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

KIT D816V is dimerization-independent and activates downstream pathways frequently perturbed in mastocytosis

KIT, a type III tyrosine kinase receptor, plays a crucial role in haematopoietic development. The KIT receptor forms a dimer after ligand binding; this activates tyrosine kinase activity leading to downstream signal transduction. The D816V KIT mutation is extensively implicated in haematological malignancies, including mastocytosis and leukaemia. KIT D816V is constitutively active, but the molecular nuances that lead to constitutive tyrosine kinase activity are unclear. For the first time, we present experimental evidence that the KIT D816V mutant does not dimerize like KIT wild type. We further show evidence of decreased stabilization of the tyrosine kinase domain in the KIT D816V mutant, a phenomenon that might contribute to its constitutive activity. Since the mechanism of KIT D816V activation varies from that of the wild type, we explored downstream signal transduction events and found that even though KIT D816V targets similar signalling moieties, the signalling is amplified in the mutant compared to stem cell factor-activated wild type receptor. Uniquely, KIT D816V induces infection-related pathways and the spliceosome pathway, providing alternate options for selective as well as combinatorial therapeutic targeting.

Behind the Scene: Exploiting MC1R in Skin Cancer Risk and Prevention

Melanoma and non-melanoma skin cancers (NMSCs) are the most frequent cancers of the skin in white populations. An increased risk in the development of skin cancers has been associated with the combination of several environmental factors (i.e., ultraviolet exposure) and genetic background, including melanocortin-1 receptor (MC1R) status. In the last few years, advances in the diagnosis of skin cancers provided a great impact on clinical practice. Despite these advances, NMSCs are still the most common malignancy in humans and melanoma still shows a rising incidence and a poor prognosis when diagnosed at an advanced stage. Efforts are required to underlie the genetic and clinical heterogeneity of melanoma and NMSCs, leading to an optimization of the management of affected patients. The clinical implications of the impact of germline MC1R variants in melanoma and NMSCs' risk, together with the additional risk conferred by somatic mutations in other peculiar genes, as well as the role of MC1R screening in skin cancers' prevention will be addressed in the current review.

Exon-1 skipping and intron-1 retaining by alternative splicing of the c-KIT gene encodes a novel splice variant in the skin of Merino sheep (Ovis aries)

c-KIT, a type III receptor protein tyrosine kinase, plays an essential role in melanocyte development, migration, and survival. Mutations within the c-KIT gene are previously shown to cause the white coat color phenotypes in pigs, mice, goats, and humans. However, up so far, the splicing isoform(s), genomic architecture of c-KIT have not been characterized well in merino sheep. Reverse transcriptase (RT)-PCR analysis with molecular prediction identified two basic splice variants: Transcript Variant-1, 2 for 12 bp insertion coding sequences (CDS) corresponding to the four amino acids 'GNSK', respectively. Using 5' RACE, here we report for the first time a novel c-KIT 'Transcript Variant-3' from the skin of merino sheep by comparative genome analyses at exon(1)-intron(1)-exon(2) boundaries. In contrast, a single product of 795 bp was characterized by 3' RACE. We also demonstrated that the c-KIT gene expression at the transcript level is not mediated via an intron-9 splicing event. Overall, beyond what was observed in other mammals, our data provide novel insights into the molecular structure of the c-KIT gene in sheep.

Engineering Stem Cell Factor Ligands with Different c-Kit Agonistic Potencies

Receptor tyrosine kinases (RTKs) are major players in signal transduction, regulating cellular activities in both normal regeneration and malignancy. Thus, many RTKs, c-Kit among them, play key roles in the function of both normal and neoplastic cells, and as such constitute attractive targets for therapeutic intervention. We thus sought to manipulate the self-association of stem cell factor (SCF), the cognate ligand of c-Kit, and hence its suboptimal affinity and activation potency for c-Kit. To this end, we used directed evolution to engineer SCF variants having different c-Kit activation potencies. Our yeast-displayed SCF mutant (SCF_M) library screens identified altered dimerization potential and increased affinity for c-Kit by specific SCF-variants. We demonstrated the delicate balance between SCF homo-dimerization, c-Kit binding, and agonistic potencies by structural studies, in vitro binding assays and a functional angiogenesis assay. Importantly, our findings showed that a monomeric SCF variant exhibited superior agonistic potency vs. the wild-type SCF protein and vs. other high-affinity dimeric SCF variants. Our data showed that action of the monomeric ligands in binding to the RTK monomers and inducing receptor dimerization and hence activation was superior to that of the wild-type dimeric ligand, which has a higher affinity to RTK dimers but a lower activation potential. The findings of this study on the binding and c-Kit activation of engineered SCF variants thus provides insights into the structure–function dynamics of ligands and RTKs.

KIT and Melanoma: Biological Insights and Clinical Implications

Melanoma, originating from epidermal melanocytes, is a heterogeneous disease that has the highest mortality rate among all types of skin cancers. Numerous studies have revealed the cause of this cancer as related to various somatic driver mutations, including alterations in KIT-a proto-oncogene encoding for a transmembrane receptor tyrosine kinase. Although accounting for only 3% of all melanomas, mutations in c-KIT are mostly derived from acral, mucosal, and chronically sun-damaged melanomas. As an important factor for cell differentiation, proliferation, and survival, inhibition of c-KIT has been exploited for clinical trials in advanced melanoma. Here, apart from the molecular background of c-KIT and its cellular functions, we will review the wide distribution of alterations in KIT with a catalogue of more than 40 mutations reported in various articles and case studies. Additionally, we will summarize the association of KIT mutations with clinicopathologic features (age, sex, melanoma subtypes, anatomic location, etc.), and the differences of mutation rate among subgroups. Finally, several therapeutic trials of c-KIT inhibitors, including imatinib, dasatinib, nilotinib, and sunitinib, will be analyzed for their success rates and limitations in advanced melanoma treatment. These not only emphasize c-KIT as an attractive target for personalized melanoma therapy but also propose the requirement for additional investigational studies to develop novel therapeutic trials co-targeting c-KIT and other cytokines such as members of signaling pathways and immune systems.

