Ectopic expression of a c-kitW42 minigene in transgenic mice: recapitulation of W phenotypes and evidence for c-kit function in melanoblast progenitors

Impact of KIT Editing on Coat Pigmentation and Fresh Meat Color in Yorkshire Pigs

The white coat color of Yorkshire pigs is caused by the dominant white I allele, which has been associated with at least one copy of the 450-kb duplication encompassing the entire KIT gene and a splice mutation (G > A) at the first base of intron 17. The splice mutation in KIT has an adverse effect on pigmentation in mice. Therefore, removing the 450 kb duplications harboring the KIT copy with splice mutations is expected to affect Yorkshire pig pigmentation. In this study, we describe the use of a Yorkshire pig kidney cell strain with the I^?/I^Be-ed genotype, previously created by CRISPR-Cas9, as donor cells for somatic cell nuclear transfer to generate gene-edited Yorkshire pigs. The removal of the 450 kb duplications harboring the KIT copy with splice mutation did not alter the white coat color of Yorkshire pigs, which was confirmed by the absence of fully mature melanocytes and melanin accumulation in the hair follicles. Except for the improved transcription of tyrosinase, and slight increase in microphthalmia transcription factor and tyrosinase-related protein 1 protein expression, there was no significant impact of the removal of splice mutations on genes and signaling pathways (PI3K/AKT) involved in melanogenesis. However, the removal of the 450 kb duplications harboring the KIT copy with splice mutation substantially improved fresh meat color accompanied by significantly increased red blood cell number, which merits further investigation. Our study provides new insights into the role of structural mutations of the KIT gene in the formation of white coat color and erythropoiesis in Yorkshire pigs.

Signaling of c-kit in dendritic cells influences adaptive immunity

The binding of the receptor tyrosine kinase, c-kit, to its ligand, stem cell factor (SCF), mediates numerous biological functions. Important roles for c-kit in hematopoiesis, melanogenesis, erythropoiesis, spermatogenesis, and carcinogenesis are well documented. Similarly, activation of granulocytes, mast cells, and of eosinophils in particular, by c-kit ligation has long been known to result in degranulation with concomitant release of pro-inflammatory mediators, including cytokines. However, recent work from a number of laboratories, including our own, highlights previously unappreciated functions for c-kit in immunologic processes. These novel findings strongly suggest that signaling through the c-kit-SCF axis could have a significant impact on the pathogenesis of diseases associated with an immunologic component. In our own studies, c-kit upregulation on dendritic cells via T helper (Th)2- and Th17-inducing stimuli led to c-kit activation and immune skewing toward these T helper subsets and away from Th1 responses. Others have shown that dendritic cell treatment with inhibitors of c-kit activation, such as imatinib mesylate (Gleevec), favored breaking of T-cell tolerance, skewing of responses toward production of Th1 cytokines, and activation of natural killer cells. These data all indicate that deeper understanding of, and ability to control, the c-kit-SCF axis could lead to improved treatment modalities aimed at redirecting unwanted and/or deleterious immune responses in a wide variety of conditions.

Emerging functions of c-kit and its ligand stem cell factor in dendritic cells: regulators of T cell differentiation

The receptor tyrosine kinase, c-kit, and its ligand, stem cell factor (SCF), function in a diverse range of biological functions. The role of c-kit in the maintenance and survival of hematopoietic stem cells and of mast cells is well recognized. c-kit also plays an important role in melanogenesis, erythropoiesis and spermatogenesis. Recent work from our laboratory highlights an important role of c-kit in the regulation of expression of two molecules in dendritic cells (DCs), interleukin-6 (IL-6) and Jagged-2 (a ligand of Notch), which are known to regulate T helper cell differentiation. Our study shows that induction of c-kit expression and its signaling in DCs promotes Th2 and Th17 responses but not Th1 response. c-kit inhibition by imatinib mesylate (Gleevec) in DCs was previously shown to promote natural killer cell activation which may be due to dampening of IL-6 production by the DCs. Since dysregulation of c-kit function has been associated with various disease states including cancer, in this perspective we have focused on known and novel functions of c-kit to include molecules such as IL-6 and Notch that were not previously recognized to be within the purview of c-kit biology. We have also reviewed the differential expression pattern of SCF and c-kit on various cell types and its variation during development or pathology. The recognition of previously unappreciated roles for c-kit will provide better insights into its function within and beyond the immune system and pave the way for developing better therapeutic strategies.

