Molecular bases of dominant negative and loss of function mutations at the murine c-kit/white spotting locus: W37, Wv, W41 and W

A cell- and developmental stage-specific promoter drives the expression of a truncated c-kit protein during mouse spermatid elongation

In the postnatal testis, the c-kit transmembrane tyrosine-kinase receptor is expressed in type A spermatogonia, and its transcription ceases at the meiotic phase of spermatogenesis. Alternative, shorter c-kit transcripts are expressed in post-meiotic germ cells. These transcripts should encode a truncated version of the c-kit protein, lacking the extracellular, the transmembrane and part of the intracellular tyrosine-kinase domains. The 5′ end of the alternative c-kit transcripts maps within an intron of the mouse c-kit gene. We now show that this intron contains a promoter active in nuclear extracts of round spermatids, and that two discrete sequences upstream of the transcriptional start site bind spermatid-specific nuclear factors. Deletion of both these sequences abolishes activity of the promoter in vitro. We have also established that this promoter is functional in vivo, in a tissue-and cell-specific fashion, since intronic sequences drive the expression of the E. coli lacZ reporter gene in transgenic mice specifically in the testis. Transgene expression is confined to haploid germ cells of seminiferous tubules, starting from spermatids at step 9, and disappearing at step 13, indicating that cryptic promoter within the 16th intron of the mouse c-kit gene is active in a short temporal window at the end of the transcriptional phase of spermiogenesis. In agreement with these data, western blot experiments using an antibody directed against the carboxy-terminal portion of the mouse c-kit protein showed that a polypeptide, of the size predicted by the open reading frame of the spermatid-specific c-kit cDNA, accumulates in the latest stages of spermatogenesis and in epididymal spermatozoa. An immunoreactive protein of the same size can be produced in both eukaryotic and prokaryotic artificial expression systems.

Recombinant human stem cell factor does exert minor stimulation of growth in small cell lung cancer and melanoma cell lines

We have previously reported on the stimulation of clonal growth of a glioblastoma cell line by rhSCF (Berdel et al., Cancer Res 1992, 52, 3498-3502). Within an extensive screening programme of haematopoietic growth factor activity on malignant cells, the effects of rhSCF were further tested on the growth of 29 different human cell lines derived from a wide range of solid tumours, among them six lung cancers and five melanomas. RhSCF (0, 1, 10, 100 ng/ml) was tested in a human tumour cloning assay (HTCA) which reliably detects growth modulation of tumour cells by cytokines. Additionally, a tritiated thymidine uptake test was used. Growth of 27 of the 29 cell lines tested was not affected by rhSCF. However, growth of the small cell lung cancer (SCLC) cell line HTB 120 was slightly stimulated (1.5 fold that of controls), and that of the melanoma cell line MeWo was stimulated up to 1.3-fold. This activity was eliminated dose-dependently by the tyrosine kinase inhibitor, genistein. We further analysed the cell lines for expression of the proto-oncogene C-KIT and its ligand SCF. All melanoma and lung cancer cell lines expressed SCF as assessed at the mRNA level. Northern blotting also revealed clear C-KIT mRNA expression in three melanoma (HAS, MeWo, SK-MEL-28), one NSCLC (HTB 53), and four SCLC cell lines (HTB 119, HTB 120, HTB 171, HTB 175). Furthermore, C-KIT protein expression was detected by flow cytometric analysis on the cell surface of MeWo, HTB 119 and HTB 120 cells. Our data indicate that SCF can be operative in growth modulation of non-haematopoietic malignant cells, especially SCLC and melanoma. However, our extensive screening of SCF/tumour cell interaction shows that this interaction is rare and makes potential hazards, such as tumour stimulation upon clinical use of rhSCF in conjunction with chemotherapy in cancer patients, unlikely for the majority of other tumour histologies.

PDGF signalling is required for gastrulation of Xenopus laevis

During Xenopus gastrulation, platelet-derived growth factor (PDGF) receptor-alpha is expressed in involuting marginal zone cells which migrate over ectodermal cells expressing PDGF-A. To investigate the role of PDGF signalling during this process, we have generated a novel point mutant of PDGF receptor-alpha analogous to the W37 mutation of c-kit. This molecule is a specific, potent, dominant inhibitor of PDGF signalling in vivo. Injection of RNA encoding this protein into Xenopus embryos prevents closure of the blastopore, leads to abnormal gastrulation and a loss of anterior structures. Convergent extension is not inhibited in these embryos, but rather, involuting mesodermal cells fail to adhere to the overlying ectoderm. PDGF may therefore be required for mesodermal cell-substratum interaction.

Regulation of cell-matrix adhesion by receptor tyrosine kinases

Cell-cell and cell-matrix adhesive interactions mediated by integrins play crucial roles in leukocyte migration to inflamed tissues, and also in cell migration during embryogenesis. Much remains to be learned about the molecular mechanisms of regulation of adhesion mediated by integrins. Recently we found that steel factor and c-kit induce adhesion to fibronectin by VLA-5 in mast cells. Activation of adhesiveness is transient, and occurs at concentrations of steel factor 100-fold lower than required for growth stimulation. This suggests that regulation of adhesion is an important biological function of steel factor and c-kit. Other receptor tyrosine kinases such as the PDGF receptor can substitute for c-kit. Signaling through receptor tyrosine kinases may offer a general mechanism for the regulation of integrin avidity.