Canine CD117-Specific Antibodies with Diverse Binding Properties Isolated from a Phage Display Library Using Cell-Based Biopanning

CD117 (c-Kit) is a tyrosine kinase receptor that is overexpressed in multiple dog tumors. There is 100% homology between the juxtamembrane domain of human and canine CD117, and many cancer-causing mutations occur in this region in both species. Thus, CD117 is an important target for cancer treatment in dogs and for comparative oncology studies. Currently, there is no monoclonal antibody (mAb) specifically designed to target the exposed region of canine CD117, although there exist some with species cross-reactivity. We panned a naïve phage display library to isolate antibodies against recombinant CD117 on whole cells. Several mAbs were isolated and were shown to bind recombinant canine CD117 at low- to sub-nanomolar affinity. Additionally, binding to native canine CD117 was confirmed by immunohistochemistry and by flow cytometry. Competitive binding assays also identified mAbs that competed with the CD117 receptor-specific ligand, the stem cell factor (SCF). These results show the ability of our cell-based biopanning strategy to isolate a panel of antibodies that have varied characteristics when used in different binding assays. These in vitro/ex vivo assessments suggest that some of the isolated mAbs might be promising candidates for targeting overexpressed CD117 in canine cancers for different useful applications.

CD117/c-kit in Cancer Stem Cell-Mediated Progression and Therapeutic Resistance

Metastasis is the primary cause of cancer patient morbidity and mortality, but due to persisting gaps in our knowledge, it remains untreatable. Metastases often occur as patient tumors progress or recur after initial therapy. Tumor recurrence at the primary site may be driven by a cancer stem-like cell or tumor progenitor cell, while recurrence at a secondary site is driven by metastatic cancer stem cells or metastasis-initiating cells. Ongoing efforts are aimed at identifying and characterizing these stem-like cells driving recurrence and metastasis. One potential marker for the cancer stem-like cell subpopulation is CD117/c-kit, a tyrosine kinase receptor associated with cancer progression and normal stem cell maintenance. Further, activation of CD117 by its ligand stem cell factor (SCF; kit ligand) in the progenitor cell niche stimulates several signaling pathways driving proliferation, survival, and migration. This review examines evidence that the SCF/CD117 signaling axis may contribute to the control of cancer progression through the regulation of stemness and resistance to tyrosine kinase inhibitors.

Mechanisms of erythrocyte development and regeneration: implications for regenerative medicine and beyond

Hemoglobin-expressing erythrocytes (red blood cells) act as fundamental metabolic regulators by providing oxygen to cells and tissues throughout the body. Whereas the vital requirement for oxygen to support metabolically active cells and tissues is well established, almost nothing is known regarding how erythrocyte development and function impact regeneration. Furthermore, many questions remain unanswered relating to how insults to hematopoietic stem/progenitor cells and erythrocytes can trigger a massive regenerative process termed 'stress erythropoiesis' to produce billions of erythrocytes. Here, we review the cellular and molecular mechanisms governing erythrocyte development and regeneration, and discuss the potential links between these events and other regenerative processes.

Current Experimental Methods for Characterizing Protein-Protein Interactions

Protein molecules often interact with other partner protein molecules in order to execute their vital functions in living organisms. Characterization of protein-protein interactions thus plays a central role in understanding the molecular mechanism of relevant protein molecules, elucidating the cellular processes and pathways relevant to health or disease for drug discovery, and charting large-scale interaction networks in systems biology research. A whole spectrum of methods, based on biophysical, biochemical, or genetic principles, have been developed to detect the time, space, and functional relevance of protein-protein interactions at various degrees of affinity and specificity. This article presents an overview of these experimental methods, outlining the principles, strengths and limitations, and recent developments of each type of method.

Current Experimental Methods for Characterizing Protein-Protein Interactions

Protein molecules often interact with other partner protein molecules in order to execute their vital functions in living organisms. Characterization of protein-protein interactions thus plays a central role in understanding the molecular mechanism of relevant protein molecules, elucidating the cellular processes and pathways relevant to health or disease for drug discovery, and charting large-scale interaction networks in systems biology research. A whole spectrum of methods, based on biophysical, biochemical, or genetic principles, have been developed to detect the time, space, and functional relevance of protein-protein interactions at various degrees of affinity and specificity. This article presents an overview of these experimental methods, outlining the principles, strengths and limitations, and recent developments of each type of method.

Chemical modifications in the seed region of miRNAs 221/222 increase the silencing performances in gastrointestinal stromal tumor cells

Most GastroIntestinal Stromal Tumors (GISTs) are characterized by KIT gene overexpression, which in turn is regulated by levels of microRNA 221 and microRNA 222. GISTs can also be distinguished by their miRNAs expression profile in which miRNAs 221/222 result reduced in comparison with GI normal tissues. In this paper, to restore normal miRNAs levels and to improve the silencing performances of miRNAs 221/222, new miRNA mimics in which guide strands are modified by Phosphorothioate (PS) and/or 2'-O-methyl RNA (2'-OMe) inside and outside the seed region, were synthesized and tested in GIST48 cells. We evaluated the positional effect of the chemical modifications on the miRNAs silencing activity, compared to natural and several commercial miRNA mimics. Our results show that chemically modified miRNAs 221/222 with alternating 2'-OMe-PS and natural nucleotides in the seed region are effective inhibitors of KIT gene expression and exhibit increased stability in rat plasma. Besides, their transfection in GIST 48 cells showed significant effects on different cellular processes in which KIT plays a functional role for tumor development (such as migration, cell proliferation, and apoptosis). Therefore, modified miRNAs 221/222 may provide an alternative therapeutic option for GIST treatment also aimed to overcome drug resistance concerns.