Pathogenesis, clinical features, and treatment advances in mastocytosis

Systemic mastocytosis (SM) is characterized by the abnormal growth and accumulation of mast cells (MC) in one or more organs. The interaction between the cytokine stem cell factor (SCF) and its cognate receptor, the c-kit receptor tyrosine kinase (KIT), plays a central role in regulating MC growth and differentiation. Whereas germline and somatically acquired activating mutations of KIT have been identified in SM, the issue as to whether individual KIT mutation(s) are necessary and sufficient to cause MC transformation remains unclear based on currently available data. Activating mutations of platelet-derived growth factor receptor-alpha (FIP1 L1-PDGFRA) are identified in a significant number of SM cases that have associated eosinophilia. To date, as with gastrointestinal stromal tumors, activating mutations of KIT and PDGFRA appear to be alternative and mutually exclusive genetic events in SM. The World Health Organization has specified criteria for classification of SM into six major subtypes: cutaneous mastocytosis, indolent systemic mastocytosis (ISM), systemic mastocytosis with an associated clonal hematological non-mast-cell disorder (SM-AHNMD), aggressive systemic mastocytosis (ASM), mast cell leukemia, and mast cell sarcoma. The ability to molecularly classify individual SM cases based on the presence or absence of specific mutations allows for molecularly targeted therapy in a growing number of cases. Imatinib mesylate therapy might result in complete remission of SM cases with wild-type KIT, certain KIT mutations, such as F522C, or the FIP1L1-PDGFRA fusion gene, but not of D816V-KIT-bearing SM. For the latter, interferon-alpha and 2-CdA are potential first- and second-line therapeutic options. Other drugs under investigation include novel tyrosine kinase inhibitors, as well as NF-kappaB inhibitors, which might display greater selectivity towards D816V-KIT as compared to wild type KIT. The pathogenesis of mastocytosis, its major clinical subtypes, and recent treatment advances are discussed in this chapter.

Mice Transgenic for KitV620A: Recapitulation of Piebaldism but not Progressive Depigmentation Seen in Humans with this Mutation

Piebaldism is an autosomal dominant genetic pigmentary disorder, characterized by congenital white hair and patches located on the forehead, anterior trunk, and extremities. Most piebald patients have a mutation of the KIT gene, which encodes a tyrosine kinase receptor involved in pigment cell development. The white hair and patches of such patients are already completely formed at birth and do not usually expand thereafter. This stability of pigmented spots also applies to Kit(W) and Kitl(Sl) mutant mice. However, two novel cases of piebaldism were reported in 2001, in which both mother and daughter having a novel Val620Ala mutation in their KIT gene showed progressive depigmentation. To prepare an animal model of this mutation, to explore undefined functions of KIT signaling for maintaining pigmented melanocytes in the skin or more specifically the integrity of the melanocyte stem cell system in the postnatal skin, we produced transgenic mice expressing Val620Ala Kit. These mice well mimicked the white spotting pattern of patients; however, no change in this pattern was observed after birth, even after increasing the transgene expression by various means. Here, we report the unexpectedly extremely stable maintenance of the melanocyte stem cell system under stringent conditions for KIT signaling.

“Out, Damned Spot!”

Mice transgenic for the Kit Val620Ala mutation, which in humans has been associated with progressive piebaldism, exhibit dominant white spotting but show no evidence of progressive depigmentation. These results are consistent with the previous hypothesis that progressive piebaldism might result from digenic inheritance, of the KIT(V620A) mutation that causes piebaldism and a second, unknown locus that causes progressive depigmentation.

Transgenic overexpression of prothymosin α induces development of polycystic kidney disease11See Editorial by Gattone, p. 2063

BACKGROUND

Polycystic kidney disease (PKD) is a genetic disorder characterized by development of renal cysts and progressive renal dysfunction. Renal tissues from both PKD patients and rodent models of PKD show elevated c-myc expression. Prothymosin alpha (ProT) is positively regulated by c-myc through binding to the E box of its promoter. Through creating transgenic mice and clinical studies, we sought to investigate whether ProT overexpression contributes to PKD development.

