant human stem cell factor (rhSCF(164)) in endotoxin-free ClearColi(®) BL21(DE3) cells harboring codon-optimized Profinity eXact™-tagged hSCF cDNA. Previously, we demonstrated that co-expression with thioredoxin increased the solubility of rhSCF in Escherichia coli BL21(DE3), and addition of l-arginine enhanced chromatography performance by removing the endotoxin-masked surface of rhSCF. Initially, we tried to express rhSCF in an endotoxin-free strain using a thioredoxin co-expression system, which resulted in significantly lower expression, possibly due to the stress imposed by overexpressed thioredoxin or antibiotics susceptibility. Therefore, we developed a new expression system without thioredoxin. External redox coupling was tested using persulfides such as glutathione persulfide or cysteine persulfide for the in vivo-folding of hSCF in the cytoplasm. Persulfides improved the protein solubility by accelerating disulfide-exchange reactions for incorrectdisulfides during folding in E. coli. Furthermore, the persulfides enhanced the expression level, likely due to upregulation of the enzymatic activity of T7 RNA polymerase. The recombinant protein was purified via affinity chromatography followed by cleavage with sodium fluoride, resulting in complete proteolytic removal of the N-terminal tag. The endotoxin-free fusion protein from ClearColi(®) BL21(DE3) could bind to the resin in the standard protocol using sodium phosphate (pH 7.2). Furthermore, purified rhSCF enhanced the proliferation and maturation of the human mast cell line LAD2. Thus, we conclude that use of the protein expression system employing E. coli by disulfide shuffling with persulfide addition could be a very useful method for efficient protein production.

Plant-produced human recombinant erythropoietic growth factors support erythroid differentiation in vitro

Clinically available red blood cells (RBCs) for transfusions are at high demand, but in vitro generation of RBCs from hematopoietic stem cells requires significant quantities of growth factors. Here, we describe the production of four human growth factors: erythropoietin (EPO), stem cell factor (SCF), interleukin 3 (IL-3), and insulin-like growth factor-1 (IGF-1), either as non-fused proteins or as fusions with a carrier molecule (lichenase), in plants, using a Tobacco mosaic virus vector-based transient expression system. All growth factors were purified and their identity was confirmed by western blotting and peptide mapping. The potency of these plant-produced cytokines was assessed using TF1 cell (responsive to EPO, IL-3 and SCF) or MCF-7 cell (responsive to IGF-1) proliferation assays. The biological activity estimated here for the cytokines produced in plants was slightly lower or within the range cited in commercial sources and published literature. By comparing EC50 values of plant-produced cytokines with standards, we have demonstrated that all four plant-produced growth factors stimulated the expansion of umbilical cord blood-derived CD34+ cells and their differentiation toward erythropoietic precursors with the same potency as commercially available growth factors. To the best of our knowledge, this is the first report on the generation of all key bioactive cytokines required for the erythroid development in a cost-effective manner using a plant-based expression system.

A novel thrombopoietin–stem-cell factor fusion protein possesses enhanced potential in stimulating megakaryocyte proliferation and differentiation

TPO (thrombopoietin) and SCF (stem-cell factor) are functionally related cytokines with overlapping but distinct haematopoietic effects. In the present study, a novel TPO-SCF fusion protein that combined the complementary biological effects of TPO and SCF into a single molecule was expressed in, and purified from, Sf9 [Spodoptera frugiperda (fall armyworm)] insect cells. The specific activity of rhTPO (recombinant human TPO)-SCF in megakaryoblastic Mo7e cell proliferation assays was 2.90+/-0.35 x 10(7) units/micromol, approx. 1.7 times as high as that of rhTPO. The specific activity of rhTPO-SCF in TF-1 cells proliferation assays was 7.10+/-0.95 x 10(6) units/micromol, approx. 1.2 times as high as that of rhSCF (recombinant human SCF). In a megakaryocyte-colony-forming assay using human peripheral-blood CD34(+) cells, the SCF moiety of rhTPO-SCF worked in a synergistic way to augment the colony number and exhibited a higher potential to stimulate megakaryocyte colony growth. According to the results of EMSA (electrophoretic mobility-shift assay) and semi-quantitative RT (reverse transcriptase)-PCR, the synergistic effects of the SCF moiety were also reflected in increased STAT5 (signal transducer and activator of transcription 5) DNA binding and enhanced up-regulation of p21 expression in Mo7e cells treated by rhTPO-SCF, suggesting that rhTPO-SCF could be more potent in promoting megakaryocyte proliferation and differentiation.