Exploring erythropoiesis of common carp (Cyprinus carpio) using an in vitro colony assay in the presence of recombinant carp kit ligand A and erythropoietin

The use of in vitro colony assays in mammals has contributed to identification of erythroid progenitor cells such as burst-forming unit-erythroid (BFU-E) and colony-forming unit-erythroid (CFU-E) progenitors, and serves to examine functions of erythropoietic growth factors like Erythropoietin (Epo) and Kit ligand. Here, we established an in vitro colony-forming assay capable of investigating erythropoiesis in carp (Cyprinus carpio), cloned and functionally characterized recombinant homologous molecules Epo and Kit ligand A (Kitla), and identified three distinct erythroid progenitor cells in carp. Recombinant carp Epo induced the formation of CFU-E-like and BFU-E-like erythroid colonies, expressing erythroid marker genes, β-globin, epor and gata1. Recombinant carp Kitla alone induced limited colony formation, whereas a combination of Kitla and Epo dramatically enhanced erythroid colony formation and colony cell growth, as well as stimulated the formation of thrombocytic/erythroid colonies expressing not only erythroid markers but also thrombocytic markers, cd41 and c-mpl. Utilizing this colony assay to examine the distribution of distinct erythroid progenitor cells in carp, we demonstrated that carp head and trunk kidney play a primary role in erythropoiesis, while the spleen plays a secondary. Furthermore, we showed that presumably bi-potent thrombocytic/erythroid progenitor cells localize principally in the trunk kidney. Our results indicate that teleost fish possess mechanisms of Epo- and Kitla-dependent erythropoiesis similar to those in other vertebrates, and also help to demonstrate the diversity of erythropoietic sites among vertebrates.

c-kit(+) cells: the tell-tale heart of cardiac regeneration?

Cardiovascular disease is the leading cause of morbidity and mortality in the developed world. Although ongoing therapeutic strategies ameliorate symptoms and prolong life for patients with cardiovascular diseases, they do not solve the critical issue related to the loss of cardiac tissue. Accordingly, stem/progenitor cell therapy has emerged as a paramount approach for cardiac repair and regeneration. In this regard, c-kit(+) cells have animated much interest and controversy. These cells are self-renewing, clonogenic, and multipotent and display a noteworthy potential to differentiate into all cardiovascular lineages. However, their functional contribution to cardiomyocyte turnover is one of the centrally debated issues concerning their regenerative potential. Regardless, plentiful preclinical and clinical studies have been conducted which provide evidence for the capacity of c-kit(+) cells to improve cardiac function. The purpose of this review is to give a comprehensive, impartial, critical description and evaluation of the literature on c-kit(+) cells from bench to bedside in order to address their true potential, benefits and controversies.

A survival Kit for pancreatic beta cells: stem cell factor and c-Kit receptor tyrosine kinase

The interactions between c-Kit and its ligand, stem cell factor (SCF), play an important role in haematopoiesis, pigmentation and gametogenesis. c-Kit is also found in the pancreas, and recent studies have revealed that c-Kit marks a subpopulation of highly proliferative pancreatic endocrine cells that may harbour islet precursors. c-Kit governs and maintains pancreatic endocrine cell maturation and function via multiple signalling pathways. In this review we address the importance of c-Kit signalling within the pancreas, including its profound role in islet morphogenesis, islet vascularisation, and beta cell survival and function. We also discuss the impact of c-Kit signalling in pancreatic disease and the use of c-Kit as a potential target for the development of cell-based and novel drug therapies in the treatment of diabetes.

Structure, domain organization, and different conformational states of stem cell factor-induced intact KIT dimers

Using electron microscopy and fitting of crystal structures, we present the 3D reconstruction of ligand-induced dimers of intact receptor tyrosine kinase, KIT. We observe that KIT protomers form close contacts throughout the entire structure of ligand-bound receptor dimers, and that the dimeric receptors adopt multiple, defined conformational states. Interestingly, the homotypic interactions in the membrane proximal Ig-like domain of the extracellular region differ from those observed in the crystal structure of the unconstrained extracellular regions. We observe two prevalent conformations in which the tyrosine kinase domains interact asymmetrically. The asymmetric arrangement of the cytoplasmic regions may represent snapshots of molecular interactions occurring during trans autophosphorylation. Moreover, the asymmetric arrangements may facilitate specific intermolecular interactions necessary for trans phosphorylation of different KIT autophosphorylation sites that are required for stimulation of kinase activity and recruitment of signaling proteins by activated KIT.

The C-kit receptor-mediated signal transduction and tumor-related diseases

As an important member of tyrosine kinase family, c-kit receptor causes specific expression of certain genes, regulates cell differentiation and proliferation, resists cell apoptosis, and plays a key role in tumor occurrence, development, migration and recurrence through activating the downstream signaling molecules following interaction with stem cell factor (SCF). The abnormality of SCF/c-kit signaling pathway is closely related to some certain tumors. The discovery of c-kit receptor-targeted drugs has promoted clinical-related cancer's diagnosis and treatment. In this paper, we review recent research progress on c-kit receptor-mediated signal transduction and its potential therapeutic application as a target in tumor-related diseases.

Analysis of high-affinity assembly for AMPA receptor amino-terminal domains

Analytical ultracentrifugation (AUC) and steady-state fluorescence anisotropy were used to measure the equilibrium dissociation constant (Kd) for formation of dimers by the amino-terminal domains (ATDs) of the GluA2 and GluA3 subtypes of AMPA receptor. Previous reports on GluA2 dimerization differed in their estimate of the monomer-dimer Kd by a 2,400-fold range, with no consensus on whether the ATD forms tetramers in solution. We find by sedimentation velocity (SV) analysis performed using absorbance detection a narrow range of monomer-dimer Kd values for GluA2, from 5 to 11 nM for six independent experiments, with no detectable formation of tetramers and no effect of glycosylation or the polypeptide linker connecting the ATD and ligand-binding domains; for GluA3, the monomer-dimer Kd was 5.6 µM, again with no detectable tetramer formation. For sedimentation equilibrium (SE) experiments, a wide range of Kd values was obtained for GluA2, from 13 to 284 nM, whereas for GluA3, the Kd of 3.1 µM was less than twofold different from the SV value. Analysis of cell contents after the ∼1-week centrifuge run by silver-stained gels revealed low molecular weight GluA2 breakdown products. Simulated data for SE runs demonstrate that the apparent Kd for GluA2 varies with the extent of proteolysis, leading to artificially high Kd values. SV experiments with fluorescence detection for GluA2 labeled with 5,6-carboxyfluorescein, and fluorescence anisotropy measurements for GluA2 labeled with DyLight405, yielded Kd values of 5 and 11 nM, consistent with those from SV with absorbance detection. However, the sedimentation coefficients measured by AUC using absorbance and fluorescence systems were strikingly different, and for the latter are not consistent with hydrodynamic protein models. Thus, for unknown reasons, the concentration dependence of sedimentation coefficients obtained with fluorescence detection SV may be unreliable, limiting the usefulness of this technique for quantitative analysis.