METHODS

ProT heterozygous and homozygous transgenic mice were generated and characterized. Morphologic, histologic, immunohistochemical, and biochemical analyses of the transgenic mice were performed.

RESULTS

Two transgenic lines that represented integration at two different loci of the chromosomes were generated. ProT overexpression in the kidneys of homozygous transgenic mice induced a PKD phenotype, which included polycystic kidneys, elevated blood urea nitrogen (BUN), and lethality at about 10 days of age. Similar overexpression pattern of ProT was noted in cystic kidneys of the transgenic mice as well as in human autosomal-recessive PKD (ARPKD) and autosomal-dominant PKD (ADPKD) kidneys. ProT protein levels in the kidneys and urine as well as renal mRNA level of epithelial growth factor receptor (EGFR) of homozygous ProT transgenic mice were significantly higher than heterozygous or nontransgenic littermates. Furthermore, the heterozygous transgenic mice at 17 months of age also developed mild cystic kidneys.

CONCLUSION

Transgenic mice overexpressing ProT represent a novel model for PKD and may provide insights into PKD development. ProT, like c-myc and EGFR, may contribute to the development of renal cysts and may be a potential noninvasive diagnostic molecule of PKD.

Inducible expression of keratinocyte growth factor (KGF) in mice inhibits lung epithelial cell death induced by hyperoxia

Oxidant-induced injury to the lung is associated with extensive damage to the lung epithelium. Instillation of keratinocyte growth factor (KGF) in the lungs of animals protects animals from oxidant-induced injury but the mechanism of protection is not well understood. An inherent problem in studying KGF function in vivo has been that constitutive overexpression of KGF in the lung causes embryonic lethality with extensive pulmonary malformation. Here we report the development of a stringently regulated, tetracycline-inducible, lung-specific transgenic system that allows regulated expression of KGF in the lung without causing developmental abnormalities from leaky KGF expression. By using this system, we show that exposure of KGF-expressing mice to hyperoxia protects the lung epithelium but not the endothelium from cell death in accordance with the selective expression of KGF receptor on epithelial and not on endothelial cells. Investigations of KGF-induced cell survival pathways revealed KGF-induced activation of the multifunctional pro-survival Akt signaling axis both in vitro and in vivo. Inhibition of KGF-induced Akt activation by a dominant-negative mutant of Akt blocked the KGF-mediated protection of epithelial cells exposed to hyperoxia. KGF-induced Akt activation may play an important role in inhibiting lung alveolar cell death thereby preserving the lung architecture and function during oxidative stress.

Mutant laboratory mice with abnormalities in pigmentation: annotated tables

Mammalian pigment cell research has recently entered a phase of significantly increased activity due largely to the exploitation of the many mutant mouse stocks that are coming on stream. Numerous transgenic, targeted mutagenesis (so-called 'knockouts'), conditional (so-called 'gene switch') and spontaneous mutant mice develop abnormal coat color phenotypes. The number of mice that exhibit such abnormalities is increasing exponentially as genetic engineering methods become routine. Since defined abnormalities in such mutant mice provide important clues to the as yet often poorly understood functional roles of many gene products, this overview includes a corresponding, annotated table of mutant mice with pigmentation alterations. These range from early developmental defects via a large array of coat color abnormalities to a melanoma metastasis model. This overview should provide helpful pointers to investigators who are looking for mouse models to explore or to compare functional activities of genes of interest and for comparing coat color phenotypes of spontaneous or genetically engineered mouse mutants with novel ones. Secondly, this review includes a table of mouse models of specific human diseases with genetically defined pigmentation abnormalities. In summary, this annotated table should serve as a useful reference for anyone interested in the molecular controls of pigmentation.

