Flt3 ligand structure and unexpected commonalities of helical bundles and cystine knots

Human Flt3 ligand (Flt3L) stimulates early hematopoiesis by activating a type III tyrosine kinase receptor on primitive bone marrow stem cells. The crystal structure of soluble Flt3L reveals that it is a homodimer of two short chain alpha-helical bundles. Comparisons of structure-function relationships of Flt3L with the homologous hematopoietic cytokines macrophage colony stimulating factor (MCSF) and stem cell factor (SCF) suggest that they have a common receptor binding mode that is distinct from the paradigm derived from the complex of growth hormone with its receptor. Furthermore, we identify recognition features common to all helical and cystine-knot protein ligands that activate type III tyrosine kinase receptors, and the closely related type V tyrosine kinase receptors.

Biological activity of human granulocyte colony stimulating factor with a modified C-terminus

Granulocyte colony-stimulating factor (G-CSF) undergoes receptor-mediated internalization into target cells which are normally restricted to neutrophilic granulocytes and their committed progenitor cells, suggesting that it may be applicable as a myeloid cell-targeting vehicle. To test this notion, we constructed a cDNA encoding a human G-CSF/murine stem cell factor (mSCF) chimeric molecule in a mammalian expression vector and transfected NIH3T3 cells with this plasmid. The resulting chimeric cytokine consisted of the entire G-CSF sequences fused to Lys148 of mSCF. It can be released from the surface membrane of NIH3T3 transformants through proteolytic cleavage at Ala164 of mSCF. The culture media conditioned by a number of stable transformants, which were confirmed by an enzyme-linked immunosorbent assay (ELISA) to secrete an hG-CSF derivative, were examined for their ability to stimulate CFU-G-derived colony formation as well as the proliferation of G-CSF-dependent NFS-60 cells. The results indicated that this C-terminus modified version of hG-CSF is as potent as recombinant hG-CSF in both assays.

The presence of novel amino acids in the cytoplasmic domain of stem cell factor results in hematopoietic defects in Steel(17H) mice

Stem cell factor (SCF) is expressed as an integral membrane growth factor that may be differentially processed to produce predominantly soluble (S) (SCF(248)) or membrane-associated (MA) (SCF(220)) protein. A critical role for membrane presentation of SCF in the hematopoietic microenvironment (HM) has been suggested from the phenotype of the Steel-dickie (Sl(d)) mice, which lack MA SCF, and by studies performed in our laboratory (and by others) using long-term bone marrow cultures and transgenic mice expressing different SCF isoforms. Steel(17H) (Sl(17H)) is an SCF mutant that demonstrates melanocyte defects and sterility in males but not in females. The Sl(17H) allele contains a intronic mutation resulting in the substitution of 36 amino acids (aa's) in the SCF cytoplasmic domain with 28 novel aa's. This mutation, which affects virtually the entire cytoplasmic domain of SCF, could be expected to alter membrane SCF presentation. To investigate this possibility, we examined the biochemical and biologic properties of the Sl(17H)-encoded protein and its impact in vivo and in vitro on hematopoiesis and on c-Kit signaling. We demonstrate that compound heterozygous Sl/Sl(17H) mice manifest multiple hematopoietic abnormalities in vivo, including red blood cell deficiency, bone marrow hypoplasia, and defective thymopoiesis. In vitro, both S and MA Sl(17H) isoforms of SCF exhibit reduced cell surface expression on stromal cells and diminished biological activity in comparison to wild-type (wt) SCF isoforms. These alterations in presentation and biological activity are associated with a significant reduction in the proliferation of an SCF-responsive erythroid progenitor cell line and in the activation of phosphatidylinositol 3-Kinase/Akt and mitogen-activated protein-Kinase signaling pathways. In vivo, transgene expression of the membrane-restricted (MR) (SCF(X9/D3)) SCF in Sl/Sl(17H) mutants results in a significant improvement in peripheral red blood cell counts in comparison to Sl/Sl(17H) mice.