Characterization of c-kit-positive neurons in the dorsal root ganglion of mouse

Previously, we showed that c-kit receptor tyrosine kinase is expressed by a subpopulation of dorsal root ganglion (DRG) neurons, and that the ligand for the c-kit receptor, stem cell factor (SCF), induces the neurite outgrowth and supports the survival of these neurons in culture [16]. However, it is unknown which class of DRG neurons express c-kit receptor and which factor regulates differentiation and survival of c-kit-positive neurons. In the present study, we attempted to characterize c-kit positive neurons in the mouse DRG. The c-kit-positive neurons were small or medium in size, and 44% of these neurons contained substance P. Central fibers of the c-kit-positive neurons terminated in laminae I and II of the gray matter of the spinal cord. These results suggest that c-kit-positive neurons in the DRG belong to a functional subpopulation. The c-kit receptor protein was presented on the membrane of processes and growth cones in neurons. When DRG cells of embryonic day 15.5 or 17.5 were cultured, the survival of c-kit-positive neurons was supported by SCF, nerve growth factor (NGF) or leukemia inhibitory factor. SCF and NGF synergistically supported the survival of c-kit-positive neurons at submaximal concentrations. c-kit-positive DRG neurons from neonatal mice survived without addition of any factor in culture, suggesting that the requirement for trophic support in c-kit-positive neurons changes during development.

c-kit-dependent development of interstitial cells and electrical activity in the murine gastrointestinal tract

In vivo injection of a neutralizing, monoclonal antibody (ACK2) to the receptor tyrosine kinase (c-kit) disrupts the normal motility patterns of the mouse small intestine. Immunohistochemical studies showed that cells expressing c-kit-like immunoreactivity (c-kit-LI) decreased in numbers in response to ACK2, but the identity of these cells is unknown. We investigated the identity and development of the cells that express c-kit-LI in the mouse small intestine and colon. Cells in the region of the myenteric plexus and deep muscular plexus of the small intestine and in the subserosa, in the myenteric plexus region, within the circular and longitudinal muscle layers, and along the submucosal surface of the circular muscle in the colon were labeled with ACK2. The distribution of cells that express c-kit-LI was the same as that of interstitial cells (ICs). In whole-mount preparations cells with c-kit-LI were interconnected, forming a network similar to the network formed by cells that stained with methylene blue, which has been used as a marker for ICs in the mouse gastrointestinal tract. Immunocytochemistry verified that ICs were labeled with ACK2. Multiple injections of animals with ACK2 between days 0 and 8 post partum (pp) caused a dramatic reduction in the number of ICs compared to control animals. From an ultrastructural point of view, the proliferation and development appeared to be suppressed in some classes of ICs, while others displayed an altered course of development. Functional studies showed that the decrease in ICs was accompanied by a loss of electrical rhythmicity in the small intestine and reduced neural responses in the small bowel and colon. Morphological experiments showed that c-kit-positive cells are ICs, and physiological evidence reinforced the concept that ICs are involved in generation of rhythmicity and translation of neural inputs in gastrointestinal smooth muscles. Controlling the development of ICs provides a powerful new tool for the investigation of the physiological role of these cells.

Mutations at the W locus affect survival of neural crest-derived melanocytes in the mouse

The development of melanoblasts in normally pigmented and dominant spotting (W) embryos was followed by in situ hybridisation to TRP-2/DT mRNA, which labels migratory melanoblasts from 10 days post coitum. Numerous melanoblasts migrate to the inner ear around 11 days. In contrast, few migratory melanoblasts are associated with the eye or skin at this stage and melanoblasts distribution within the trunk and tail is patchy. The distribution of melanoblasts in 10.5-11-day-old Wv/Wv, Wsh/Wsh and W41/W41 mutants was similar to that in controls but melanoblasts density was lower and by 12 days was severely reduced. These results suggest that mutations of the c-kit receptor tyrosine kinase encoded at the W locus do not alter early migration or differentiation of melanoblasts but severely affect melanoblasts survival.

A Xenopus c-kit-related receptor tyrosine kinase expressed in migrating stem cells of the lateral line system

The mammalian c-kit receptor tyrosine kinase gene is required during embryogenesis for the survival and/or proliferation of three migrating stem cell populations: primordial germ cells, haematopoietic stem cells and neural crest-derived melanoblasts. We have cloned a Xenopus gene, XKrk1, whose closest relative is c-kit. Differences in the expression pattern suggest that XKrk1 is not the Xenopus homologue of c-kit; however, it is expressed in a migrating stem cell population, the precursor cells for the mechanosensory lateral line system. XKrk1 is the first reported marker for lateral line stem cells.

