Identification of the control regions for mouse c-kit gene transcription induced by retinoic acid

The c-kit gene encodes a receptor tyrosine kinase that reveals the pleiotropic effects both in the developing embryo and in the adult animal. In this study, we characterized transcriptional control of the c-kit gene. F9 teratocarcinoma cells differentiated and expressed the c-kit gene upon exposure to retinoic acid (RA). We isolated c-kit genomic DNA from approximately 20 kb upstream to approximately 10 kb downstream of the transcribed region. We identified two control regions for c-kit gene transcription by fusing genomic sequences to bacterial chloramphenicol acetyl transferase reporter genes and introducing the resultant constructs into differentiated F9 cells. The fragments, Sac I-Apa I in the 9th intron and Xho I-Kpn I in the 13th intron induced reporter gene activities. Nucleotide sequencing of the regions showed in that there were no consensus sequences of the RA receptor or cyclic AMP-responsive elements. Cyclic AMP augmented the expression of the c-kit gene induced by RA; when combined with RA, it was less effective on the reporter gene activity. In contrast to other reports, continuous exposure to the reagents was not required for c-kit expression. These findings suggested that transcription of the c-kit gene is regulated by complex controls and that there are different control regions responsive to RA and cyclic AMP.

Overexpression of retinoic acid receptor alpha suppresses myeloid cell differentiation at the promyelocyte stage

Retinoic acid receptor (RAR) alpha is required to heterodimerize with retinoid X receptor (RXRs) in order to regulate myeloid differentiation. If so, it is expected that overexpression of normal RAR alpha may perturb the RAR alpha/RXR heterodimer formation and also the differentiation of myeloid cells. We have described here the morphology and the RA response of human RAR alpha cDNA transduced murine bone marrow cells using a retroviral vector. Most of RAR alpha transduced cells displayed promyelocyte like morphology and their proportion of c-kit expressing population was increased remarkably compared with the control (Neor gene transduced cells). Furthermore, this morphology was observed even after these cells were brought into the semisolid culture containing IL-3 alone. Interestingly, immature RAR alpha transduced cells differentiated into mature granulocytes under the condition of the high concentration of RA(10(-6) M). We did not observe any effect of RAR alpha on monocytes. These results indicate that overexpression of normal RAR alpha is sufficient for inducing maturation arrest of myeloid cell lineage that is similar to the phenotype found in the acute promyelocytic leukemia bearing PML-RAR alpha translocation.

Cell cycle control of c-kit+IL-7R+ B precursor cells by two distinct signals derived from IL-7 receptor and c-kit in a fully defined medium

An important goal for the investigation of the proliferation of mammalian cells is to establish a fully defined condition for culturing them in vitro. Here, we report establishment of a fully defined culture condition that supports the primary culture of normal c-kit+IL-7 receptor (IL-7R)+ B precursor cells without the aid of stromal cell lines. This defined culture condition contains IL-7, the ligand for c-kit, transferrin, insulin, and bovine serum albumin as protein components. By using the cell lines derived from RAG2(-/-) mice, which do not differentiate into c-kit- stage, we have evaluated the role of each protein in the cell cycle progression of c-kit+IL-7R+ B precursor cells. Since B precursor cells can grow without insulin, c-kit remains a sole functional receptor tyrosine kinase for their growth. While both c-kit ligand (KL) and IL-7 are the requisite molecules for sustained proliferation of B precursor cells, each molecule plays distinct roles. IL-7 starvation results in prompt arrest of the cells at G1. An accumulation of the cells in the mitotic phase was also detected. Thus, the major role of IL-7 is to regulate the G1/S transition and the process of cytokinesis of B precursor cells. Although prolonged KL starvation over 48 h resulted in accumulation of G1 cells, its effect could not be detected within 24 h, which is long enough for all the cells to complete one cell cycle. This suggests that KL might be involved in the cell cycle progression of B precursor cells in a manner that its signal could still be effective in the one or two cell cycles that follow. Although molecular nature of the signals underlying the present observation awaits future investigation, the method described in this report would provide a useful model system for investigating the signaling pathways that are involved in the cell cycle progression of B precursor cells.