Consequences of exclusive expression in vivo of Kit-ligand lacking the major proteolytic cleavage site

Membrane growth factors that are processed to produce soluble ligands may function both as soluble factors and as membrane factors. The membrane growth factor Kit-ligand (KL), the ligand of the Kit receptor tyrosine kinase, is encoded at the Sl locus, and mice carrying Sl mutations have defects in hematopoiesis, gametogenesis, and melanogenesis. Two alternatively spliced KL transcripts encode two cell-associated KL protein products, KL-1 and KL-2. The KL-2 protein lacks the major proteolytic cleavage site for the generation of soluble KL, thus representing a more stable cell-associated form of KL. We investigated the consequences of exclusive expression of KL-2 in vivo. The KL gene in embryonic stem cells was modified and KL exon 6 was replaced with a PGKneoNTRtkpA cassette by homologous recombination, and mice carrying the SlKL2 allele were obtained. SlKL2/SlKL2 mice had only slightly reduced levels of soluble KL in their serum, suggesting that in vivo KL-2 may be processed to produce soluble KL-2S. The steady-state characteristics of the hematopoietic system and progenitor numbers were normal, and the mutant animals were not anemic. However, mast cell numbers in the skin and peritoneum were reduced and the mutant animals displayed increased sensitivity to sublethal doses of gamma-irradiation. Therefore, KL-2 may substitute for KL-1 in most situations with the exception of the production of mast cells, and induced proteolytic cleavage of KL-1 to produce soluble KL may have a role in the regeneration of hematopoietic tissue after radiation injury.

Changes in conformation and stability upon SCF/sKit complex formation

Stem cell factor (SCF) is thought to be a member of the four-helical bundle cytokine superfamily, and exists in solution as a noncovalent homodimer. It is the ligand for Kit, a tyrosine kinase type III receptor. The interaction of SCF and Kit affects early hematopoietic progenitors, as well as gametocytes, melanocytes, and mast cells. Upon binding of SCF the Kit undergoes dimerization and transphosphorylation. Circular dichroism (CD), intrinsic fluorescence, and Fourier transform infrared (FTIR) spectroscopy were used for conformational analyses of free SCF, soluble Kit (sKit), and the complex. The sKit consisted of the extracellular domain of Kit, contained five Ig-like domains, and was prepared from the conditioned media of transfected Chinese hamster ovary cells. With these techniques, a reproducible conformational change was seen upon ligand/receptor binding. The far-UV CD and FTIR spectroscopy indicated a slight increase in the alpha-helical content. The near-UV CD and fluorescence spectra showed changes in the environments of the aromatic amino acids. The thermal denaturation of SCF was not affected by complex formation, while the melting temperature of sKit increased only a few degrees when binding SCF. This indicates that binding is temperature dependent, consistent with titration calorimetry results published previously which demonstrated that there is a large enthalpy of binding. The conformational changes which accompany SCF/sKit binding could play a role in the receptor dimerization and signal transduction which follow.

Role of dimerization of the membrane-associated growth factor kit ligand in juxtacrine signaling: the Sl17H mutation affects dimerization and stability-phenotypes in hematopoiesis

The Kit ligand (KL)/Kit receptor pair functions in hematopoiesis, gametogenesis, and melanogenesis. KL is encoded at the murine steel (Sl) locus and encodes a membrane growth factor which may be proteolytically processed to produce soluble KL. The membrane-associated form of KL is critical in mediating Kit function in vivo. Evidence for a role of cytoplasmic domain sequences of KL comes from the Sl17H mutation, a splice site mutation that replaces the cytoplasmic domain with extraneous amino acids. Using deletion mutants and the Sl17H allele, we have investigated the role of the cytoplasmic domain sequences of KL in biosynthetic processing and cell surface presentation. The normal KL protein products are processed for cell surface expression, where they form dimers. Both Sl17H and the cytoplasmic deletion mutants of KL were processed to the cell surface; however, the rate of transport and protein stability were affected by the mutations. Deletion of cytoplasmic domain sequences of KL did not affect dimerization of KL. In contrast, dimerization of the Sl17H protein was reduced substantially. In addition, we have characterized the hematopoietic cell compartment in Sl17H mutant mice. The Sl17H mutation has only minor effects on hematopoiesis. Tissue and peritoneal mast cell numbers were reduced in mutant mice as well as in myeloid progenitors. Interestingly, long-term bone marrow cultures from Sl17H mice did not sustain the long-term production of hematopoietic cells. In addition, homing of normal hematopoietic progenitors to the spleen of irradiated Sl17H/Sl17H recipient mice was diminished in transplantation experiments, providing evidence for a role of Kit in homing or lodging. These results demonstrate that the membrane forms of KL exist as homodimers on the cell surface and that dimerization may play an important role in KL/Kit-mediated juxtacrine signaling.