Soluble and cell-bound forms of steel factor activity play distinct roles in melanocyte precursor dispersal and survival on the lateral neural crest migration pathway

Trunk neural crest cells segregate from the neuroepithelium and enter a ‘migration staging area’ lateral to the embryonic neural tube. After some crest cells in the migration staging area have begun to migrate on a medial pathway, a subpopulation of crest-derived cells remaining in the migration staging area expresses mRNAs for the receptor tyrosine kinase, c-kit, and tyrosinase-related protein-2, both of which are characteristic of melanocyte precursors. These putative melanocyte precursors are subsequently observed on the lateral crest migration pathway between the dermatome and overlying epithelium, and then dispersed in nascent dermal mesenchyme. Melanocyte precursors transiently require the c-kit ligand, Steel factor for survival. Although Steel factor mRNA is transiently expressed in the dorsal dermatome before the onset of trunk neural crest cell dispersal on the lateral pathway, it is no longer produced by dermatomal cells when melanocyte precursors have dispersed in the dermal mesenchyme. To assess the role of Steel factor in migration of melanocyte precursors on the lateral pathway, we analyzed melanocyte precursor dispersal and fate on the lateral pathway of two different Sl mutants, Sl, a null allele, and Sld, which lacks cell surface-associated Steel factor but produces a soluble form. No melanocyte precursors were detected in the dermatome of embryos homozygous for the Sl allele or in W mutants that lack functional c-kit. In contrast, in embryos homozygous for the Sld allele, melanocyte precursors appeared on the lateral pathway, but subsequently disappear from the dermis. These results suggest that soluble Steel factor is required for melanocyte precursor dispersal on the lateral pathway, or for their initial survival in the migration staging area. In contrast, membrane-bound Steel factor appears to promote melanocyte precursor survival in the dermis.

Characterization of a novel spermatogenic cell antigen specific for early stages of germ cells in mouse testis

To study the mechanism of spermatogenesis during the premeiotic phase, a hybridoma producing monoclonal antibody (mAb) specific for early stages of spermatogenic cells was obtained. In immunohistochemical staining of adult testis, this mAb, designated as EE2, was able to react with type A to B spermatogonia and early meiotic cells, but not with Sertoli cells, Leydig cells, and other somatic tissues. Precursor cells of type A spermatogonia (gonocytes) were also positive for EE2 in perinatal mouse testis. The antigenic molecule recognized by mAb EE2 was a novel glycoprotein with molecular weight of 114 kDa, which had affinity with Con A and WGA lectins, and was susceptible to N-glycanase, suggesting the presence of asparagine-linked sugar chains. Furthermore, EE2 antigen was found to localize on the germ cell surface. The specific expression of this antigenic molecule suggests that it may play an important role in early spermatogenesis, of which only a little information is available at present.

The role of mast cells in the development of skin fibrosis in tight-skin mutant mice

Chronic inflammatory conditions can evolve a fibrotic phenotype often associated with an increase in the number of mast cells (MC) near or within the granulation tissue. Despite the potential of MC to mediate fibrosis, it is unclear whether these cells play a central role in the pathogenesis of fibrosis or whether their presence is simply circumstantial. The tight-skin (Tsk) mouse develops an inherited fibrotic disease (sharing many similarities with the human disease scleroderma, systemic sclerosis) in which the lesions are associated with increased numbers and heightened granule release implicating MC in the pathogenesis of fibrosis. Despite their close association with the skin fibrosis of Tsk mice, the precise role of the MC in the pathogenesis of this inherited disease is unknown. Therefore, to assess directly whether MC are key elements in the pathogenesis of Tsk fibrosis, we generated MC deficient mice carrying the Tsk locus by utilizing selective interbreeding between Tsk and mutant mice deficient in mast cells (W, dominant white-spotting). We found that in the absence of MC, the early natural history of Tsk fibrosis was not altered. Furthermore, in older (5-7 months) Tsk mice, we found that the number of cutaneous MC was correlated with a more pronounced fibrosis. Therefore, we conclude that Tsk skin lesions are a pleiotropic manifestation of the Tsk gene in which MC are involved/recruited by an uncharacterized mechanism and that subsequent proliferation and activation of MC leads to augmentation of fibrosis.

A 1.8-Mb YAC contig spanning three members of the receptor tyrosine kinase gene family (Pdgfra, Kit, and Flk1) on mouse chromosome 5

We constructed a yeast artificial chromosome (YAC) contig spanning the genes encoding Kit (Kit), the platelet-derived growth factor alpha receptor (Pdgfra), and fetal liver kinase 1 (Flk1), three members of a receptor tyrosine kinase gene family located in the central portion of mouse chromosome 5. The orientation of YAC clones and the extent of their overlap was determined by "probe content mapping," that is, hybridization analysis of YAC clones using the available gene probes and YAC end sequences. For four YAC clones, which constitute a minimal set spanning 1.8 Mb, a detailed restriction map was constructed. This map, in conjunction with the previously published long-range restriction map, indicates the order, the physical distances, and the relative transcriptional orientations of the Pdgfra, Kit, and Flk1 genes. The YAC clones and corresponding YAC end probes presented here provide an important resource for the molecular analysis of a cluster of developmental mutations, namely dominant white spotting (W), patch (Ph), recessive spotting (rs), and rump-white (Rw), associated with this chromosomal region.

The fourth immunoglobulin domain of the stem cell factor receptor couples ligand binding to signal transduction

Receptor dimerization is ubiquitous to the action of all receptor tyrosine kinases, and in the case of dimeric ligands, such as the stem cell factor (SCF), it was attributed to ligand bivalency. However, by using a dimerization-inhibitory monoclonal antibody to the SCF receptor, we confined a putative dimerization site to the nonstandard fourth immunoglobulin-like domain of the receptor. Deletion of this domain not only abolished ligand-induced dimerization and completely inhibited signal transduction, but also provided insights into the mechanism of the coupling of ligand binding to dimer formation. These results identify an intrinsic receptor dimerization site and suggest that similar sites may exist in other receptors.