Identification of two transcripts of AML1/ETO-fused gene in t(8;21) leukemic cells and expression of wild-type ETO gene in hematopoietic cells

The t(8;21) is a common chromosomal abnormality, preferentially associated with acute leukemia showing features of myeloid differentiation. Recently, two genes--AML1, which has a unique runt domain, and ETO (MTG8)--have been isolated from the chromosomal breakpoint. In this study, we isolated and identified two fused transcripts from a leukemic cell line carrying t(8;21). AML1 and ETO were fused at the same position in these transcripts. One of the transcripts codes a unique domain, including two zinc finger domains and three proline- and one leucine-rich region. The other transcript codes only for one proline- and leucine-rich region but lacks zinc finger domains. We demonstrated by polymerase chain reaction (PCR) analysis that 1) these two transcripts are consistently expressed in leukemic cells with t(8;21) obtained from patients and 2) expression of AML1 was not restricted to the particular stage of hematopoietic differentiation but was present in all hematopoietic cells investigated. We also provide evidence that two wild types of ETO transcripts containing the region of the ETO gene in fused transcripts are expressed in hematopoietic cells from different lineages. The widespread expression of AML1 and ETO in hematopoietic cells suggests a fundamental role of these proteins in hematopoiesis. Furthermore, the differences in the carboxy termini of ETO may modulate the activity of fused proteins resulting from the chromosomal translocation t(8;21).

Exogenous expression of human granulocyte colony-stimulating factor receptor in a B-lineage acute lymphoblastic leukemia cell line: a possible model for mixed lineage leukemia

We report evidence that the granulocyte colony-stimulating factor (G-CSF) receptor is present and may be functioning on blast cells from some patients with myeloid surface antigen positive (My+) acute lymphoblastic leukemia (ALL). In the present study, a human G-CSF receptor expression plasmid was transfected into a newly established B-lineage ALL cell line 'Tanoue' and its subclone 'ST' by lipofection to investigate whether expression of the G-CSF receptor and G-CSF stimulation would induce myeloid characteristics on myeloid surface antigen negative (My-) ALL cells. The G-CSF receptor became detectable on the transfected cells (GR-Tanoue and GR-ST), with dissociation constant values of 50-130 pmol/l, and maximal binding sites (Bmax of 77-6100 sites/cell on receptor binding assays. Short term culture with recombinant human G-CSF induced myeloid differentiation (a two to three-fold increase in CD33 and CD15 expression), and a moderate 3H-thymidine uptake (stimulation index, 1.75) only in the GR-ST clone no. 15 which expressed a high number of G-CSF receptors (Bmax, 6100 sites/cell). Our data show that (a) exogenous expression of the G-CSF receptor and G-CSF stimulation can induce myeloid characteristics on ALL cells; and (b) in the G-CSF receptor-expressing cells, there is a correlation between the number of G-CSF receptors and cell responsiveness to G-CSF in either proliferation or differentiation.

Detection of human immunodeficiency virus-1 nucleic acid on inactivated filter paper disks by polymerase chain reaction and microtiter plate assay

Human immunodeficiency virus type 1 (HIV-1) in cultured cells, peripheral blood samples and sera were adsorbed on filter paper disks and inactivated by heat or ethanol. Two procedures, the polymerase chain reaction (PCR) and microtiter plate assay (HMPA) were used to detect the nucleic acid. The sensitivity after different heat treatments with nested PCR for HIV-1 DNA (or nested reverse transcription-PCR for HIV-1 RNA) was identical regardless of whether the samples were examined immediately or one month later. Inactivation by ethanol treatment resulted in a slight loss of sensitivity. The HMPA proved to be as reliable and specific as the conventional PCR technique. We conclude that the heat-treated filter paper disk assay is suitable for identifying HIV nucleic acid in clinical samples sent to the laboratory from a distance, e.g. in an envelope.