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.

Increased growth promoting but not mast cell degranulation potential of a covalent dimer of c-Kit ligand

The native form of soluble c-kit ligand (KL) is a noncovalent dimer. We have isolated a soluble, disulfide-linked dimer of murine KL (KL-CD) by expressing KL in Escherichia coli and refolding the denatured protein under conditions that promote the formation of both noncovalent dimers (KL-NC) and KL-CD. KL-CD exhibits a 10- to 15-fold increase in the ability to stimulate the growth of both the human megakaryocytic cell line MO7e and murine bone marrow-derived mast cells relative to KL-NC. Colony-forming assays of murine bone marrow progenitor cells also reflected this increased potency. However, KL-CD and KL-NC are equally able to prime mast cells for enhanced IgE-dependent degranulation in vitro and activate mast cells in vivo. Improving the growth-promoting activity of KL without changing its mast cell activation potential suggests that KL-CD or a related molecule could be administered in the clinic at doses that stimulate hematopoietic recovery while avoiding significant mast cell activation.

Effects of alpha-phenyl N-tert-butylnitrone, a spin trap reagent, on the proliferation of murine hematopoietic progenitor cells in vitro

We investigated the effect of alpha-phenyl N-tert-butylnitrone (PBN), a spin trap reagent, on the proliferation of murine hematopoietic progenitor cells in vitro. During the addition of PBN to the liquid cultures of murine bone marrow cells containing a combination of interleukin-3, interleukin-6 and the c-kit ligand/stem cell factor, colony-forming cells in vitro (CFC) and the colony-forming unit in the spleen (CFU-S) increased about 1.6-fold and 2.0-fold, respectively, higher than the control culture. These effects were not observed when using dimethyl sulfoxide, which has the ability to scavenge radicals, and 5,5-dimethyl-1-pyrroline N-oxide, another spin trap reagent. Analysis of cultured cells from a 7-day liquid culture with PBN revealed that the ratio of the intracellular glutathione (GSH) and GSH/GSSG (oxidized GSH) content was higher than the control. Adding thiol N-acetylcysteine, a thiol reagent and a precursor of intracellular GSH, also showed similar effects on the liquid culture of murine hematopoietic progenitor cells and the level of intracellular GSH. In contrast, adding DL-buthionine-[S,R]-sulfoximine, a gamma-glutamylcysteine synthetase inhibitor, decreased the intracellular GSH level and did not increase the number of CFC and CFU-S. These results suggest that PBN regulates the content of intracellular thiol molecules, and the possibility of a relationship between the intracellular redox state and the proliferation and differentiation of hematopoietic stem cells.

Detection of a point mutation (A to G) in exon 5 of the murine Mgf gene defines a novel allele at the Steel locus with a weak phenotype

A new mutation at the locus encoding the mast cell growth factor (Mgf) is described and designated as MgfSl-3Neu. Homozygous mutants have a light grey fur, sometimes with white patches. Homozygotes are fertile, but with reduced litter size, when mated inter se. Analysis of haematological parameters indicated no difference between mutant and wild-type mice. Sequence analysis of the cDNA obtained from the brain of homozygous mutants revealed an A-->G exchange at position 400 leading to a predicted amino acid exchange from Asn-->Leu at position 122. As a consequence of the predicted amino acid exchange an extension of the alpha-helical context and a decreased hydropathicity of the region at positions 101-125 can be deduced. This single amino acid exchange is outside of the known important domains of MGF and explains the weak phenotype of MgfSl-3Neu.