Novel mutations and deletions of the KIT (steel factor receptor) gene in human piebaldism

Piebaldism is an autosomal dominant genetic disorder of pigmentation characterized by white patches of skin and hair. Melanocytes are lacking in these hypopigmented regions, the result of mutations of the KIT gene, which encodes the cell surface receptor for steel factor (SLF). We describe the analysis of 26 unrelated patients with piebaldism-like hypopigmentation--17 typical patients, 5 with atypical clinical features or family histories, and 4 with other disorders that involve white spotting. We identified novel pathologic mutations or deletions of the KIT gene in 10 (59%) of the typical patients, and in 2 (40%) of the atypical patients. Overall, we have identified pathologic KIT gene mutations in 21 (75%) of 28 unrelated patients with typical piebaldism we have studied. Of the patients without apparent KIT mutations, none have apparent abnormalities of the gene encoding SLF itself (MGF), and genetic linkage analyses in two of these families are suggestive of linkage of the piebald phenotype to KIT. Thus, most patients with typical piebaldism appear to have abnormalities of the KIT gene.

Mutagenesis and human genetic disease: dominant mutation frequencies and a characterization of mutational events in mice and humans

Dominant deleterious traits are generally regarded to be the most relevant genetic endpoints when the expected increased mutational load of genetic diseases associated with exposure to mutagenic agents is considered in humans. At present, human risk estimation procedures rely on results from laboratory mammal germ-cell mutagenicity experiments as well as on data from human epidemiology and medical genetics. A comparison of the mouse and human data indicates that a small subset of loci, which when mutated result in a dominant phenotype, is contributing disproportionately to the observed mutation frequency. This is likely due to the fact that those loci with an observed high mutation frequency are inherently unstable, the function of such loci is critical, and/or the wild-type phenotype requires two copies of the normal gene (haploinsufficiency). The locus specificity of the observed spontaneous and induced mutation frequencies implies that efforts must be made to closely match those genetic endpoints screened in the mouse with the human genetic endpoints considered relevant in estimating the genetic risk after exposure to mutagenic agents. The contributions to our understanding of the organization, function, and stability of the mouse and human genomes provided by molecular biological techniques should make compliance with this restriction feasible.

The Genomic Structure of the Proto-Oncogene c-kit Encoded at the Murine White Spotting Locus

The proto-oncogene c-kit encodes a transmembrane receptor with tyrosine kinase activity, which transduces signal from kit ligand (KL), and is responsible for hematogenesis, melanogenesis and gametogenesis during fetal development and adult life. Partial or complete loss of c-kit function due to mutation of the c-kit or KL gene accounts for the phenotypes of the murine White-spotting and Steel mutations, respectively. The c-kit protein has the structural features of extracellular immunoglobulin-like domains and intracellular kinase domain with a hydrophilic 'insert'. These features have categorized c-kit along with platelet-derived growth factor receptors, colony-stimulating factor 1 receptor (c-fms) and others to subclass III of the receptor tyrosine kinases. We report the structure of the murine c-kit gene. The c-kit gene consists of 21 exons and spans at least 70 kb. The 5' and 3' flanking exons encode the untranslated sequences as well as part of the coding sequence. The internal exons are typically small with each of them encoding a structurally important subunit of the protein. Comparison of gene structures of members of the subclass III receptor tyrosine kinases has improved our understanding of the structure-functional relationship of the c-kit protein. Copyright 1995 S. Karger AG, Basel

Comparative cytokine release from human monocytes, monocyte-derived immature mast cells, and a human mast cell line (HMC-1)

To obtain further information regarding the role of cytokines during mast cell differentiation, we have investigated changes of cytokine secretion in mast cells developing from the human peripheral blood monocytic cell fraction during culture with fibroblast-derived conditioned media. The influence of stem cell factor and an antibody to the respective receptor in our culture system was studied as well. Interleukin (IL)-1 alpha, IL-1 beta, IL-6, and tumor necrosis factor (TNF)alpha were spontaneously secreted by cultured cells at day 1 and decreased markedly by day 14. Similar changes occurred also during culture with stem cell factor and were partially abrogated by an anti-receptor antibody. IL-8 was secreted at a high level throughout the culture, whereas no spontaneous secretion of IL-2, IL-3, IL-4, and IL-7 was measured at all. Upon stimulation with phorbol myristate acetate and A23187, cultured cells showed substantially more release of IL-3 and TNF-alpha after 14 d of culture, compared to peripheral blood monocytic cells. Preformed TNF-alpha was found in one of three monocytic cell preparations from peripheral blood, but not in monocytic cell-derived mast cells. During mast cell differentiation, cytokines from monocytic cells are therefore downregulated while the cells assume a pattern typically found in mast cells.