Synergistic effect of recombinant CD4-immunoglobulin in combination with azidothymidine, dideoxyinosine and 0.5 beta-monoclonal antibody on human immunodeficiency virus infection in vitro

Data are presented which indicate that combinations of rCD4 immunoglobulin with azidothymidine, dideoxyinosine or 0.5 beta mouse monoclonal antibodies directed against the V3 region of HIV-1, were more effective in treatment of acute HIV infection in vitro than each compound alone. It is suggested that combination therapy with these compounds is more beneficial in treatment of HIV-infected patients than monotherapy, especially with respect to a reduction of the known side effects and the formation of resistant HIV strains after treatment with nucleoside analogues.

HIV-1 gp120 and NMDA induce protein kinase C translocation differentially in rat primary neuronal cultures

Exposure of rat cortical neurons to the human immunodeficiency virus type 1 (HIV-1) coat protein gp120 in vitro causes a rise in the intracellular Ca2+ level and a subsequent translocation of protein kinase C (PKC) from the cytosol to the membrane. Such a translocation persists for at least 2 h, but only in cultures with media not depleted of endogenous glutamate. Enzymatic degradation of glutamate in the medium by the enzyme glutamate-pyruvate transaminase (GPT) abolishes the long-lasting effect of gp120 on the association state of PKC; under this incubation condition the translocation period is < 1 h. Memantine and the ganglioside GM1 prevent N-methyl D-aspartate receptor-mediated long-term translocation of PKC and gp120-mediated neurotoxicity (in the absence of GPT); they have no effect on short-term translocation of PKC. We suggest that gp120-caused neuronal death involves an indirect sensitization step of the NMDA receptors, which ultimately induces neuronal death.

Expression and function of c-Kit in fetal hemopoietic progenitor cells: transition from the early c-Kit-independent to the late c-Kit-dependent wave of hemopoiesis in the murine embryo

The protooncogene c-kit encodes a receptor type tyrosine kinase and is allelic with the W locus of mice. SLF, the c-Kit ligand which is encoded by the Sl locus, has growth promoting activity for hemopoietic stem cells. Previous studies demonstrated that c-Kit is functionally required for the proliferation of hemopoietic progenitor cells at various differentiation stages in adult bone marrow. However, the absence of functional SLF and c-Kit in fetuses with mutant alleles of Sl and W loci produces only minor effects on the myeloid and early erythroid progenitor cells in the fetal liver, although the level of the late erythroid progenitor cells is significantly affected. We used an anti-c-Kit monoclonal antibody to investigate the expression and function of c-Kit in murine fetal hemopoietic progenitor cells. Flow-cytometric analysis showed that hemopoiesis in the yolk sac and fetal liver started from cells that express c-Kit. The c-Kit expression decreased upon maturation into erythrocytes in each organ. By fluorescence activated cell sorting, the c-Kit+ cell population was enriched with the hemopoietic progenitor cells clonable in vitro (CFU-E, BFU-E and GM-CFC). To elucidate whether c-Kit functions in these progenitor cells in vivo, we took advantage of the antagonistic anti-c-Kit monoclonal antibody, ACK2, which can block the function of c-Kit. Administration of ACK2 after 12.5 days of gestation rapidly eliminated BFU-E and GM-CFC as well as CFU-E from the fetal liver. However, the number of these progenitor cells in the yolk sac and fetal liver was less affected when the fetuses were given ACK2 before 12.5 days of gestation. Our results provide evidence that there are two waves of hemopoiesis in murine embryos relative to c-Kit dependency. The c-Kit has an essential role on the growth of hemopoietic progenitor cells in the fetal liver after 12.5 days of gestation, whereas the progenitor cells in the liver and yolk sac of the earlier embryo do not depend on c-Kit and its ligand SLF.