The majority of stem cell factor exists as monomer under physiological conditions. Implications for dimerization mediating biological activity

Soluble Escherichia coli-derived recombinant human stem cell factor (rhSCF) forms a non-covalently associated dimer. We have determined a dimer association constant (Ka) of 2-4 x 10(8) M-1, using sedimentation equilibrium and size exclusion chromatography. SCF has been shown previously to be present at concentrations of approximately 3.3 ng/ml in human serum. Based on the dimerization Ka, greater than 90% of the circulating SCF would be in the monomeric form. When 125I-rhSCF was added to human serum and the serum analyzed by size exclusion chromatography, 72-49% of rhSCF was monomer when the total SCF concentration was in the range of 10-100 ng/ml, consistent with the Ka determination. Three SCF variants, SCF(F63C), SCF (V49L,F63L), and SCF(A165C), were recombinantly expressed in Escherichia coli, purified, and characterized. The dimer Ka values, biophysical properties, and biological activities of these variants were studied. Dimerization-defective variants SCF(F63C)S-CH2CONH2 and SCF(V49L,F63L) showed substantially reduced mitogenic activity, while the activity of the Cys165-Cys165 disulfide-linked SCF(A165C) dimer was 10-fold higher than that of wild type rhSCF. The results suggest a correlation between dimerization affinity and biological activity, consistent with a model in which SCF dimerization mediates dimerization of its receptor, Kit, and subsequent signal transduction.

[cDNA cloning of human stem cell factor and its high level expression in E. coli]

The total RNA of HepG2 cell was extracted as the template by ultrocentrifuge method. The full length cDNA (0.8 kb) encoding the human stem cell factor (hSCF) was amplified by RT-PCR method. The cDNA encoding mature hSCF (0.5 kb) was sequenced and was recombined into the expression vector (PBV-220). The expression level of rhSCF in E. coli DH5 alpha was about 15% of the total protein.

Airway epithelial cells produce stem cell factor

Airway epithelial cells modulate the inflammatory response in asthmatic, allergic and fibrotic lung diseases through the secretion of cytokines that regulate the movement and activation of inflammatory cells. Mast cells play an important role in the pathogenesis of these lung diseases. In this study we report that normal airway epithelial cells express stem cell factor which is a critical mediator of mast cell growth and differentiation and that transforming growth factor-beta inhibits secretion of stem cell factor by airway epithelial cells.

Computational simulations of stem-cell factor/c-kit receptor interaction

Stem-cell factor (SCF) is a noncovalent homodimeric cytokine that exhibits profound biological function in the early stages of hematopoiesis by binding to a cell surface tyrosine kinase receptor that is encoded by the c-Kit proto-oncogene. The results obtained from a combined implementation of homology-based molecular modeling and computational simulations in the study of species-specific SCF/ c-Kit interactions are reported. The structural models of the human and rat SCF ligands are based on the close structural similarity to the cytokine M-CSF, whose C alpha structure has recently become available. The constant domains of the human Fc fragment are used as a template for the ligand binding domains of the c-Kit receptor. The factors responsible for the stabilization of the SCF quaternary structure and the molecular determinants for ligand recognition and ligand specificity have been identified by assessing the conformational, topographical, and dynamic features of the isolated ligands and of the ligand-receptor complexes.

Structure-function relationships of stem cell factor: an analysis based on a series of human-murine stem cell factor chimera and the mapping of a neutralizing monoclonal antibody

Although much is now known about the biological properties of the c-kit receptor and its ligand, stem cell factor (SCF), little is known of the structural basis for the binding and function of this hematopoietic cytokine. By analyzing the activities of chimeric interspecies and homologue muteins and epitope mapping of a monoclonal antibody (MoAb) to the human protein, we have found that three distinct regions of SCF are essential for full biological function. Homologue and interspecies swapping of polypeptide sequences between the amino terminus and G35, between L79 and N97, and between R121 and D128 reduced or eliminated the ability of the chimera to act in synergy with murine granulocyte-macrophage colony-stimulating factor (GM-CSF) to promote hematopoietic colony formation. Moreover, a nonconformation-dependent MoAb that neutralizes human, but not murine SCF, was found to bind to residues within the L79-N97 segment of the human homologue. As these three regions localize to the putative first, third, and fourth helices of the protein, findings remarkably similar to previous studies of cytokines as diverse as growth hormone, GM-CSF, and interleukin (IL)-4, our results suggest that cytokines of multiple classes share a common functional organization.