Expression, renaturation and simultaneous purification of recombinant human stem cell factor in Escherichia coli

Recombinant human stem cell factor (rhSCF) was produced as an inclusion body by Escherichia coli DH5alpha grown in a 5 l fermentor. Inclusion bodies of rhSCF were purified and solubilized in urea solution, then renatured with simultaneous purification using a high performance hydrophobic interaction chromatographic (HPHIC) squat column. The refolded rhSCF had a purity of 94% and a bioactivity of 1.2 x 10(6 )IU mg(-1)of rhSCF protein. The method described is fast and simple to implement.

[Cloning, expression and purification of human stem cell growth factor cDNA and its species-specificity in hematopoiesis]

Stem cell growth factor (SCGF) is an early-acting hematopoitic cytokine that has two isoforms including hSCGF with full length molecules and hSCGFbeta, 78 amino acids of which lost in the conserved calcium-dependent carbohydrate-recognition domain (CRD). It has been demonstrated that hSCGFbeta is strictly species-specific in regulating he-matopoiesis. This study was aimed to explore whether human SCGF can exert synergistic stimulatory effect on heterogenous murine CFU-GM progenitor. Firstly, hSCGF cDNA was amplified from human fetal liver cDNA library by using two-step PCR. The hSCGF mature peptide coding sequence was subsequently placed at downstream of glutathione S-transferase (GST) sequence in GST gene fusion expression vector. The results indicated that there existed an additional 60 kD protein compared with mock BL21 when the cells hosting recombinant plasmid were induced with IPTG at 37 degrees C. SDS-PAGE analysis demonstrated that the GST-hSCGF fusion protein mainly existed in insoluble form. When induced at low temperature (28 degrees C), the recombinant protein was mostly soluble. The GST-fusion recombinant protein was subsequently purified by using affinity chromatography. The clonogenic assay revealed that, unlike hSCGFbeta, hSCGF had the granulocyte/macrophage promoting activity (GPA) for murine bone marrow GM progenitor. It is concluded that, in contrast to human SCGFbeta, the intact molecular hSCGF may have no species specificity, implying that CRD domain in human SCGFbeta does not directly bind to corresponding SCGF receptor, but may have certain biological function.

Expression, purification, and characterization of a novel recombinant fusion protein, rhTPO/SCF, in Escherichia coli

Thrombopoietin (TPO) is the principal regulatory cytokine of megakaryopoiesis and thrombopoiesis and promotes all aspects of megakaryocyte development. Stem cell factor (SCF) is mainly a pleiotropic cytokine acting on hematopoiesis by promoting the survival and proliferation of hematopoietic stem cells and has a potent synergistic effect on megakaryopoiesis in the presence of TPO. Here, we report the construction, expression, and purification of a novel recombinant human thrombopoietin/stem cell factor (rhTPO/SCF) fusion protein, which consists of a truncated human thrombopoietin (1-157 a.a.) plus a truncated human stem cell factor (1-145 a.a.), linked by a peptide (GGGGSPGGSGGGGSGG). The TPO/SCF gene was cloned into the Escherichia coli expression vector pET28a and expressed in BL21(DE3) strain. The rhTPO/SCF constituted up to 6% of the total bacterial protein. Co-expression with E. coli chaperones, Trigger Factor (TF) and GroES/GroEL, and lowering cultivation temperature cooperatively improved the solubility of expressed rhTPO/SCF, resulting in about fourfold increase in the yield soluble rhTPO/SCF. The rhTPO/SCF was purified to homogeneity using anion exchange followed by metal affinity chromatography. Western blot analysis confirmed the identity of the purified protein. rhTPO/SCF stimulated a dose-dependent cell proliferation in both TF1 and Mo7e cell lines.