Cell type-specific deficiency of c-kit gene expression in mutant mice of mi/mi genotype

The mi locus of mice encodes a novel member of the basic-helix-loop-helix-leucine zipper protein family of transcription factors (hereafter called mi factor). In addition to microphthalmus, osteopetrosis, and lack of melanocytes, mice of mi/mi genotype are deficient in mast cells. Since the c-kit receptor tyrosine kinase plays an important role in the development of mast cells, and since the c-kit expression by cultured mast cells from mi/mi mice is deficient in both mRNA and protein levels, the mast cell deficiency of mi/mi mice has been attributed at least in part to the deficient expression of c-kit. However, it remained to be examined whether the c-kit expression was also deficient in tissues of mi/mi mice. In the present study, we examined the c-kit expression by mi/mi skin mast cells using in situ hybridization and immunohistochemistry. Moreover, we examined the c-kit expression by various cells other than mast cells in tissues of mi/mi mice. We found that the c-kit expression was deficient in mast cells but not in erythroid precursors, testicular germ cells, and neurons of mi/mi mice. This suggested that the regulation of the c-kit transcription by the mi factor was dependent on cell types. Mice of mi/mi genotype appeared to be a useful model to analyze the function of transcription factors in the whole-animal level.

Dominant-negative and targeted null mutations in the endothelial receptor tyrosine kinase, tek, reveal a critical role in vasculogenesis of the embryo

The receptor tyrosine kinases (RTKs) expressed on the surface of endothelial cells are likely to play key roles in initiating the program of endothelial cell growth during development and subsequent vascularization during wound healing and tumorigenesis. Expression of the Tek RTK during mouse development is restricted primarily to endothelial cells and their progenitors, the angioblasts, suggesting that Tek is a key participant in vasculogenesis. To investigate the role that Tek plays within the endothelial cell lineage, we have disrupted the Tek signaling pathway using two different genetic approaches. First, we constructed transgenic mice expressing a dominant-negative form of the Tek receptor. Second, we created a null allele of the tek gene by homologous recombination in embryonic stem (ES) cells. Transgenic mice expressing dominant-negative alleles of Tek or homozygous for a null allele of the tek locus both died in utero with similar defects in the integrity of their endothelium. By crossing transgenic mice that express the lacZ reporter gene under the transcriptional control of the endothelial cell-specific tek promoter, we found that the extraembryonic and embryonic vasculature was patterned correctly. However, homozygous tek embryos had approximately 30% and 75% fewer endothelial cells at day 8.5 and 9.0, respectively. Homozygous null embryos also displayed abnormalities in heart development, consistent with the conclusion that Tek is necessary for endocardial/myocardial interactions during development. On the basis of the analysis of mice carrying either dominant-negative or null mutations of the tek gene, these observations demonstrate that the Tek signaling pathway plays a critical role in the differentiation, proliferation, and survival of endothelial cells in the mouse embryo.

Genetics and molecular biology of mouse pigmentation

The formation of mouse coat color is a relatively complex developmental process that is affected by a large number of mutations, both naturally occurring and induced. The cloning of the genes in which these mutations occur and the elucidation of the mechanisms by which these mutations disrupt the normal pigmentation pattern is leading to an understanding of the way interactions between gene products lead to a final phenotype.

The c-kit ligand, stem cell factor, promotes mast cell survival by suppressing apoptosis

Stem cell factor (SCF) and its receptor (SCFR), a member of the receptor tyrosine kinase III family that is encoded by the c-kit gene, critically regulate several complex biological programs including hematopoiesis, mast cell development, cutaneous pigmentation, and gametogenesis. We show herein that mouse mast cells die rapidly after the withdrawal of SCF in vivo or in vitro, and provide morphological evidence that such mast cells undergo programmed cell death or apoptosis. We also show that when in vitro-derived mouse mast cells maintained in SCF are removed from SCF-containing medium for only 5 or 6 hours, the cells' genomic DNA exhibits the ladder-like pattern of oligonucleosome-sized fragments typical of apoptosis. These findings demonstrate that SCF can regulate the survival of a cellular lineage which expresses the SCFR by suppressing apoptosis. They also identify a mechanism that can result in striking and rapid reductions in the size of tissue mast cell populations without histological evidence of the concomitant induction of a significant inflammatory response.

Effects of mutations at the W locus (c-kit) on inner ear pigmentation and function in the mouse

The W locus encodes a tyrosine kinase receptor, c-kit, which affects survival of melanoblasts from the neural crest. The primary cochlear defect in Viable Dominant Spotting (Wv/Wv) mutants is a lack of melanocytes within the stria vascularis (SV) associated with an endocochlear potential (EP) close to zero and hearing impairment. In this study, we compare inner ear pigmentation with cochlear potentials in three other W alleles (Wx, Wsh, and W41) and reveal an unequivocal correlation between presence of strial melanocytes and presence of an EP. Asymmetry was common, and 8.3% of Wsh/Wx, 25% of Wsh/Wsh, 60% of W41/Wx, and 69.2% of W41/W41 ears had a pigmented stria and an EP, while the remainder had no strial melanocytes and no EP. In those mutants that partially escaped the effects of the mutation, strial melanocytes rarely extended the entire length of the stria, but were confined to the middle and/or basal turns of the cochlea. The extent of strial pigmentation was unrelated to the EP value, which was measured from the basal turn only. Compound action potential (CAP) responses recorded from ears with an EP were variable and they showed greatly raised thresholds or were absent in all ears where the EP was close to zero. In controls, melanocytes in the vestibular part of the ear were found in the utricle, crus commune, and ampullae, whereas in many mutants only one or two of these regions were pigmented. There was a broad correlation between pigmentation of the stria and pigmentation of the vestibular region but this was not absolute. All W41/Wx, Wsh/Wsh, and W41/W41 mutants had some pigment on the pinna but, in contrast to controls where melanocytes were found in the epidermis and dermis of the pinna, pigment cells were reduced in number and generally restricted to the dermis. Injection of normal neural crest cells into 9.5-day-old mutant embryos increased the extent of skin pigmentation on the head and coat of adult chimeras and was associated with a small increase in the proportion of pigmented strias.