In vitro methionine oxidation of Escherichia coli-derived human stem cell factor: effects on the molecular structure, biological activity, and dimerization

The effect of oxidation of the methionine residues of Escherichia coli-derived recombinant human stem cell factor (huSCF) to methionine sulfoxide on the structure and activity of SCF was examined. Oxidation was performed using hydrogen peroxide under acidic conditions (pH 5.0). The kinetics of oxidation of the individual methionine residues was determined by quantitation of oxidized and unoxidized methionine-containing peptides, using RP-HPLC of Asp-N endoproteinase digests. The initial oxidation rates for Met159, Met-1, Met27, Met36, and Met48 were 0.11 min-1, 0.098 min-1, 0.033 min-1, 0.0063 min-1, and 0.00035 min-1, respectively, when SCF was incubated in 0.5% H2O2 at room temperature. Although oxidation of these methionines does not affect the secondary structure of SCF, the oxidation of Met36 and Met48 affects the local structure as indicated by CD and fluorescence spectroscopy. The 295-nm Trp peak in the near-UV CD is decreased upon oxidation of Met36, and lost completely following the oxidation of Met48, indicating that the Trp44 environment is becoming significantly less rigid than it is in native SCF. Consistent with this result, the fluorescence spectra revealed that Trp44 becomes more solvent exposed as the methionines are oxidized, with the hydrophobicity of the Trp44 environment decreasing significantly. The oxidations of Met36 and Met48 decrease biological activity by 40% and 60%, respectively, while increasing the dissociation rate constant of SCF dimer by two- and threefold. These results imply that the oxidation of Met36 and Met48 affects SCF dimerization and tertiary structure, and decreases biological activity.

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.

Characterization of ovine stem cell factor messenger ribonucleic acid and protein in the corpus luteum throughout the luteal phase

Stem cell factor (SCF) is a growth factor known to have profound effects on the proliferation, migration, differentiation, and survival of numerous cell types, including those of the ovary. The objectives of the present study were to identify and characterize expression of this growth factor in the ovine corpus luteum (CL). A 952-bp cDNA was amplified from Day 3 (Day 0 = estrus) ovine luteal total cellular (tc) RNA by reverse transcriptase-polymerase chain reaction and determined to encode SCF. Northern analysis of Day 10 luteal poly(A)+ RNA indicated one major transcript of approximately 6.5 kb. SCF mRNA was localized within Day 3 and Day 10 CL by in situ hybridization and was expressed throughout luteal tissue on both days examined. To asses expression throughout the luteal phase, SCF mRNA was quantified by ribonuclease protection assay in tcRNA collected on Day 3, 7, 10, 13, and 16; values did not differ across days (p > 0.10). Similarly, SCF mRNA was quantified in tcRNA isolated from pools of Day 10 large and small steroidogenic cells (n = 4 and 3, respectively); levels did not differ (p > 0.10) between cell types. In addition, SCF protein was detected in CL on Days 3 and 10, and was expressed in a cell-specific manner in cells with morphological characteristics of large and small luteal cells. These data indicate that SCF may be involved in communication among steroidogenic cells and/or between steroidogenic and nonsteroidogenic cells of the CL.

Human stem cell factor dimer forms a complex with two molecules of the extracellular domain of its receptor, Kit

Stem cell factor (SCF) is a cytokine that is active toward hematopoietic progenitor cells and other cell types, including germ cells, melanocytes, and mast cells, which express its receptor, the tyrosine kinase, Kit. SCF exists as noncovalently associated dimer at concentrations where it has been possible to study its quaternary structure; it stimulates dimerization and autophosphorylation of Kit at the cell surface. We have used recombinant versions of human SCF and human Kit extracellular domain (sKit) to study SCF-Kit interactions. By size exclusion chromatography, plus various physical chemical methods including light scattering, sedimentation equilibrium, and titration calorimetry, we demonstrate the formation of complexes containing a dimer of SCF (unglycosylated SCF1-165) plus two molecules of sKit. The concentrations of SCF and sKit in these studies were in the range of 0.35-16.2 microM. The data are analyzed and discussed in the context of several possible models for complex formation. In particular, the sedimentation data are not consistent with a model involving cooperative binding. The Kd estimate for SCF-sKit interaction, obtained by sedimentation equilibrium, is about 17 nm at 25 degrees C. With glycosylated SCF1-165, the Kd is considerably higher.