Molecular and functional characterization of kita and kitla of the goldfish (Carassius auratus L.)

Kit ligand and its type III tyrosine kinase receptor Kit promotes the survival, proliferation and differentiation of progenitor cells involved in mammalian myelopoiesis. In this study we report on the molecular and functional characterization of kit receptor A (kita) and kit ligand A (kitla) from the goldfish. Both kita and kitla were ubiquitously expressed in goldfish tissues, with higher mRNA levels observed in the kidney and spleen, the major hematopoietic organs of fish. Furthermore, both kita and kitla expressions decreased in a time-dependent manner in goldfish primary kidney macrophage (PKM) cultures, as progenitor to macrophage development progressed, and the highest expressions of both the receptor and ligand were observed in sorted progenitor cell populations. Activation of mature macrophage cultures increased both kita and kitla expressions. Kit ligand A induced chemotactic response, proliferation and survival of PKM cells in a dose-dependent manner, but did not induce differentiation of early PKM cells. These results are consistent with the role of kita and kitla during myelopoiesis of higher vertebrates and suggest a conserved mechanism of macrophage development throughout vertebrates.

Deregulation of signaling pathways in acute myeloid leukemia

In acute myeloid leukemia (AML), aberrant signal transduction enhances the survival and proliferation of hematopoietic progenitor cells. Activation of signal transduction in AML may occur through a variety of genetic alterations affecting different signaling molecules, such as the FLT3 and KIT receptor tyrosine kinases (RTKs) and members of the RAS family of guanine nucleotide-binding proteins. These mutant signaling proteins are attractive therapeutic targets; however, developing targeted therapies for each genotypic variant and determining the relationships between different genotypes and critical functional dependencies of the leukemic cells remain major challenges. As the large number of mutant signaling proteins that have been identified in AML are likely to reflect activation of a more limited number of downstream effector pathways, such as the RAF/MEK/ERK and PI3K/AKT cascades, targeting these unifying pathways may represent a more broadly applicable therapeutic strategy. Furthermore, integrative genomic studies combining DNA sequencing, DNA copy number analysis, transcriptional profiling, and functional genetic approaches hold great promise for identifying additional signaling abnormalities in AML that are relevant to leukemogenesis and can be exploited therapeutically. Eventually, it may become possible to use pathogenesis-oriented combinations of signal transduction inhibitors to improve the cure rate in AML patients.

Size-exclusion chromatography with on-line light scattering

This unit describes the use of size-exclusion chromatography with on-line light scattering, UV absorbance, and refractive index detectors (SEC-LS/UV/RI) to determine: (a) the molecular weight of simple proteins containing no carbohydrates, (b) the molecular weight of glycoproteins, and (c), most importantly, the molecular weight and stoichiometry of protein-protein complexes or protein-carbohydrate complexes. Multiangle light scattering is also discussed.

Protein isolated from biopharmaceutical formulations cannot be used for comparative studies: Follow-up to "a case study using Epoetin Alfa from Epogen and EPREX"

In the biotechnology area, the issue of comparability with an innovator product is complex. Ideally, a side-by-side comparison of physical properties would be part of the demonstration of comparability. However, biogeneric companies do not have access to the bulk drug substance from the innovator company for biophysical comparison, and isolation of protein from marketed product cannot be guaranteed to produce material that is identical to the bulk drug substance from which it was prepared. In a recently published study, protein was isolated from marketed product and comparative studies performed. In a follow-up investigation of the published work, we demonstrate here that even a simple isolation procedure can significantly compromise the protein, which raises serious questions about the interpretation of that study, and in a broader context the value of any studies done with such "out-of-process" protein.

Structural basis for activation of the receptor tyrosine kinase KIT by stem cell factor

Stem Cell Factor (SCF) initiates its multiple cellular responses by binding to the ectodomain of KIT, resulting in tyrosine kinase activation. We describe the crystal structure of the entire ectodomain of KIT before and after SCF stimulation. The structures show that KIT dimerization is driven by SCF binding whose sole role is to bring two KIT molecules together. Receptor dimerization is followed by conformational changes that enable lateral interactions between membrane proximal Ig-like domains D4 and D5 of two KIT molecules. Experiments with cultured cells show that KIT activation is compromised by point mutations in amino acids critical for D4-D4 interaction. Moreover, a variety of oncogenic mutations are mapped to the D5-D5 interface. Since key hallmarks of KIT structures, ligand-induced receptor dimerization, and the critical residues in the D4-D4 interface, are conserved in other receptors, the mechanism of KIT stimulation unveiled in this report may apply for other receptor activation.

Structural basis for stem cell factor-KIT signaling and activation of class III receptor tyrosine kinases

Stem cell factor (SCF) binds to and activates the KIT receptor, a class III receptor tyrosine kinase (RTK), to stimulate diverse processes including melanogenesis, gametogenesis and hematopoeisis. Dysregulation of KIT activation is associated with many cancers. We report a 2.5 A crystal structure of the functional core of SCF bound to the extracellular ligand-binding domains of KIT. The structure reveals a 'wrapping' SCF-recognition mode by KIT, in which KIT adopts a bent conformation to facilitate each of its first three immunoglobulin (Ig)-like domains to interact with SCF. Three surface epitopes on SCF, an extended loop, the B and C helices, and the N-terminal segment, contact distinct KIT domains, with two of the epitopes undergoing large conformational changes upon receptor binding. The SCF/KIT complex reveals a unique RTK dimerization assembly, and a novel recognition mode between four-helix bundle cytokines and Ig-family receptors. It serves as a framework for understanding the activation mechanisms of class III RTKs.