Cellular compartmentalization in insulin action: altered signaling by a lipid-modified IRS-1

While most receptor tyrosine kinases signal by recruiting SH2 proteins directly to phosphorylation sites on their plasma membrane receptor, the insulin receptor phosphorylates intermediary IRS proteins that are distributed between the cytoplasm and a state of loose association with intracellular membranes. To determine the importance of this distribution to IRS-1-mediated signaling, we constructed a prenylated, constitutively membrane-bound IRS-1 by adding the COOH-terminal 9 amino acids from p21(ras), including the CAAX motif, to IRS-1 (IRS-CAAX) and analyzed its function in 32D cells expressing the insulin receptor. IRS-CAAX migrated more slowly on sodium dodecyl sulfate-polyacrylamide gel electrophoresis than did IRS-1 and demonstrated increased levels of serine/threonine phosphorylation. Insulin-stimulated tyrosyl phosphorylation of IRS-CAAX was slightly decreased, while IRS-CAAX-mediated phosphatidylinositol 3'-kinase (PI3'-kinase) binding and activation were decreased by approximately 75% compared to those for wild-type IRS-1. Similarly, expression of IRS-CAAX desensitized insulin-stimulated [(3)H]thymidine incorporation into DNA by about an order of magnitude compared to IRS-1. By contrast, IRS-CAAX-expressing cells demonstrated increased signaling by mitogen-activated protein kinase, Akt, and p70(S6) kinase in response to insulin. Hence, tight association with the membrane increased IRS-1 serine phosphorylation and reduced coupling between the insulin receptor, PI3'-kinase, and proliferative signaling while enhancing other signaling pathways. Thus, the correct distribution of IRS-1 between the cytoplasm and membrane compartments is critical to the normal balance in the network of insulin signaling.

Severe B cell hyperplasia and autoimmune disease in TALL-1 transgenic mice

TALL-1/Blys/BAFF is a member of the tumor necrosis factor (TNF) ligand superfamily that is functionally involved in B cell proliferation. Here, we describe B cell hyperplasia and autoimmune lupus-like changes in transgenic mice expressing TALL-1 under the control of a beta-actin promoter. The TALL-1 transgenic mice showed severe enlargement of spleen, lymph nodes, and Peyer's patches because of an increased number of B220+ cells. The transgenic mice also had hypergammaglobulinemia contributed by elevations of serum IgM, IgG, IgA, and IgE. In addition, a phenotype similar to autoimmune lupus-like disease was also seen in TALL-1 transgenic mice, characterized by the presence of autoantibodies to nuclear antigens and immune complex deposits in the kidney. Prolonged survival and hyperactivity of transgenic B cells may contribute to the autoimmune lupus-like phenotype in these animals. Our studies further confirm TALL-1 as a stimulator of B cells that affect Ig production. Thus, TALL-1 may be a primary mediator in B cell-associated autoimmune diseases.

Severe B cell hyperplasia and autoimmune disease in TALL-1 transgenic mice

TALL-1/Blys/BAFF is a member of the tumor necrosis factor (TNF) ligand superfamily that is functionally involved in B cell proliferation. Here, we describe B cell hyperplasia and autoimmune lupus-like changes in transgenic mice expressing TALL-1 under the control of a beta-actin promoter. The TALL-1 transgenic mice showed severe enlargement of spleen, lymph nodes, and Peyer's patches because of an increased number of B220+ cells. The transgenic mice also had hypergammaglobulinemia contributed by elevations of serum IgM, IgG, IgA, and IgE. In addition, a phenotype similar to autoimmune lupus-like disease was also seen in TALL-1 transgenic mice, characterized by the presence of autoantibodies to nuclear antigens and immune complex deposits in the kidney. Prolonged survival and hyperactivity of transgenic B cells may contribute to the autoimmune lupus-like phenotype in these animals. Our studies further confirm TALL-1 as a stimulator of B cells that affect Ig production. Thus, TALL-1 may be a primary mediator in B cell-associated autoimmune diseases.

Expression in transgenic mice of dominant interfering Fas mutations: a model for human autoimmune lymphoproliferative syndrome