Expression of a novel recombinant dual human stem cell factor in insect cells

Stem cell factor (SCF) is a hematopoietic cytokine that promotes the survival, proliferation, and differentiation of hematopoietic cells. A dual human stem cell factor (dhSCF) cDNA was constructed, which consisted of a full-length human stem cell factor cDNA plus a truncated hSCF cDNA (1-145aa), linked by a peptide (GGGGSGGGGSGG) coding region. The dhSCF gene was cloned into baculovirus transfer vector pAcSecG2T under the control of polyhedrin promoter. The Sf9 cells infected with the recombinant virus expressed rdhSCF up to 6000 U/10(6) cell in flask and 8300 U/10(6) cell in spinner flask. The rdhSCF was purified by two-step chromatography. The molecular mass of rdhSCF was examined by western blotting and HPLC analysis. The specific activity of rdhSCF was up to 3.1x10(6) U/mg, about 8.7 times as high as that of monomer rhSCF from Escherichia coli.

A novel recombinant dual human SCF expressed in and purified from silkworm, Bombyx mori, possesses higher bioactivity than recombinant monomeric human SCF

A novel recombinant dual human stem cell factor (rdhSCF) gene was constructed which consisted of a full-length hSCF cDNA plus a truncated hSCF cDNA (1-145 aa), linked by a peptide (GGGGSGGGGSGG) coding region. The rdhSCF gene was cloned into baculovirus transfer vector pAcSecG2T under the polyhedrin promoter control. Silkworm larvae infected with the recombinant virus expressed rdhSCF up to 15,800 units/mL in haemolymph. The specific activity of rdhSCF purified from the haemolymph was up to 3.0 x 10(6) units/mg, about 8.6 times as high as that of monomer rhSCF from Escherichia coli, and about 9.1 times as high as that of monomer rhSCF from insect cell. The binding affinity of rdhSCF to the cell surface receptor was higher than that of monomer rhSCF.

Production of a biologically active recombinant marsupial growth factor using the methylotrophic yeast Pichia pastoris

The cytokine stem cell factor (SCF) and its interaction with its receptor c-kit plays an important role in the development of germ cells in eutherians. To investigate the putative roles of the SCF/c-kit system in marsupials, recombinant Australian brushtail possum (Trichosurus vulpecula) SCF was purified after secretion by the methylotrophic yeast Pichia pastoris. The purification procedure utilized Ni2+ affinity chromatography with a poly-histidine tag engineered onto the C-terminus of the recombinant SCE The recombinant possum SCF had a molecular weight of 48 kDa, as determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, and was biologically active with respect to its ability to maintain and induce proliferation of marsupial primordial germ cells in vitro. Furthermore, the recombinant possum SCF stimulated proliferation of the cell line TF1 and this bioactivity could be inhibited using an antibody directed against recombinant mouse SCF. This source of biologically active marsupial SCF may prove useful in future studies of marsupial development.

Identification of leukemia inhibitory factor as a potent mast cell growth-enhancing factor produced by mouse keratinocyte cell line, KCMH-1

Inoculation of KCMH-1 cells, a keratinocyte-derived cell line established from a chemically induced skin tumor, into the skin of mice results in accumulation of mast cells around the resulting tumors. The conditioned medium of KCMH-1 cells enhances the growth of mast cells in vitro when they are cultured in the presence of NIH/3T3 fibroblasts, suggesting an important role for keratinocytes in mast cell hyperplasia in the skin. The aim of this study was to identify this mast cell growth-enhancing factor (MCGEF) by screening a KCMH-1 cDNA library. We first established a polyclonal antibody raised against the partially purified factor obtained from KCMH-1-conditioned medium which neutralized the MCGEF activity in KCMH-1-conditioned medium. Expression cloning of 1 x 10(6) cDNAs from the KCMH-1 cDNA library led to 16 cDNAs. One of these cloned cDNAs was found to be leukemia inhibitory factor (LIF). Both LIF produced by COS cells and the recombinant protein obtained commercially showed MCGEF activity when added to mast cell/fibroblast cocultures. MCGEF activity in KCMH-1-conditioned medium was completely neutralized by an anti-LIF monoclonal antibody. These results suggest that MCGEF produced by KCMH-1 cells is identical to LIF.