Dynamic localization and persistent stimulation of factor-dependent cells by a stem cell factor / cellulose binding domain fusion protein

The extracellular matrix provides structural components that support the development of tissue morphology and the distribution of growth factors that modulate the overall cellular response to those growth factors. The ability to manipulate the presentation of factors in culture systems should provide an additional degree of control in regulating the stimulation of factor-dependent cells for tissue engineering applications. Cellulose binding domain (CBD) fusion protein technology facilitates the binding of bioactive cytokines to cellulose materials, and has permitted the analysis of several aspects of cell stimulation by surface-localized growth factors. We previously reported the synthesis and initial characterization of a fusion protein comprised of a CBD and murine stem cell factor (SCF) (Doheny et al. [1999] Biochem J 339:429-434). A significant advantage of the CBD fusion protein system is that it permits the stimulation of factor-dependent cells with localized growth factor, essentially free of nonfactor-derived interactions between the cell and matrix. In this work, the long-term stability and bioactivity of SCF-CBD fusions adsorbed to microcrystalline cellulose under cell culture conditions is demonstrated. Cellulose-bound SCF-CBD is shown to stimulate receptor polarization in the cell membrane and adherence to the cellulose matrix. In addition, cellulose-surface presentation of the SCF-CBD attenuates c-kit dephosphorylation kinetics, potentially modulating the overall response of the cell to the SCF signal.

Design of Recombinant Stem Cell Factor–macrophage Colony Stimulating Factor Fusion Proteins and their Biological Activity In Vitro

Stem cell factor (SCF) and macrophage colony stimulating factor (M-CSF) can act in synergistic way to promote the growth of mononuclear phagocytes. SCF-M-CSF fusion proteins were designed on the computer using the Homology and Biopolymer modules of the software packages InsightII. Several existing crystal structures were used as templates to generate models of the complexes of receptor with fusion protein. The structure rationality of the fusion protein incorporated a series of flexible linker peptide was analyzed on InsightII system. Then, a suitable peptide GGGGSGGGGSGG was chosen for the fusion protein. Two recombinant SCF-M-CSF fusion proteins were generated by construction of a plasmid in which the coding regions of human SCF (1-165aa) and M-CSF (1-149aa) cDNA were connected by this linker peptide coding sequence followed by subsequent expression in insect cell. The results of Western blot and activity analysis showed that these two recombinant fusion proteins existed as a dimer with a molecular weight of approximately 84 KD under non-reducing conditions and a monomer of approximately 42 KD at reducing condition. The results of cell proliferation assays showed that each fusion protein induced a dose-dependent proliferative response. At equimolar concentration, SCF/M-CSF was about 20 times more potent than the standard monomeric SCF in stimulating TF-1 cell line growth, while M-CSF/SCF was 10 times of monomeric SCF. No activity difference of M-CSF/SCF or SCF/M-CSF to M-CSF (at same molar) was found in stimulating the HL-60 cell linear growth. The synergistic effect of SCF and M-CSF moieties in the fusion proteins was demonstrated by the result of clonogenic assay performed with human bone mononuclear, in which both SCF/M-CSF and M-CSF/SCF induced much higher number of CFU-M than equimolar amount of SCF or M-CSF or that of two cytokines mixture.

Normal and Oncogenic Forms of the Receptor Tyrosine Kinase Kit

Kit is a receptor tyrosine kinase (RTK) that binds stem cell factor. This receptor ligand combination is important for normal hematopoiesis, as well as pigmentation, gut function, and reproduction. Structurally, Kit has both an extracellular and intracellular region. Theintra-cellular region is comprised of a juxtamembrane domain (JMD), a kinase domain, a kinase insert, and a carboxyl tail. Inappropriate expression or activation of Kit is associated with a variety of diseases in humans. Activating mutations in Kit have been identified primarily in the JMD and the second part of the kinase domain and have been associated with gastrointestinal stromal cell tumors and mastocytosis, respectively. There are also reports of activating mutations in some forms of germ cell tumors and core binding factor leukemias. Since the cloning of the Kit ligand in the early 1990s, there has been an explosion of information relating to the mechanism of action of normal forms of Kit as well as activated mutants. This is important because understanding this RTK at the biochemical level could assist in the development of therapeutics to treat primary and secondary defects in the tissues that require Kit. Furthermore, understanding the mechanisms mediating transformation of cells by activated Kit mutants will help in the design of interventions for human disease associated with these mutations. The objective of this review is to summarize what is known about normal and oncogenic forms of Kit. We will place particular emphasis on recent developments in understanding the mechanisms of action of normal and activated forms of this RTK and its association with human disease, particularly in hematopoietic cells.

Studying multiprotein complexes by multisignal sedimentation velocity analytical ultracentrifugation

Protein interactions can promote the reversible assembly of multiprotein complexes, which have been identified as critical elements in many regulatory processes in cells. The biophysical characterization of assembly products, their number and stoichiometry, and the dynamics of their interactions in solution can be very difficult. A classical first-principle approach for the study of purified proteins and their interactions is sedimentation velocity analytical ultracentrifugation. This approach allows one to distinguish different protein complexes based on their migration in the centrifugal field without isolating reversibly formed complexes from the individual components. An important existing limitation for systems with multiple components and assembly products is the identification of the species associated with the observed sedimentation rates. We developed a computational approach for integrating multiple optical signals into the sedimentation coefficient distribution analysis of components, which combines the size-dependent hydrodynamic separation with discrimination of the extinction properties of the sedimenting species. This approach allows one to deduce the stoichiometry and to assign the identity of the assembly products without prior assumptions of the number of species and the nature of their interaction. Although chromophoric labels may be used to enhance the spectral resolution, we demonstrate the ability to work label-free for three-component protein mixtures. We observed that the spectral discrimination can synergistically enhance the hydrodynamic resolution. This method can take advantage of differences in the absorbance spectra of interacting solution components, for example, for the study of protein-protein, protein-nucleic acid or protein-small molecule interactions, and can determine the size, hydrodynamic shape, and stoichiometry of multiple complexes in solution.