Most humans with autoimmune lymphoproliferative syndrome (ALPS) carry heterozygous dominant mutations in one allele of the gene encoding Fas/APO-1/CD95. ALPS patients, like Fas-deficient MRL lpr/lpr mice, have lymphoproliferation, autoimmunity, increased CD4(-)/CD8(-) T lymphocytes, and apoptosis defects. Consistent with the phenotypic variability of lpr/lpr mice of different background strains, human genetic studies indicate that a Fas mutation is insufficient to induce ALPS in all mutation carriers. To investigate the dominant function of human Fas mutations and the additional genetic factor(s) involved in the development of ALPS, we generated transgenic mice expressing, in addition to endogenous Fas, mouse Fas molecules bearing mutations in the intracellular death domain corresponding to mutations identified in ALPS patients. Transgenic mice developed mild features of ALPS, including hepatosplenomegaly, elevated proportions of lymphocytes in spleen and lymph nodes, apoptotic defects, and hepatic lymphocytic infiltrates. Therefore defective murine Fas proteins act in a dominant manner to impair apoptosis of activated lymphocytes and disrupt lymphocyte homeostasis. The influence of genetic background on phenotype was studied by comparing transgenic mice on FVB/N and (FVB/N x MRL) backgrounds with syngenetic control mice and with MRL and MRL lpr/lpr mice. While expression of transgenic mutant Fas contributed mainly to hepatosplenomegaly and accumulation of lymphocytes, MRL background genes played a major role in the production of autoantibodies and elevated serum immunoglobulin levels. Moreover, compared to FVB/N (+/+) mice, a substantial Fas-specific apoptotic defect was found in MRL (+/+) mice, suggesting a mechanism for the known tendency of this strain to develop autoimmunity.

Dominant lethality of the mouse skeletal mutation tail-short (Ts) is determined by the Ts allele from mating partners

Mice with the Tail-short (Ts) mutation have a short, kinky tail and numerous skeletal abnormalities, including a homeotic anteroposterior patterning problem involving the axial skeleton. The viability of Ts heterozygotes varies dramatically, depending on the mouse strain crossed with the mutant strain. At the extremes, the heterozygotes are viable or lethal prenatally. In this study, we found that laboratory mouse strains could be divided into two groups. A cross with strains from the first group yielded viable Ts heterozygotes, whereas a cross with the second group resulted in dominant lethality in utero. We planned to map the gene(s) that controls strain differences in the viability of the Ts heterozygotes. The result clearly indicated that a single chromosomal region, genetically inseparable from the Ts locus, is responsible for these differences. This suggests that allelism at the Ts locus generates variable manifestation of the mutant phenotype. Morphological and histological analyses indicated that embryos from the lethal cross exhibit severe developmental defects from the gastrulation stage through the early fetal stage. In particular, the umbilical vein does not develop properly. All of these results suggest that the phenotype of the Ts mutant is modified by the Ts alleles of the mating partners.

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.

Stepwise requirement of c-kit tyrosine kinase in mouse ovarian follicle development

Ovarian follicle development is controlled by the cycling variation of gonadotrophins derived from the central nervous system. Intragonadal signals are also required, especially in the autonomous development of small follicles. Receptor tyrosine kinase c-kit and its ligand SLF (Steel factor) are expressed on the surface of specific populations of follicle-forming cells in a contiguous manner and are thought to have important roles in follicular development. We blocked the interaction of c-kit and its ligand by administering the function-blocking antibody ACK2 to developing mice at various times after birth and monitored ovarian follicle development. A blockade of c-kit function disturbed the onset of primordial follicle development, primary follicle growth, follicular fluid formation of preantral follicles, and penultimate-stage ovarian follicle maturation before ovulation. Ovarian follicle growth was dependent on c-kit during the first 5 days after birth when the functional FSH receptor is not yet expressed in mouse ovary. In contrast, primordial follicle formation and survival, small preantral or antral follicle development, ovulation, and luteinization of the ovulated follicle were not affected by this antibody. These findings indicate the stepwise requirement of c-kit and its ligand interaction system in the developing ovarian follicle and that c-kit with its ligand supports the autonomous development of ovarian follicle independent of gonadotrophins.

Functional correction of renal defects in a mouse model for ARPKD through expression of the cloned wild-type Tg737 cDNA

Autosomal recessive polycystic kidney disease (ARPKD) is characterized by the formation of large collecting tubule and ductular cysts that often result in renal insufficiency within the first decade of life. Understanding the process leading to cyst formation will require the identification and characterization of genes involved in the etiology of this disease. In this regard, we previously described the generation of a mouse model (TgN737Rpw) for ARPKD and the cloning of a candidate gene. Here we show direct involvement of the Tg737 gene in collecting duct cyst formation by expressing the wild-type Tg737 cDNA as a transgene in TgN737Rpw mutants. In contrast to TgN737Rpw mutants, the "rescued" animals survive longer, have normal renal function and normal localization of the EGFr to the basolateral surfaces of collecting duct epithelium.