Point mutations affecting the tyrosine kinase domain of the RET proto-oncogene in Hirschsprung's disease

Hirschsprung's disease is a genetic disorder of neural crest development affecting 1 in 5,000 births. It is characterized by the absence of intramural ganglion cells in the hindgut, which often results in partial to complete intestinal obstruction during the first years of life. An autosomal dominant gene causing this disease was recently mapped to chromosome 10q11.2 (refs 1,2), using an interstitial deletion of this region isolated in a cell hybrid. It was subsequently localized to a 250-kilobase interval which contains the RET proto-oncogene. Using flanking intronic sequences as primers to amplify 12 of the 20 exons of RET from genomic DNA of 27 Hirschsprung's disease patients, we have now identified four mutations (one frameshift and three missense) that totally disrupt or partially change the structure of the tyrosine kinase domain of the RET protein (Ret). Mutations in the extracellular cysteine-rich domain of Ret have been identified previously in patients with multiple endocrine neoplasia type 2A, and a targeted mutation in the tyrosine kinase domain of the same gene produces intestinal aganglionosis and kidney agenesis in homozygous transgenic mice. Our results support the hypothesis that RET, in addition to its potential role in tumorigenesis, plays a critical role in the embryogenesis of the mammalian enteric nervous system.

Growth factor regulation of mouse primordial germ cell development

This chapter focused on three key regulators of PGC survival and proliferation; SLF, LIF, and bFGF. The survival of all animal cells may require multiple polypeptide factors and PGCs seem to be no exception (Fig. 7). A number of lines of evidence suggest that membrane-bound forms of SLF may be required for PGC survival. These data suggest an exquisite mechanism for controlling both PGC survival and migration. Thus PGCs that stray from the normal migratory pathway might be eliminated through programmed cell death. SLF, together with LIF, can stimulate PGC proliferation in culture and it seems likely that LIF or a related cytokine may function in vivo to regulate PGC survival and proliferation. Animals doubly deficient in LIF and its relatives may soon allow the roles of these cytokines in PGC development to be determined. Although bFGF is a potent PGC mitogen in vitro, whether PGCs ever encounter bFGF in vivo remains questionable since in culture it alters both the proliferative and developmental potential of PGCs. TGF beta or MIS may be important negative regulators of PGC development, and mice lacking these factors should allow their role in PGC development to be assessed.

Deficient differentiation of mast cells in the skin of mi/mi mice. Usefulness of in situ hybridization for evaluation of mast cell phenotype

The staining property of skin mast cells changed from Alcian blue+/berberine sulfate- to Alcian blue+/berberine sulfate+ in the skin of normal (+/+) and Wv/Wv mice. In contrast, this change did not occur in the skin of mi/mi mice. Heparin content and histamine content per a mi/mi skin mast cell were estimated to be 34% and 18% those of a +/+ skin mast cell, respectively. The low heparin content of mi/mi skin mast cells seemed to be consistent with the Alcian blue+/berberine sulfate- staining property. Expression of genes encoding mast cell-specific proteolytic enzymes was examined by Northern blotting and in situ hybridization. Messenger RNA of mast cell carboxypeptidase A was expressed most of all by +/+, Wv/Wv, and mi/mi skin mast cells, but mRNA of mouse mast cell protease (MMCP)-6 was expressed by approximately a half of +/+ and Wv/Wv skin mast cells and by only 3% of mi/mi skin mast cells. A significant amount of MMCP-2 mRNA was not expressed in the skin of all +/+, Wv/Wv and mi/mi mice. This shows the presence of at least three phenotypes in skin mast cells of mice: berberine sulfate+/MMCP-6+, berberine sulfate+/MMCP-6-, and berberine sulfate-/MMCP-6-. The in situ hybridization of mRNA of mast cell-specific proteolytic enzymes seemed to be useful to describe abnormalities of mast cell differentiation in the skin of mi/mi mice.

Characterization of the promoter region of the human c-kit proto-oncogene

The c-kit proto-oncogene encodes a tyrosine kinase receptor for stem cell factor and plays a critical role in the growth and differentiation of various types of cells including hematopoietic stem cells. To investigate the mechanisms of its transcriptional regulation, we isolated the 5' flanking region of the human c-kit gene and characterized its promoter activity in hematopoietic cells. Nucleotide sequence analysis revealed that the 1.2 kb 5' flanking region lacked a typical "TATA box," but had a relatively high G + C content and four potential Sp1-binding sites. Putative binding sites for AP-2, basic helix-loop-helix proteins, Ets-domain proteins, Myb and GATA-1 were also found. Primer extension and S1 nuclease protection analyses of hematopoietic cells indicated that the major transcription start sites are 62 bp and 58 bp upstream of the translation start site. Essentially the same start sites were detected in non-hematopoietic cells such as small cell lung carcinoma and glioblastoma: this single promoter in c-kit is different from the multiple promoter system of c-fms, a c-kit-related gene, in which at least two promoters are differently used in hematopoietic and non-hematopoietic cells. An analysis of the c-kit 5' flanking region using the bacterial chloramphenicol acetyltransferase gene (CAT assay) in human erythroleukemia HEL cells, which express the endogenous c-kit mRNA at high levels, showed that a region from -180 to -22 is important for the expression of the c-kit gene. In addition, a negative regulatory element(s) is suggested to be involved in the regulation of the c-kit gene expression in mammals.