Conditions required for myelopoiesis in murine spleen

While the spleen is an active site for myelopoiesis during the late embryonal and perinatal stages, the activity is gradually lost later. However, myelopoiesis in the adult spleen can be reactivated by irradiation or various stimulants. In this study we investigated factors which determine the myelopoiesis-supporting activity in the adult spleen. To address this question, we used scid mouse because virtually no lymphocytes, which might compete in the splenic microenvironment with hematopoietic progenitors, are present there. The results demonstrated: 1. Even in scid mouse, the myelopoiesis-supporting activity in the spleen is lost within a week after birth as in normal mice. 2. While myelopoiesis does not occur in the spleen of unstimulated scid mouse by bone marrow transfer alone, myelopoiesis in the spleen is reactivated by irradiation or lipopolysaccharide (LPS) application. 3. Myelopoiesis in the spleen induced by irradiation is dependent on c-kit and its ligand steel factor (SLF), because it was suppressed completely by the monoclonal antibody (mAb) against c-kit. 4. The expression of SLF transcripts in the spleen was enhanced after irradiation. These results suggest that the factor which determines myelopoietic activity in the spleen resides primarily in the status of the splenic microenvironment.

Characterization of cells of the myeloid-monocytic lineage (ML-1, HL-60, THP-1, U-937) chronically infected with the human immunodeficiency virus-1

The myeloid-monocytic cells ML-1, HL-60, THP-1 and U-937 were chronically infected (for more than 2 years) with the lymphotropic HIV-1 strain HTLV-IIIB. Reinfection experiments revealed that viruses obtained from chronically infected ML-1/HIV-1 and HL-60/HIV-1 cells show a low infectivity if tested with uninfected ML-1 and HL-60 cells in contrast to virus preparations from chronically infected THP-1/HIV-1 and U-937/HIV-1 with their corresponding uninfected cell lines. Analyses of selected cell surface markers showed a differential expression of CD4, CD8, CD11c, CD14, CD15, CD20, HLA-DR and HLA-DQ in non- or chronically infected cells. During chronical infection, the myeloid-monocytic cells lost their reactivity with peroxidase and esterase. In chronically infected cells, the steady-state levels for TNF-alpha mRNA remained unchanged while those for IL-6 decreased. The half-lives of transcripts of both TNF-alpha (t1/2: 70 min) and IL-6 (t1/2: 100 min) were nearly the same in uninfected and chronically infected HL-60 cells.

Effect of lignins on HIV-induced cytopathogenicity and myeloperoxidase activity in human myelogenous leukemic cell lines

We have previously reported the potent stimulation effect of lignin on the iodination of myeloperoxidase (MPO)-positive cells. We investigated here the anti-HIV (human immunodeficiency virus) activity of lignins in the MPO-positive (HL-60) and -negative (U-937) human myelogenous leukemic cell lines. Natural lignified material and dehydrogenation polymers, but not their precursors, effectively inhibited the cytopathic effect of HIV infection in both these cells as well as in MT-4 and MOLT-4 cells. HIV infection caused significant reduction of MPO activity in HL-60 cells, regardless of the presence or absence of lignins. These data suggest that MPO might not be involved in the anti-HIV activity induction by lignins.

Characterization of human immunodeficiency virus-1-infected cells of myeloid-monocytic lineage (ML-1, HL-60, THP-1, U-937)

The myeloid-monocytic cells ML-1, HL-60, THP-1, and U-937 were chronically infected (for > 2 years) with the lymphotropic human immunodeficiency virus type 1 (HIV-1) strain HTLV-IIIB. Reinfection experiments revealed that viruses obtained from chronically infected ML-1/HIV-1 and HL-60/HIV-1 cells showed a low infectivity if tested with uninfected ML-1 and HL-60 cells in contrast to virus preparations from chronically infected THP-1/HIV-1 and U-937/HIV-1 with their corresponding uninfected cell lines. Analyses of selected cell surface markers revealed a differential expression of CD4, CD8, CD11c, CD14, CD15, CD20, HLA-DR, and HLA-DQ in non- or chronically infected cells. In chronically infected cells, the steady-state levels for tumor necrosis factor-alpha, interleukin (IL)-1 beta, and granulocyte-macrophage colony-stimulating factor mRNA remained unchanged whereas the one for IL-6 dropped.