Expression and purification of biologically active recombinant quail stem cell factor in E. coli

Stem cell factor (SCF) is a multifunctional cytokine involved in hematopoiesis, melanogenesis and gametogenesis. Previous studies have demonstrated that avian SCF is a requirement for the proliferation and survival of various cell types in vivo and in vitro. In the current study, recombinant quail stem cell factor was produced in Escherichia coli using a prokaryotic expression system. SCF was expressed as a fusion protein with a histidine hexamer tag at the N-terminal end of the protein. Following expression, the protein was purified by affinity chromatography on the Ni-NTA column. The uninduced and induced protein lysates and the purified protein were separated by SDS-PAGE and transferred onto nitrocellulose membrane. Western blot analysis with the monoclonal antibody to the histidine tag identified SCF in the induced cell lysates and the purified sample. The recombinant SCF was approximately 22-23 kD in size. This protein was generated devoid of the signal peptide, the transmembrane domain, and the intracellular domain and, hence, resembles the soluble form of SCF. Biological activity was assayed using the in vitro survival of E12 chicken dorsal root ganglion-derived sensory neurons. The addition of recombinant quail SCF improved neuronal survival. Survival (20.6%) was the highest at the 50 ng/ml concentration of SCF. The availability of quail SCF will be a valuable tool to further resolve the function of stem cell factor in birds.

Stage-specific expression of the Kit receptor and its ligand (KL) during male gametogenesis in the mouse: a Kit-KL interaction critical for meiosis

The Kit receptor and its ligand KL, which together constitute an essential effector at various stages of embryonic development, are both present during adult gametogenesis. In the testis, KL is expressed in Sertoli cells, and Kit in germ cells, starting at the premeiotic stages. A series of observations indicated previously a role in spermatogonia survival, without excluding a possible function at later stages. We identified a complex pattern of expression of the two components in the adult murine testis, suggestive of a role in the meiotic progression of spermatocytes. At stages VII-VIII of the cycle of the seminiferous epithelium, the time when spermatocytes enter meiosis, the membrane-associated form of KL extends on the Sertoli cell from the peripheral to the adluminal compartment of the tubule. We also found that the receptor is present on the surface of germ cells up to the pachytene stage. The availability of differentiated Sertoli cell lines, which express the KL protein and support part of the maturation of germ cells in coculture, allowed us to ask whether, in the in vitro reconstructed system, transit of spermatocytes through meiosis requires the Kit-KL interaction. Addition of a blocking monoclonal antibody against the Kit receptor (ACK2) inhibited extensively the appearance of haploid cells and the expression of a haploid-phase-specific gene (Prm1). Recognition of the supporting Sertoli cell by germ cells was not affected, indicating a requirement for the activity of the receptor for either entering or completing meiosis. Involvement of the membrane-associated form of the ligand was suggested by the observation that addition of the soluble form of KL was equally inhibitory.

Selective deamidation of recombinant human stem cell factor during in vitro aging: isolation and characterization of the aspartyl and isoaspartyl homodimers and heterodimers

During in vitro aging, deamidation of recombinant human stem cell factor produced in Escherichia. coli was detected by HPLC analysis and by the release of soluble ammonia. The deamidation rate is very slow in buffers at low pH or at low temperatures; however, the rate is significantly accelerated in alkaline buffers such as sodium bicarbonate in combination with elevated temperatures. HPLC isolation of various deamidated forms followed by peptide mapping and mass spectrometric analyses revealed that the deamidation involves Asn10 in the sequence -T9NNV- near the N-terminus of the protein. Following peptide mapping analysis, significant amounts of aspartyl and isoaspartyl peptides were identified, indicating the conversion of asparagine into both aspartate and isoaspartate residues. As a result of spontaneous association-dissociation of stem cell factor dimer, a total of five deamidated forms, including two homodimers and three heterodimers, were detected and isolated. Cell proliferation assays showed that two rhSCF heterodimeric species, derived from dimerization between isoaspartyl and other stem cell factor monomers, retain only approximately half of the biological activity. The homodimer with isoaspartic acid in place of Asn10 is 50-fold less potent, while the aspartyl homodimer, either isolated during deamidation experiments or recombinantly prepared by site-directed mutagenesis (e.g., N10D and N10D/N11D variants), exhibits higher activity than the standard molecule. In comparison, synthetic N10A and N10E variants, though missing the deamidation site, are significantly less active. All these variants lacking the Asn10 deamidation site are relatively more stable than those containing the asparagine residue. The results indicate that the biological function and chemical stability of stem cell factor are influenced by the nature of the residue at position 10.