Stem cell factor induces outgrowth of c-kit-positive neurites and supports the survival of c-kit-positive neurons in dorsal root ganglia of mouse embryos

The c-kit receptor tyrosine kinase is highly expressed by about 10% of the neurons in the dorsal root ganglia (DRGs) of mouse embryos. We investigated the in vitro effect of stem cell factor (SCF), the ligand for c-kit receptor, on DRGs. Recombinant murine SCF (rmSCF) induced the outgrowth of c-kit-positive neurites from DRGs of normal (+/+) embryos. The effect of SCF was dose dependent and completely abolished by anti-c-kit ACK2 monoclonal antibody (mAb). Some neurites whose outgrowth was induced by nerve growth factor (NGF) were c-kit-positive, but anti-NGF mAb did not inhibit the rmSCF-induced neurite outgrowth. rmSCF did not induce neurite outgrowth from DRGs of W/W embryos that did not express c-kit receptors on the cell surface and of W42/W42 mutant embryos that expressed c-kit receptors without tyrosine kinase activity. rmSCF also had a trophic effect on c-kit-positive neurons in the culture of dissociated DRG cells. Most c-kit-positive neurons appeared to respond to NGF as well, and the SCF-responsive subpopulation represented about 10% of NGF-responsive neurons. rmSCF did not support the survival of DRG neurons from embryos of W/W and W42/W42 genotypes. These results suggest that the stimulus through the c-kit receptor tyrosine kinase has an important role in development of the peripheral nervous system.

Stem cell renewal and determination during clonal expansion in normal and leukaemic haemopoiesis

Normal haemopoiesis is a cellular hierarchy headed by pluripotent stem cells capable of both self renewal and, after determination, the generation of differentiating lineages that end in terminal functional cells. The role of stem cells is crucial because only these have the capacity to generate clonal populations during development or after injury. During clonal expansion the cells are affected by many sets of receptors and ligands. These belong to at least two classes: one consists of growth factors that bind cell surface receptors and initiate signalling events; the other class contains receptors which act as ligand-dependent transcription factors such as the intracellular steroid superfamily. In spite of this elaborate regulatory apparatus, control during clonal expansion is lax, perhaps stochastic, as evident from the great heterogeneity disclosed by examining the cellular compositions of haemopoietic clones. It may be that the large number of signals impinging on binary possible outcomes (for example self-renewal or determination) serve to set probabilities rather than to determine outcomes. In leukaemia, many of the features of normal haemopoiesis are retained. The disease begins as transformations in normal stem cells; after additional leukaemogenic events clonal expansion yields malignant populations which are clonal in each affected individual. These dominant clonal populations retain the hierarchical organization found in the normal, the major difference is that post-deterministic divisions in leukaemia yield descendants that retain primitive (blast) morphology although proliferative capacity is lost. In acute myeloblastic leukaemia (AML) cell culture methods are available that permit the measurement of clonogenic blast stem cells. These methods have shown that regulatory mechanisms active in normal haemopoiesis are retained in AML, including lax regulation during clonal expansion. The biological features of blast stems cells displayed by the culture technique reflect in part, events in vivo, as associations have been found between results in cell culture and clinical outcome. Thus, study of leukaemic populations provides a challenge for basic science and an opportunity for successful application in control of disease.

From white spots to stem cells: the role of the Kit receptor in mammalian development

The Kit tyrosine kinase membrane receptor is essential for melanogenesis, gametogenesis and hematopoiesis during embryonic development and postnatal life. This review summarizes the genetic evidence implicating Kit and its ligand, Steel factor, in the control of stem cell proliferation, migration and survival, with emphasis on mutations in the human and mouse genes.

W-sash affects positive and negative elements controlling c-kit expression: ectopic c-kit expression at sites of kit-ligand expression affects melanogenesis