An intron 1 regulatory region from the murine adenosine deaminase gene can activate heterologous promoters for ubiquitous expression in transgenic mice

Ubiquitously expressed genes contain regulatory features which allow expression in virtually all cell types. In an effort to understand the molecular basis for this regulatory feature, the chromatin structure of the murine adenosine deaminase gene was examined by DNase I digestion in nuclei of several tissues. The promoter contained a strong hypersensitive site in all tissues examined, including those with very high and very low levels of ADA expression. Transgenic mouse studies revealed that a 3.3 kb EcoRI (3.3EE) fragment from intron I was required to generate a strong promoter DNase I hypersensitive site, and to produce ubiquitous expression. The 3.3EE fragment also contained a thymic enhancer activity which mapped to sequences conserved with the human ADA gene T-lymphocyte enhancer. Mutational analysis indicated that ubiquitous expression was not dependent on the presence of a functional thymic enhancer. Both the thymic enhancer and the ubiquitous activator within the 3.3EE fragment functioned with heterologous promoters in transgenic mice.

Identification of a unique membrane-bound molecule on a hemopoietic stem cell line and on multipotent progenitor cells

Hemopoietic stem cells are a distinct population of cells that can differentiate into multilineages of hemopoietic cells and have long-term repopulation capability. A few membrane-bound molecules have been found to be preferentially, but not uniquely, present on the surface of these primitive cells. We report here the identification of a unique 105-kDa glycoprotein on the surface of hemopoietic stem cell line BL3. This molecule, recognized by the absorbed antiserum, is not present on the surface of myeloid progenitors 32D and FDC-P1 cells, EL4 T cells, and NIH 3T3 fibroblasts. This antiserum can also be used to block the proliferation of BL3 cells even in the presence of mitogen-stimulated spleen cell conditioned medium, which is known to have a stimulating activity on BL3 cells. It can also inhibit development of in vitro, fetal liver cell-derived multilineage colonies, but not other types of colonies, and of in vivo bone marrow cell-derived colony-forming unit spleen foci. These data suggest that gp105 plays an important role in hemopoietic stem cell differentiation.

c-KIT expression enhances the leukemogenic potential of 32D cells

The growth of human leukemic cells in culture and in vivo is dependent upon the presence of hematopoietic growth factors. Most populations of human leukemic acute myeloblastic leukemia (AML) cells express c-Kit on their surface and respond to Kit ligand (KL) in culture. To determine if this interaction was of potential significance in vivo we used a mouse model system. 32D cells, a murine IL-3-dependent myeloid cell line, were rendered KL responsive by transfection of the murine c-Kit. After injection of 32D or 32D-Kit cells into syngeneic hosts, animals bearing 32D-Kit cells, but not 32D cells, became moribund and were killed. These animals had circulating leukemic blast cells, infiltration of bone marrow, spleen, brain, liver, lung, and kidney. Cells recovered from some of the animals continued to be dependent upon IL-3 or KL for growth while in other cases the cells were factor independent. This model illustrates that the constitutive expression of c-Kit enhances the leukemic potential of 32D cells. The model will be useful for studying the progression of leukemia in vivo and testing whether interruption of the interaction of Kit and KL can affect the growth of leukemic cells.

Ectopic expression of the agouti gene in transgenic mice causes obesity, features of type II diabetes, and yellow fur

Mice that carry the lethal yellow (Ay) or viable yellow (Avy) mutation, two dominant mutations of the agouti (a) gene in mouse chromosome 2, exhibit a phenotype that includes yellow fur, marked obesity, a form of type II diabetes associated with insulin resistance, and an increased susceptibility to tumor development. Molecular analyses of these and several other dominant "obese yellow" a-locus mutations suggested that ectopic expression of the normal agouti protein gives rise to this complex pleiotropic phenotype. We have now tested this hypothesis directly by generating transgenic mice that ectopically express an agouti cDNA clone encoding the normal agouti protein in all tissues examined. Transgenic mice of both sexes have yellow fur, become obese, and develop hyperinsulinemia. In addition, male transgenic mice develop hyperglycemia by 12-20 weeks of age. These results demonstrate conclusively that the ectopic agouti expression is responsible for most, if not all, of the phenotypic traits of the dominant, obese yellow mutants.