Stem cell factor stimulates chicken osteoclast activity in vitro

Stem cell factor (SCF) is a polypeptide growth factor active on multiple cell types, mainly of hematopoietic origin. We studied the effects of avian SCF on the differentiation of chicken osteoclasts from their putative progenitors as well as on the bone-resorbing activity of terminally differentiated osteoclasts. Osteoclast formation was analyzed in long-term cocultures of osteoblasts and nonadherent, osteoclast-depleted bone marrow cells. Osteoclast activity was studied in short-term (48 h) cultures of bone marrow cell populations enriched for osteoclasts, on dentine slices. SCF strongly enhanced osteoclast differentiation. The IL-6-related chicken myelomonocytic growth factor (cMGF) had a similar effect, and the effects of SCF and cMGF were additive. SCF, but not cMGF, also stimulated the bone-resorbing activity of existing osteoclasts. As osteoblasts have been found to regulate osteoclast activity and formation, chicken osteoblasts were tested for their ability to express and secrete SCF. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed that osteoblasts express SCF mRNA and that parathyroid hormone increases expression levels about fourfold. SCF did not accumulate in the culture medium, but remained cell (osteoblasts) surface associated.

Granulocyte-colony stimulating factor and stem cell factor are the crucial factors in long-term culture of human primitive hematopoietic cells supported by a murine stromal cell line

The findings that murine marrow stromal cell line MS-5 supported the proliferation of human lineage-negative (Lin-) CD34+CD38- bone marrow cells in long-term culture have been reported. In this study, we analyzed this proliferating activity of MS-5-conditioned medium (CM) on human primitive hematopoietic cells. When Lin-CD34+CD38- cells of normal human cord blood cells were co-cultured with MS-5, colony forming cells (CFCs) were maintained over 7 weeks in vitro. Prevention of contact between MS-5 and Lin-CD34+CD38- cells by using membrane filter (0.45 micron) was negligible for this activity. This indicated that the activity of MS-5 on human primitive hematopoietic cells is a soluble factor(s) secreted from MS-5, which is not induced by the contact between MS-5 and Lin-CD34+CD38- cells. We tried to purify this soluble activity. An active material with a molecular weight of about 150 kDa, determined by gel filtration chromatography, solely supported the growth of Lin-CD34+CD38- cells and Mo7e, a human megakaryocytic cell line. This activity not only reacted with anti-mouse stem cell factor (mSCF) antibody on Western blots, but it was also neutralized in the presence of anti-mSCF antibody. Another active material with a molecular weight of about 20-30 kDa synergized with mSCF to stimulate the growth of Lin-CD34+CD38- cells but failed to do so alone, although this synergy was inhibited in the presence of soluble mouse granulocyte-colony stimulating factor (mG-CSF) receptor, which is a chimeric protein consisting of the extracellular domain of mG-CSF receptor and the Fe region of human IgG1. In addition, the latter molecule supported the growth of the G-CSF dependent cell line FD/GR3, which is a murine myeloid leukemia cell line, FDC-P2, transfected with mG-CSF receptor cDNA. Adding of anti-mSCF antibody and soluble mG-CSF receptor to the culture completely abrogated the activity of MS-5-CM. Recombinant (r) mSCF and rmG-CSF had synergistic activity on the growth of Lin-CD34+CD38- cells. These results indicated that the activity on Lin-CD34+CD38- cells included in MS-5-CM is based upon the synergistic effects of mSCF and mG-CSF.