The receptor tyrosine kinase c-kit and its cognate ligand KL are encoded at the white spotting (W) and steel (Sl) loci of the mouse, respectively. Mutations at both the W and the Sl locus cause deficiencies in gametogenesis, melanogenesis and hematopoiesis (erythrocytes and mast cells). The W-sash mutation differs from most W mutations in that it affects primarily mast cells and melanogenesis but not other cellular targets of W and Sl mutations. Thus, Wsh/Wsh mice are fertile and not anemic, but they lack mast cells in their skin and intestine and are devoid of coat pigment. Heterozygotes are black with a broad white sash/belt in the lumbar region. In order to determine the basis for the phenotypes of W-sash mice, we investigated c-kit RNA and protein expression patterns in adult Wsh/Wsh mice and during embryonic development. We show that c-kit expression is absent in bone-marrow-derived Wsh/Wsh mast cells, the fetal and the adult lung, and the digestive tract at embryonic day 13 1/2 (E13 1/2), tissues that normally express c-kit. Unexpectedly, in E10 1/2 and 11 1/2d Wsh/Wsh embryos, we found c-kit expression in the dermatome of the somites, the mesenchyme around the otic vesicle and the floorplate of the neural tube, structures known to express the c-kit ligand in wild-type embryos. The ectopic c-kit expression in Wsh homozygous embryos does not affect c-kit ligand expression. The presumed Wsh/Wsh melanoblasts appeared to be normal and, at E10 1/2, similar numbers were found in normal and homozygous mutant embryos. At E13 1/2 +/+ embryos had a graded distribution of melanoblasts from cranial to caudal with a minimum in the lumbar region. Whereas E13 1/2 homozygous Wsh/Wsh embryos essentially lacked c-kit-positive cells in the skin, E13 1/2 heterozygous Wsh/+ embryos had reduced numbers of melanoblasts compared to +/+ with few or none in the lumbar region (future sash). It is proposed that ectopic c-kit expression in the somitic dermatome affects early melanogenesis in a dominant fashion. Molecular analysis of Wsh chromosomal DNA revealed a deletion or rearrangement in the vicinity of the c-kit gene. These results provide an explanation for the Wsh phenotype and have implications for the control of c-kit expression.

Novel mutations of the KIT (mast/stem cell growth factor receptor) proto-oncogene in human piebaldism

Piebaldism is an autosomal dominant genetic disorder of pigmentation characterized by congenital patches of white skin and hair that lack melanocytes. Piebaldism results from mutations of the KIT proto-oncogene, which encodes the cellular receptor transmembrane tyrosine kinase for mast/stem cell growth factor. Here we describe two novel KIT mutations associated with human piebaldism. These amino acid substitutions, located in the most highly conserved sections of the KIT kinase domain, would be expected to dominant-negatively inhibit KIT-dependent signal transduction, resulting in aberrant melanocyte proliferation or migration during embryologic development.

The c-kit receptor, stem cell factor, and mast cells. What each is teaching us about the others

Many years ago, alert observers noticed among thousands of laboratory mice a few individuals that, unlike their littermates, exhibited areas of white spotting on their fur. No one could have predicted then that an effort to understand the basis for these abnormalities would ultimately contribute to the characterization of a receptor (c-kit) and a corresponding ligand (stem cell factor, SCF) that are critical not only to the migration and development of melanocytes, but also to hematopoiesis, gametogenesis, mast cell development, and, perhaps, development of the central nervous system. Nor could anyone have foretold then that this receptor and ligand would be shown to regulate the development of multiple distinct cellular lineages not only in mice, but also in humans and other primates, or that c-kit and its ligand would be found to influence the secretory function of cells bearing this receptor, as well as their development. Investigation of the effects of SCF on a single cell type, the mast cell, has produced the most complete picture of the spectrum of biological processes that can be regulated by interactions between c-kit and its ligand. This work shows that SCF critically regulates the migration and survival of mast cell precursors, promotes the proliferation of both immature and mature mast cells, enhances mast cell maturation, directly induces secretion of mast cell mediators, and can regulate the extent of mediator release in mast cells activated by IgE-dependent mechanisms. Indeed, SCF may well prove to be one of the most important of the factors influencing mast cell numbers, phenotype, and function in both health and disease. It now seems virtually certain that further studies of c-kit and SCF will produce important new insights into problems as diverse as the regulation of lineage commitment during normal hematopoiesis or the development and function of the central nervous system. And even though an effect on mast cell development was one of the last phenotypic abnormalities to be recognized in mice with mutations affecting the genes encoding c-kit or SCF, mast cells will continue to represent an important model system for analyzing the biology of c-kit and its ligand.

High affinity VEGF binding and developmental expression suggest Flk-1 as a major regulator of vasculogenesis and angiogenesis

Examination of flk-1 receptor tyrosine kinase mRNA expression by in situ hybridization analysis revealed specific association with endothelial cells at all stages of mouse development, including the blood islands in the yolk sac of day 8.5-10.5 embryos, in which the early progenitors of this lineage originate. flk-1 transcripts were abundant in proliferating endothelial cells of vascular sprouts and branching vessels of embryonic and early postnatal brain, but were drastically reduced in adult brain, where proliferation has ceased. Identification of the angiogenic mitogen, vascular endothelial growth factor (VEGF), as the high affinity ligand of Flk-1 and correlation of the temporal and spatial expression pattern of Flk-1 and VEGF suggest a major role of this ligand-receptor signaling system in vasculogenesis and angiogenesis.

Steel factor and c-kit receptor: from mutants to a growth factor system

Mutations within the Steel and Dominant Spotting loci of mice have led to the recent identification of a growth factor/receptor system required for the normal development of germ cells, pigment cells and hematopoietic cells. Interactions between the products of these genes, Steel factor and c-Kit respectively, have now been demonstrated to influence various developmental processes, including survival, proliferation, and/or differentiation of cells in a tissue specific manner. In addition, our current understanding of the molecular basis of various Steel and Dominant Spotting alleles coupled with the emerging information on the expression pattern of steel factor and c-kit transcripts during development, is now beginning to explain the pleiotropic affects of these mutations.