Immobilized anti-KIT monoclonal antibody induces ligand-independent dimerization and activation of Steel factor receptor: biologic similarity with membrane-bound form of Steel factor rather than its soluble form

Interaction of a tyrosine kinase type receptor and its ligand induces receptor-dimerization or -oligomerization followed by transphosphorylation and activation of its intrinsic kinase, which leads to a series of intracellular signals. We have previously reported that the membrane-bound form of Steel factor (SLF) induces more persistent tyrosine kinase activation and longer life span of c-kit encoded protein (KIT) than its soluble form (Miyazawa et al, Blood 85:641, 1995). In this study, we used YB5.B8 monoclonal antibody (MoAb) that recognizes the extracellular domain of KIT to investigate whether immobilized anti-KIT MoAb can substitute for SLF as a potent activator of KIT by cross-linking receptors and further compared its effect with each SLF isoform in a factor-dependent cell line M07e. YB5.B8 MoAb in a soluble state suppressed SLF-induced M07e cell proliferation in a dose-dependent manner. By contrast, once this antibody was immobilized on the goat-antimouse MoAb (GAM)-coated culture plates, it supported the growth of M07e cells in the absence of any growth factors, whereas culture the cells in GAM alone or YB5.B8 without GAM-coated plates resulted in rapid cell-death within 24 hours. As with the natural ligand SLF, immobilized YB5.B8 MoAb synergized with granulocyte-macrophage colony-stimulating factor (GM-CSF) in inducing cell proliferation compared with either YB5.B8 MoAb or GM-CSF alone. Immunoblotting with antiphosphotyrosine MoAb showed that interaction of M07e cells with immobilized YB5.B8 induced tyrosine phosphorylation of a series of intracellular proteins including KIT (145 kD). In addition, cross-linking studies using a water-soluble cross linking reagent bis-sulfosuccinimidyl-suberate showed that immobilized YB5.B8 MoAb induced dimerization and activation of KIT. However, as with stimulation by the membrane-bound form of SLF, the kinetics of KIT activation with YB5.B8 MoAb was more prolonged compared with the cells treated with recombinant soluble SLF. Flow cytometry showed that, unlike the cells treated with soluble SLF, no downmodulation of cell-surface KIT expression was observed in M07e cells cultured with immobilzed YB5.B8 MoAb. These data suggest that immobilized antibodies against hematopoietic receptors may replace their ligand-stimulators; however, their activities may resemble the membrane-bound form rather than the soluble form of natural ligands.

Cloning, sequencing and expression of stem cell factor (c-kit ligand) cDNA of brushtail possum (Trichosurus vulpecula)

By means of reverse transcription polymerase chain reaction (RT-PCR), three stem cell factor (SCF) cDNAs (822-738 bp in size) were amplified from brushtail possum ovarian poly (A)+ RNA. The largest and smallest of these cDNAs were cloned and sequenced. Characterization of these cDNAs has revealed that possum SCF has approximately 75% and 66% homology to SCF of eutherian mammals at the nucleotide level and the predicted amino acid level respectively. Nucleotide sequencing shows that the 738-bp cDNA represents an mRNA splice variant, equivalent to that found in eutherian mammals, in which an exon (84 bp) encoding a potential proteolytic cleavage site is removed. Comparison of the predicted possum SCF amino acid sequence with the predicted SCF amino acid sequences from eutherian mammals reveals conservation of all cysteine residues and 3 of 4 potential N-linked glycosylation sites. In addition, the hydropathicity profile of the possum SCF protein is similar to that of eutherian SCF suggesting that protein conformation is conserved. Northern analysis was used to characterize possum SCF gene expression in adult ovary and testis. A major transcript of 9 kb was observed in both ovarian and testicular tissue. The conservation of the SCF gene and its predicted protein, suggests that SCF in the possum has similar biological activities to SCF in eutherian mammals.

The cloning and sequencing of cDNAs encoding two isoforms of feline stem cell factor

cDNA clones encoding two isoforms of feline stem cell factor (fSCF) have been isolated using RT-PCR and their sequences determined. The cDNAs encode a predicted full length fSCF protein of 274 amino-acids and a shorter isoform of 246 amino acids. Feline SCF shows a high degree of homology to the SCFs of other species at both the nucleic acid and protein level.