Refolding and oxidation of recombinant human stem cell factor produced in Escherichia coli

Oxidative folding of recombinant human stem cell factor (rhSCF) produced in Escherichia coli was investigated in vitro. Folding of denatured and reduced rhSCF involves at least five intermediate forms, I-1 to I-5, detectable by their differences in hydrophobicity using reverse-phase high performance liquid chromatography. Both I-1 and I-2 contain a native-like disulfide bond, Cys4-Cys89 and Cys43-Cys138, respectively, and I-3 forms a mispaired disulfide, Cys43-Cys89. These forms appear to reach steady state equilibrium and are important folding intermediates. I-1 was found to be the prominent intermediate that directly folds into native rhSCF (N); and the thermodynamically less stable I-2 favors rearrangment into I-1. I-3 may serve as an intermediate for disulfide rearrangment between I-1 and I-2. I-4 and I-5, which are disulfide-linked dimers, are in equilibrium with reduced rhSCF and other intermediates and may not play an important role in rhSCF folding. Both trifluoroacetic acid-trapped I-1 and I-2, after isolation by high performance liquid chromatography, proceed with the remaining oxidative folding process after reconstitution. Iodoacetate-trapped I-1 and I-2 contain low alpha-helical content and some tertiary structure, while I-3 and reduced rhSCF have little ordered structure. Gel filtration/light-scattering experiments indicate that reduced rhSCF and iodoacetate-trapped I-1, I-2, and I-3 exist as dimeric forms, indicating that rhSCF dimerization precedes formation of disulfide bonds. I-1, I-2, I-3, and the C43,138A analog lacking Cys43-Cys138 bond are not biologically active or exhibit significantly lower activity. The two disulfide bonds in rhSCF seem to be essential for the molecule to maintain an active conformation required for its receptor binding and biological activities.

Isolation and characterization of a disulfide-linked human stem cell factor dimer. Biochemical, biophysical, and biological comparison to the noncovalently held dimer

Distinct from the noncovalently linked recombinant human stem call factor (rhSCF) dimer, we report here the isolation and identification of an SDS-nondissociable dimer produced during folding/oxidation of rhSCF. Experimental evidence using various cleavage strategies and analyses shows that the isolated dimer is composed of two rhSCF monomers covalently linked by four disulfide bonds. The cysteines are paired as in the noncovalently associated dimer except that all pairings are intermolecular rather than intramolecular. Other structural models, involving intertwining of intramolecular disulfide loops, are ruled out. The molecule behaves similarly to the noncovalently associated dimer during ion-exchange or gel permeation chromatography. However, the disulfide-linked dimer exhibits increased hydrophobicity in reverse-phase columns and in the native state does not undergo spontaneous dimer dissociation-association as seen for the noncovalent dimer. Spectroscopic analyses indicate that the disulfide-linked and noncovalently associated rhSCF dimers have grossly similar secondary and tertiary structures. In vitro, the disulfide-linked dimer exhibits approximately 3-fold higher biological activity in supporting growth of a hematopoietic cell line and stimulating hematopoietic cell colony formation from enriched human CD34+ cells. The molecule binds to the rhSCF receptor, Kit, with an efficiency only half that of the noncovalently associated dimer. Formation of intermolecular disulfides in the disulfide-linked dimer with retention of biological activity has implications for the three-dimensional structure of noncovalently held dimer and disulfide-linked dimer.

Purification and activity of recombinant chicken stem cell factor produced by using a baculovirus vector

Stem cell factor is a pleiotropic cytokine that plays an essential role in the development of hematopoietic cells, germ cells, and melanocytes. To obtain recombinant soluble chicken stem cell factor (chSCF), a baculovirus containing the cDNA encoding chSCF polypeptide from amino acids -25 to 170 was constructed. The chSCF produced in insect cells infected with the virus was purified by ion exchange column chromatography. The ability of the purified protein to induce the outgrowth of neurites from chicken dorsal root ganglia cultured in vitro was demonstrated.

The new generation of recombinant human hematopoietic cytokines

In the past year, the most exciting development in the field of hematopoietic growth factors has been the identification of the platelet-inducing factor Mpl ligand. Administration of recombinant Mpl ligand may alleviate the potential for hemorrhagic complications following cancer therapies. Stem cell factor continues to be studied clinically in the mobilization of peripheral blood cells for transplantation.