Systemic gene therapy with p53 reduces growth and metastases of a malignant human breast cancer in nude mice

We report on an in vivo delivery system that attenuates the growth, in nude mice, of a malignant human breast cancer cell line containing a p53 mutation. Nude mice, inoculated with breast carcinoma cells, were injected every 10-12 days with a liposome-p53 complex via the tail vein. A significant reduction of greater than 60% in primary tumor volume was observed as compared to the control groups. Furthermore, when individual growth patterns of the tumors were assessed, we found that primary tumor size regressed in the majority of p53-treated animals (8/15), whereas only one tumor in the control groups (1/22) regressed. The eight tumors that regressed with the liposome-p53 complex showed no evidence of relapse for 1 month after the cessation of treatment. We also determined that the administration of the liposome-p53 complex reduced the incidence of metastases. The MDA-MB-435 tumor cells, transduced with the lacZ gene, facilitated quantitation of beta-galactosidase activity and tumor burden in the lungs. The number of metastatic cells in the lung was significantly lower in the p53-treated group (0.53 +/- 0.43 x 10(6), p < 0.01) than in either the vector-treated (8.1 +/- 3.7 x 10(6)) or untreated control groups (15.8 +/- 5.9 x 10(6)). Thus, systemic administration of the liposome-p53 complex reduced not only the size of the primary tumors but, more importantly, prevented the relapse and metastases of these tumors.

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.

Cloning and expression of the chicken c-kit proto-oncogene

The Kit protein is a cell-surface tyrosine kinase receptor encoded by the c-kit proto-oncogene. cDNA clones encoding chicken Kit were isolated from a chicken brain cDNA library, and the nucleotide (nt) sequence of a cDNA clone containing the entire protein-coding region was determined. The deduced amino acid (aa) sequence of chicken Kit shows 63% identity to mouse Kit, and suggests that chicken Kit shares common structural and functional features with mouse Kit. RNA blot analysis indicated that the expression pattern of the chicken c-kit transcript in chicken organs was similar to that of mouse c-kit in mice, suggesting that chicken Kit has biological roles analogous to those of mouse Kit.

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.

Developmental failure of chimeric embryos expressing high levels of H-2Dd transplantation antigens

The absence of expression of class I products of the major histocompatibility complex at early stages of development is thought to play a key role in maternal tolerance of the fetal allograft. To test this, we developed a strategy that would allow us to describe the consequences of overexpression of the H-2Dd transplantation antigen in the developing embryo. A construct containing the H-2Dd gene under control of the human beta-actin promoter was transfected into pluripotent embryonic stem (ES) cells. Particularly in this case, since overexpression of major histocompatibility complex class I gene products may profoundly affect embryonic development, an important advantage of the ES cell system is the ability to analyze gene expression and study effects on cell growth and differentiation in vitro. ES cells do not constitutively express beta 2-microglobulin. Consistent with this, H-2Dd H chains expressed by ES cell transformants were not associated with beta 2-microglobulin or transported to the cell surface. Significant levels of beta 2-microglobulin and H-2Dd membrane glycoproteins were expressed following differentiation in vitro. H-2Dd-transfected ES cells gave rise to a wide range of differentiated cell types, and there was no evidence to suggest that expression of the introduced H-2Dd gene affects the differentiation abilities of ES cells in vitro. When introduced into blastocysts, H-2Dd-transfected ES cells extensively contribute to embryonic and extraembryonic tissues, but this results in the failure of chimeric conceptuses at midgestation. Considering that transgenic chimeras cannot be rescued by transfer into syngeneic foster females, it seems likely that nonimmunological mechanisms are responsible for these prenatal lethalities.

The kit ligand encoded at the murine Steel locus: a pleiotropic growth and differentiation factor

The c-kit receptor and its recently identified ligand are allelic with the murine White Spotting and Steel loci, respectively. These observations brought to light the functions of the c-kit receptor system in melanogenesis, gametogenesis and hematopoeisis during embryogenesis and in postnatal life. The recent molecular analysis of several White Spotting and Steel alleles has provided insights into the mechanism of c-kit ligand-mediated processes, including cell proliferation, cell migration and cell survival. Furthermore, the availability of the kit ligand has allowed in vitro investigations of the pleiotropic functions of c-kit in development and cell differentiation to be carried out.