Expression of c-kit, a proto-oncogene of the murine W locus, in cerebella of normal and neurological mutant mice: immunohistochemical and in situ hybridization analysis

The c-kit proto-oncogene encodes a receptor tyrosine kinase and is allelic with the murine white-spoting (W) locus. Although no apparent defects in the brain have been reported in W mutant mice, brain tissue, especially cerebellum, shows a high level of c-kit transcription. In the present study, sites of c-kit expression in the cerebellum were exained by immunohistochemical and in situ hybridization techniques. Immunohistochemistry with a monoclonal antibody against c-Kit protein revealed that the c-Kit protein was localized close to the Purkinje cell soma in the region facing the granular cell layer. Similar distribution of the c-Kit protein was observed in cerebella of mutant mice in which the Purkinje cell (pcd) or the granular cell layer (weaver) is missing. These data suggest that the c-Kit protein is produced not by the Purkinje cell nor by the granular cell but by the cells present in the molecular layer and that the protein is then transported to the region around the Purkinje cell soma. This interpretation was supported by in situ hybridization analysis: cells containing the c-kit transcripts were found only in the molecular layer, while the granular and Purkinje cells were negative.

Primary structure of guinea-pig Hageman factor: sequence around the cleavage site differs from the human molecule

The guinea-pig and human Hageman factors differ in their sensitivity to activation by particular bacterial proteinases. To understand this difference, the primary structure and cleavage site on activation of the guinea-pig molecule were determined and compared with the human molecule. By the use of a synthetic oligodeoxyribonucleotide probe which encoded a part of human Hageman factor cDNA, a cDNA clone was isolated from a lambda gt11 cDNA library of guinea-pig liver and sequenced. The cDNA clone was identified as that of guinea-pig Hageman factor by the complete identity of the deduced amino-acid sequence with the actual sequence of the amino-terminal portion of guinea-pig Hageman factor molecule and the active form. The cDNA included part of a leader sequence and the entire coding region of the Hageman factor molecule. Guinea-pig Hageman factor was composed of the same domain structures as the human counterpart with an overall 72% homology in the amino-acid sequence. However, the sequences around the cleavage site were surprisingly different; -Met351-Thr-Arg-Val-Val-Gly-Gly-Leu-Val359-(human) and -Leu338-Ser-Arg-Ile-Val-Gly-Gly-Leu-Val346-(guinea-pig). The amino-acid substitutions around the cleavage site might explain the difference in sensitivity to activation between the human and guinea-pig molecules.

Role of c-kit in mouse spermatogenesis: identification of spermatogonia as a specific site of c-kit expression and function

Recent studies have shown that the dominant white spotting (W) locus encodes the proto-oncogene c-kit, a member of the tyrosine kinase receptor family. One symptom of mice bearing mutation within this gene is sterility due to developmental failure of the primordial germ cells during early embryogenesis. To elucidate the role of the c-kit in gametogenesis, we used an anti-c-kit monoclonal antibody, ACK2, as an antagonistic blocker for c-kit function to interfere with the development of male and female germ cells during postnatal life. ACK2 enabled us to detect the expression of c-kit in the gonadal tissue and also to determine the functional status of c-kit, which is expressed on the surface of a particular cell lineage. Consistent with our immunohistochemical findings, the intravenous injection of ACK2 into adult mice caused a depletion in the differentiating type A spermatogonia from the testis during 24–36 h, while the undifferentiated type A spermatogonia were basically unaffected. Intraperitoneal injections of ACK2 into prepuberal mice could completely block the mitosis of mature (differentiating) type A spermatogonia, but not the mitosis of the gonocytes and primitive type A spermatogonia, or the meiosis of spermatocytes. Our results indicate that the survival and/or proliferation of the differentiating type A spermatogonia requires c-kit, but the primitive (undifferentiated) type A spermatogonia or spermatogenic stem cells are independent from c-kit. Moreover, the antibody administration had no significant effect on oocyte maturation despite its intense expression of c-kit.