Effect of the intermolecular disulfide bond on the conformation and stability of glial cell line-derived neurotrophic factor

Glial cell line-derived neurotrophic factor (GDNF) is a member of the TGF-beta superfamily of proteins. It exists as a covalent dimer in solution, with the 15 kDa monomers linked by an interchain disulfide bond through the Cys101 residues. Sedimentation equilibrium and velocity experiments demonstrated that, after removal of the interchain disulfide bond, GDNF remains as a non-covalent dimer and is stable at pH 7.0. To investigate the effect of the intermolecular disulfide on the structure and stability of GDNF, we compared the solution structures of the wild-type protein and a cysteine-101 to alanine (C101A) mutant using Fourier transform infrared (FTIR), FT-Raman and circular dichroism (CD) spectroscopy and sedimentation analysis. The elimination of the intermolecular disulfide bond causes only minor changes (approximately 4%) in the secondary structures of GDNF. The far- and near-UV CD spectra demonstrated that the secondary and tertiary structures were similar for both wild-type and C101A GDNF. Heparin binding and sedimentation velocity experiments also indicated that the folded structure of the wild-type and C101A GDNF are indistinguishable. The thermal stability of GDNF does not appear to be affected by the absence of the interchain disulfide bond and the biological activity of the C101A mutant is identical with that of the wild-type protein. However, small but significant changes in side chain conformations of tyrosine and aliphatic residues were observed by FT-Raman spectroscopy upon removal of the intermolecular disulfide bond, which may reflect structural changes in the area of dimeric contact. By comparing the Raman spectrum of wild-type GDNF with that of the C101A analog, we identified the conformation of the intermolecular disulfide as trans-gauche-trans geometry. These results indicate that GDNF is an active, properly folded molecule in the absence of the interchain disulfide bond.

The fifth immunoglobulin-like domain of the Kit receptor is required for proteolytic cleavage from the cell surface

Stem cell factor (SCF) initiates its biological effects by binding to its receptor Kit. Cell surface Kit is proteolytically cleaved to generate soluble Kit. Structure-function analysis of the extracellular region of Kit has implicated the first three immunoglobulin-like domains in SCF binding, and the fourth immunoglobulin-like domain in receptor dimerization. However, the role of the fifth immunoglobulin-like domain is unknown. To test the hypothesis that the fifth immunoglobulin-like domain is important for proteolytic cleavage of Kit from the cell surface, we constructed a mutant form of Kit in which the first four immunoglobulin-like domains are linked to the transmembrane and cytoplasmic domains (designated Kit-Del5). Kit-wild type (Kit-WT) and Kit-Del5 were expressed in the murine mast cell line IC2. Flow cytometry demonstrated that both Kit-WT and Kit-Del5 are displayed on the IC2 cell surface, and immunoblotting confirmed the presence of Kit proteins of the expected molecular weights, 154 kDa and 134 kDa, respectively. Although IC2-Kit-WT cells proteolytically cleave cell surface Kit, generating a 98 kDa soluble form of Kit, IC2-Kit-Del5 cells do not. These findings demonstrate that the fifth immunoglobulin-like domain of Kit is required for proteolytic cleavage of Kit from the cell surface.

Copper staining method for extracting biologically active proteins from native gels

An attempt was made to make protein bands visible on native gel using copper staining, since such a mild staining procedure would make the entire native gel electrophoresis process non-denaturing. Copper staining not only was able to detect various proteins on native gel with reasonable sensitivity, but also made extraction and recovery of active proteins possible from the gel using a gentle procedure.

Soluble Kit receptor blocks stem cell factor bioactivity in vitro

Stem cell factor (SCF) is a growth factor that promotes the survival, proliferation, and differentiation of hematopoietic cells. SCF and its receptor, Kit, are normally present in both cell surface and soluble forms. Both forms of Kit can bind SCF. However, the function of soluble Kit is unknown. In order to determine if soluble Kit can modulate SCF activity, we produced a fusion protein, Kit-Fc, comprised of the extracellular domain of murine Kit and the Fc portion of human IgG(1) and investigated its ability to bind 125I-SCF and to inhibit SCF-stimulated hematopoietic colony growth in vitro. Stable cell lines expressing Kit-Fc were generated and Kit-Fc was purified to greater than 95% purity. Scatchard analysis demonstrated that Kit-Fc binds iodinated SCF with high affinity (Kd 570 pM). Kit-Fc also bound to transmembrane SCF displayed on the surface of fibroblasts. The murine mast cell line IC2 was engineered to express murine Kit on the cell surface and was demonstrated to proliferate in the presence of SCF. Kit-Fc completely blocked SCF-stimulated proliferation of IC2-Kit cells, but not IL-3-stimulated growth of IC2-Kit cells, demonstrating the specificity of Kit-Fc. We investigated the ability of Kit-Fc to block SCF-stimulated murine hematopoietic colony growth. Kit-Fc blocked SCF-stimulated erythroid colony growth as effectively as a neutralizing anti-Kit monoclonal antibody, ACK2, but did not block erythropoietin-stimulated erythroid colony growth. Likewise, Kit-Fc blocked SCF-stimulated myeloid colony growth as effectively as ACK2 antibody, but did not block IL-3- or GM-CSF-stimulated myeloid colony growth. These results indicate that a form of soluble Kit binds SCF with high affinity, and can specifically block the ability of SCF to stimulate hematopoietic colony growth, suggesting that one function of soluble Kit may be to modulate SCF bioactivity.

Structure of the active core of human stem cell factor and analysis of binding to its receptor kit

Stem cell factor (SCF) is an early-acting hematopoietic cytokine that elicits multiple biological effects. SCF is dimeric and occurs in soluble and membrane-bound forms. It transduces signals by ligand- mediated dimerization of its receptor, Kit, which is a receptor tyrosine kinase related to the receptors for platelet-derived growth factor (PDGF), macrophage colony-stimulating factor, Flt-3 ligand and vascular endothelial growth factor (VEGF). All of these have extracellular ligand-binding portions composed of immunoglobulin-like repeats. We have determined the crystal structure of selenomethionyl soluble human SCF at 2.2 A resolution by multiwavelength anomalous diffraction phasing. SCF has the characteristic helical cytokine topology, but the structure is unique apart from core portions. The SCF dimer has a symmetric 'head-to-head' association. Using various prior observations, we have located potential Kit-binding sites on the SCF dimer. A superimposition of this dimer onto VEGF in its complex with the receptor Flt-1 places the binding sites on SCF in positions of topographical and electrostatic complementarity with the Kit counterparts of Flt-1, and a similar model can be made for the complex of PDGF with its receptor.

Deficiency of Trp53 rescues the male fertility defects of Kit(W-v) mice but has no effect on the survival of melanocytes and mast cells

Mutations of the receptor tyrosine kinase, Kit, or its ligand, mast growth factor (Mgf), affect three unrelated cell populations: melanocytes, germ cells, and mast cells. Kit signaling is required initially to prevent cell death in these lineages both in vitro and in vivo. Mgf appears to play a role in the survival of some hematopoietic cells in vitro by modulating the activity of p53. Signaling by Mgf inhibits p53-induced apoptosis of erythroleukemia cell lines and suppresses p53-dependent radiation-induced apoptosis of bone marrow cells. We tested the hypothesis that cell survival in Kit mutant mice would be enhanced by p53 deficiency in vivo. Double-mutant mice, which have greatly reduced Kit receptor tyrosine kinase activity and also lack Trp53, were generated and the affected cell lineages examined. Mast cell, melanoblast, and melanocyte survival in the double Kit(W-v/W-v):Trp53(-/-) mutants was not increased compared to the single Kit(W-v/W-v):Trp53(+/+) mutants. However, double-mutant males showed an increase in sperm viability and could father litters, in contrast to their homozygous Kit mutant, wild-type p53 littermates. This germ cell rescue appears to be male specific, as female ovaries were similar in mice homozygous for the Kit mutant allele with or without p53. We conclude that defective Kit signaling in vivo results in apoptosis by a p53-independent pathway in melanocyte and mast cell lineages but that in male germ cells apoptosis in the absence of Kit is p53-dependent.

Early signaling pathways activated by c-Kit in hematopoietic cells

c-Kit is a receptor tyrosine kinase that binds stem cell factor (SCF). Structurally, c-Kit contains five immunoglobulin-like domains extracellularly and a catalytic domain divided into two regions by a 77 amino acid insert intracellularly. Studies in white spotting and steel mice have shown that functional SCF and c-Kit are critical in the survival and development of stem cells involved in hematopoiesis, pigmentation and reproduction. Mutations in c-Kit are associated with a variety of human diseases. Interaction of SCF with c-Kit rapidly induces receptor dimerization and increases in autophosphorylation activity. Downstream of c-Kit, multiple signal transduction components are activated, including phosphatidylinositol-3-kinase, Src family members, the JAK/STAT pathway and the Ras-Raf-MAP kinase cascade. Structure-function studies have begun to address the role of these signaling components in SCF-mediated responses. This review will focus on the biochemical mechanism of action of SCF in hematopoietic cells.

Heparin is essential for a single keratinocyte growth factor molecule to bind and form a complex with two molecules of the extracellular domain of its receptor

Keratinocyte growth factor (KGF or FGF-7) is a member of the heparin binding fibroblast growth factor (FGF) family and is a paracrine mediator of proliferation and differentiation of a wide variety of epithelial cells. To examine the stoichiometry of complexes formed between KGF and its receptor, we have utilized a soluble variant of the extracellular region of the KGF receptor containing two tandem immunoglobulin-like loops, loops II and III (sKGFR). Ligand-receptor complexes were examined by size exclusion chromatography, light scattering, N-terminal protein sequencing, and sedimentation velocity. In the presence of low-molecular mass heparin ( approximately 3 kDa), we demonstrate the formation of complexes containing two molecules of sKGFR and one molecule of KGF. In the absence of heparin, we were unable to detect any KGF-sKGFR complexes using the above techniques, and additional studies in which sedimentation equilibrium was used show that the binding is very weak (Kd >/= 70 microM). Furthermore, using heparin fragments of defined size, we demonstrate that a heparin octamer or decamer can promote formation of a 2:1 complex, while a hexamer does not. Utilizing the highly purified proteins and defined conditions described in this study, we find that heparin is obligatory for formation of a KGF-sKGFR complex. Finally, 32D cells, which appear to lack low-affinity FGF binding sites, were transfected with a KGFR-erythropoeitin receptor chimera and were found to require heparin to achieve maximal KGF stimulation. Our data are consistent with the previously described concept that cell- or matrix-associated heparan sulfate proteoglycans (HSPGs) and FGF ligands participate in a concerted mechanism that facilitates FGFR dimerization and signal transduction in vivo.

Mode of receptor binding and activation by plasminogen-related growth factors

Hepatocyte growth factor/scatter factor (HGF/SF) and macrophage stimulating protein (MSP) are plasminogen-related kringle proteins that lost serine protease domain enzymatic activity and became ligands for cell surface tyrosine kinase receptors. They are activated by cleavage to disulfide-linked alphabeta chains. Surprisingly, despite structural similarities, the high affinity receptor binding regions of the two proteins are different: alpha chain for HGF, and beta chain for MSP. We propose that after cleavage exposes a beta chain binding site (high affinity for MSP, low affinity for HGF), monomeric ligand induces receptor dimerization and activation via alpha and beta chain binding sites of different affinity.

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.

Oligomerization of the extracellular domain of Boss enhances its binding to the Sevenless receptor and its antagonistic effect on R7 induction

In the developing compound eye of Drosophila, neuronal differentiation of the R7 photoreceptor cell is induced by the interaction of the receptor tyrosine kinase Sevenless with its ligand Bride of sevenless (Boss), which is expressed on the neighboring R8 cell. Boss is an unusual ligand of a receptor tyrosine kinase: it is composed of a large extracellular domain, a transmembrane domain with seven membrane-spanning segments and a cytoplasmic tail. Expression of a monomeric, secreted form of the extracellular domain of Boss is not sufficient for Sevenless activation, and instead acts as a weak antagonist. Because oligomerization appears to be a critical step in the activation of receptor tyrosine kinases, we used oligomerized forms of the Boss extracellular domain to test their ability to bind to Sevenless in vivo and restore R7 induction in vivo. Oligomerization was achieved by fusion to the leucine zipper of the yeast transcription factor GCN4 or to the tetramerization helix of Lac repressor. Binding of these multivalent proteins to Sevenless could be detected in vitro by immunoprecipitation of cross-linked ligand/receptor complexes and in vivo by receptor-dependent ligand localization. However, neither R8-specific or ubiquitous expression of multivalent Exboss ligands rescued the boss phenotype. Instead, these ligands acted as competitive inhibitors for wild-type Boss protein and thereby suppressed R7 induction. Therefore the role of the transmembrane or cytoplasmic domains of Boss in the activation of the Sev receptor cannot be replaced by oligomerization.