In utero manipulation of coat color formation by a monoclonal anti-c-kit antibody: two distinct waves of c-kit-dependency during melanocyte development

Previous studies on mice bearing various mutations within the c-kit gene, dominant white spotting (W), indicate the functional role of this tyrosine kinase receptor in the development of melanocytes, germ cells and hematopoietic cells. Despite the availability of mice defective in the c-kit gene and a respectable understanding of the molecular nature of c-kit, however, it is not clear at what stage of gestation c-kit is functionally required for the development of each of these cell lineages. To address this question, we have used a monoclonal anti-c-kit antibody, ACK2, as an antagonistic blocker of c-kit function to interfere with the development of melanocytes during embryonic and postnatal life. ACK2 injected intradermally into pregnant mice entered the embryos where it blocked the proper development of melanocytes. This inhibitory effect was manifested as coat color alteration in the offspring. Furthermore, ACK2 injection also altered the coat color of neonatal and adult mice. Based on the coat color patterns produced by ACK2 administration at various stages before or after birth, the following conclusions are drawn: (i) during mid-gestation, c-kit is functionally required during a restricted period around day 14.5 post-coitum when a sequence of events leading to melanocyte entry into the epidermal layer occurs; (ii) during postnatal life, c-kit is required for melanocyte activation which occurs concomitantly with the hair cycle which continues throughout life after neonatal development of the first hair.

Expression and function of c-kit in hemopoietic progenitor cells

The expression and function of a receptor tyrosine kinase, c-kit, in the adult bone marrow of the mouse were investigated by using monoclonal antibodies (mAbs) against the extracellular domain of murine c-kit. In adult C57BL/6 mouse, 7.8% of total bone marrow cells express c-kit on their surface. Half of the c-kit+ cells do not express lineage markers including Mac-1, Gr-1, TER-119, and B220, while the remainder coexpress myeloid lineage markers such as Mac-1 and Gr-1. After c-kit+ cells were removed from the bone marrow cell preparation, hemopoietic progenitor cells reactive to IL-3, GM-CSF, or M-CSF and also those which give rise to spleen colonies in irradiated recipients disappeared almost completely. Thus, most hemopoietic progenitors in the adult bone marrow express c-kit. To investigate whether or not c-kit has any role in the hemopoiesis of adult bone marrow, we took the advantage of one of the anti-c-kit mAbs that can antagonize the function of c-kit. As early as two days after the injection of 1 milligram of an antagonistic antibody, ACK2, almost all hemopoietic progenitor cells disappeared from the bone marrow, which eventually resulted in the absence of mature myeloid and erythroid cells in the bone marrow. These results provide direct evidence that c-kit is an essential molecule for constitutive intramarrow hemopoiesis, especially for the self-renewal of hemopoietic progenitor cells at various stages of differentiation.

Exon skipping by mutation of an authentic splice site of c-kit gene in W/W mouse

The murine mutation dominant white spotting (W) is in the proto-oncogene, c-kit. The receptor tyrosine kinase encoded by this gene has pleiotropic effects on murine development including hemopoietic cells, pigment cells, and germ cells. In this study, mutation in W homozygous mouse was identified as a single base substitution (GT----AT) at the 5'-splice donor site of the exon which encodes the transmembrane domain. Two types of aberrant exon skipping resulted from this mutation, occurred in a tissue specific manner. Either transcript lost the exon coding for transmembrane region and therefore the product might not be functional for signal transduction. Any unusual cryptic splice sites were not activated by this mutation as beta-globin gene in beta-thalassaemia. In addition, twelve base pair sequence of the 3'-end of the exon prior to the exon coding for transmembrane domain was found to be alternatively spliced. These findings should provide the genetic base for not only the receptor function but the splicing mechanism.