Analysis of c-kit Receptor Dimerization by Fluorescence Resonance Energy Transfer

Stem cell factor (SCF) binding to the c-kit receptor triggers homodimerization and intermolecular tyrosine phosphorylation of the c-kit receptor, thus initiating signal transduction. Receptor dimerization is a critical early step in this process. Prior biochemical studies of c-kit receptor dimerization have mainly used affinity cross-linking techniques, which are beset with problems including low efficiency of cross-linking and the usual requirement for radiolabeled SCF to detect the cross-linked complex. We used the fluorescence resonance energy transfer (FRET) technique to examine the effects of SCF and other hematopoietic cytokines on c-kitreceptor dimerization. The nonneutralizing anti–c-kit receptor monoclonal antibody 104D2 was directly conjugated to fluorescein isothiocyanate (FITC) or to the carbocyanine dye Cy3 and used to label cytokine-responsive human hematopoietic cell lines. The ability of SCF to induce c-kit receptor dimerization was assessed by flow cytometric analysis of FRET between the donor fluorochrome FITC and the acceptor fluorochrome Cy3. SCF induced a dose-dependent increase inc-kit receptor dimerization that correlated well with the concentrations of SCF required to stimulate cell proliferation. Receptor dimerization was detectable within 3 minutes after the addition of SCF and was maximal 30 minutes after the addition of SCF. Confocal microscopy showed redistribution of the c-kit receptor (from a diffuse distribution on the cell surface to “caps” at one end of the cell) within 3 minutes after SCF addition, followed by receptor internalization. Reappearance of the c-kit receptor on the cell surface required new protein synthesis, suggesting that thec-kit receptor is not recycled to the cell surface after internalization. Finally, erythropoietin (Epo), but not the structurally and functionally related cytokine thrombopoietin (Tpo), stimulated c-kit receptor dimerization detectable by FRET, and tyrosine phosphorylation of the c-kit receptor. These results suggest that exposure to Epo can activate the c-kit receptor and provide further evidence for cross-talk between the Epo andc-kit receptors in human hematopoietic cell lines. Studies with progeny of burst-forming unit-erythroid (BFU-E) suggest that the FRET technique is sufficiently sensitive to detectc-kit receptor dimerization on normal human hematopoietic cells.

Analysis of c-kit Receptor Dimerization by Fluorescence Resonance Energy Transfer

Stem cell factor (SCF) binding to the c-kit receptor triggers homodimerization and intermolecular tyrosine phosphorylation of the c-kit receptor, thus initiating signal transduction. Receptor dimerization is a critical early step in this process. Prior biochemical studies of c-kit receptor dimerization have mainly used affinity cross-linking techniques, which are beset with problems including low efficiency of cross-linking and the usual requirement for radiolabeled SCF to detect the cross-linked complex. We used the fluorescence resonance energy transfer (FRET) technique to examine the effects of SCF and other hematopoietic cytokines on c-kitreceptor dimerization. The nonneutralizing anti–c-kit receptor monoclonal antibody 104D2 was directly conjugated to fluorescein isothiocyanate (FITC) or to the carbocyanine dye Cy3 and used to label cytokine-responsive human hematopoietic cell lines. The ability of SCF to induce c-kit receptor dimerization was assessed by flow cytometric analysis of FRET between the donor fluorochrome FITC and the acceptor fluorochrome Cy3. SCF induced a dose-dependent increase inc-kit receptor dimerization that correlated well with the concentrations of SCF required to stimulate cell proliferation. Receptor dimerization was detectable within 3 minutes after the addition of SCF and was maximal 30 minutes after the addition of SCF. Confocal microscopy showed redistribution of the c-kit receptor (from a diffuse distribution on the cell surface to “caps” at one end of the cell) within 3 minutes after SCF addition, followed by receptor internalization. Reappearance of the c-kit receptor on the cell surface required new protein synthesis, suggesting that thec-kit receptor is not recycled to the cell surface after internalization. Finally, erythropoietin (Epo), but not the structurally and functionally related cytokine thrombopoietin (Tpo), stimulated c-kit receptor dimerization detectable by FRET, and tyrosine phosphorylation of the c-kit receptor. These results suggest that exposure to Epo can activate the c-kit receptor and provide further evidence for cross-talk between the Epo andc-kit receptors in human hematopoietic cell lines. Studies with progeny of burst-forming unit-erythroid (BFU-E) suggest that the FRET technique is sufficiently sensitive to detectc-kit receptor dimerization on normal human hematopoietic cells.

Activation of the receptor tyrosine kinase Kit is required for the proliferation of melanoblasts in the mouse embryo

The development of neural crest-derived melanocytes, as well as haematopoietic and germ cells, is affected by mutations of the Kit and Mgf genes, which lead to dominant spotting (W) or steel (Sl) phenotypes. Mgf codes for the ligand of the receptor tyrosine kinase encoded by the Kit locus. KitW-v, a point mutation exerting a dominant negative effect, causes a substantial reduction in tyrosine kinase activity of the Kit receptor and leads to a characteristic pigmentation phenotype, namely dilute coat colour and a white ventral and head spot with reduced pigmentation of the feet and tail in the heterozygous animal, as well as slight anaemia. Homozygous animals lack coat pigmentation and are severely anaemic and infertile. Dct is a marker for cells of the melanoblast lineage. In order to study these cells in detail we have generated transgenic mouse lines carrying the lacZ reporter under the control of the Dct promoter and have used the embryonic expression of the reporter to identify early melanoblasts before they begin to produce pigment. Our transgenic lines have simplified the study of melanoblasts in the mouse embryo, and by crossing our mice with KitW-v mutants we have been able to identify the midgestation stages at which melanoblasts rely critically on Mgf/Kit interactions. We conclude that the survival of immature melanoblasts depends crucially upon Kit signalling up until E11, and later in development Kit plays a vital role in melanoblast proliferation. Our data do not describe a dependence upon Kit for melanoblast migration or differentiation.

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.