Increased susceptibility to SV40 transformation with development and in vitro aging

The incidence of most cancers increases with aging. To examine whether this increased risk might be related to a higher susceptibility of older cells to neoplastic transformation, we transfected rat fibroblasts aged in vivo and in vitro with origin-defective SV40 DNA and measured the number of transformed foci. Substantial increases in the number of transformed foci were observed in cells from adult rats when compared with those of cells from embryos or weanlings. Much higher numbers of foci were also obtained at late passage, when 68% or more of the in vitro lifespan had been completed, while no foci were produced from cells at early or middle passage. To control for changes with aging in uptake, integration, or expression of exogenous DNA, parallel cultures were transfected with a G418 resistance gene. The number of G418-resistant colonies did not increase with aging and, in fact, decreased in late passage embryonic cell cultures. Therefore, increased susceptibility to SV40 transformation appears to be a feature of development and in vitro aging in rat cells.

The murine mutation osteopetrosis is in the coding region of the macrophage colony stimulating factor gene

Mice homozygous for the recessive mutation osteopetrosis (op) on chromosome 3 have a restricted capacity for bone remodelling, and are severely deficient in mature macrophages and osteoclasts. Both cell populations originate from a common haemopoietic progenitor. As op/op mice are not cured by transplants of normal bone marrow cells, the defects in op/op mice may be associated with an abnormal haematopoietic microenvironment rather than with an intrinsic defect in haematopoietic progenitors. To investigate the molecular and biochemical basis of the defects caused by the op mutation, we established primary fibroblast cell lines from op/op mice and tested the ability of these cell lines to support the proliferation of macrophage progenitors. We show that op/op fibroblasts are defective in production of functional macrophage colony-stimulating factor (M-CSF), although its messenger RNA (Csfm mRNA) is present at normal levels. This defect in M-CSF production and the recent mapping of the Csfm structural gene near op on chromosome 3 suggest that op is a mutation within the Csfm gene itself. We have sequenced Csfm complementary DNA prepared from op/op fibroblasts and found a single base pair insertion in the coding region of the Csfm gene that generates a stop codon 21 base pairs downstream. Thus, the op mutation is within the Csfm coding region and we conclude that the pathological changes in this mutant result from the absence of M-CSF.

Stepwise progression of B lineage differentiation supported by interleukin 7 and other stromal cell molecules

Growth of early B precursor cells was investigated in vitro by using rIL-7 and IL-7-defective stromal cell line PA6 as separate growth signals. B cell development proceeds through three sequential stages different from the growth signal requirement. The cells in the first stage require PA6 alone for the proliferation, and differentiate into the second stage, which requires both PA6 and IL-7 for its growth. When IL-7 is available for the cells in the second stage, they proliferate extensively on the PA6 layer, and some acquire the ability to proliferate in response to IL-7 alone. This sequential change of growth signal requirement, however, does not proceed autonomously along the time schedule. The possibility that it is primarily directed by the result of Ig gene rearrangement is considered. This mode of growth control may explain why only functional B cells are selected in the error-prone process of Ig gene rearrangement during B lineage differentiation.

Ultraviolet light-induced DNA damage in transcribed sequences: no change in repair with age

We studied the repair of a plasmid vector containing the chloramphenicol acetyltransferase (CAT) gene by treating the plasmid with UV light and then transfecting this plasmid into fibroblasts from human fetal lung (in vitro aging) and into primary cultured fibroblasts from rat lung and skin. This methodology allows us to examine the repair of specific transcribed DNA sequences. There was no age-related change in the repair of UV damage in these cells. Rat embryo fibroblasts at different passages transfected with the plasmid also revealed no significant alteration in UV repair as a function of passage number.