Isolation of rat Schwann cell lines: use of SV40 T antigen gene regulated by synthetic metallothionein promoters

Manipulation of the phenotype of immortalised rat hepatocytes by different culture configurations and by dimethyl sulphoxide

The liver-specific phenotype of immortalised rat hepatocytes is not irretrievably lost as they age in culture but can be manipulated by modifying the culture environment. Testosterone metabolism was used to investigate the profile of cytochrome P450 isoenzymes present in two immortalised cell lines, P9 and LQC, and in primary cultures of rat hepatocytes, cultured on collagen films, gels and double gel cultures (sandwich configuration). The extent of testosterone metabolism, and the range of metabolites produced, was increased in immortalised cells by the presence of collagen as a substratum film or gel but survival was poorer and the range of metabolites was reduced in sandwich culture. In contrast, testosterone metabolism was retained in primary hepatocytes in sandwich cultures at a higher level than in collagen film or gel cultures. Expression of alpha class glutathione-S-transferases (GSTs) increased and that of GSTP1 decreased (changes which indicate a recovery of normal liver GST phenotype) when the medium of immortalised cell cultures was supplemented with dimethyl sulphoxide (DMSO). DMSO also improved ethoxyresorufin O-deethylation (EROD) and testosterone metabolism in immortalised cells. It also markedly inhibited proliferation, DNA, RNA and protein synthesis. Maximal testosterone metabolism was observed in immortalised cells cultured on collagen gels in the presence of 1% (v/v) DMSO. Development of a protocol for treating immortalised liver cells cultured on collagen gels with DMSO to switch between proliferation and differentiation may provide a convenient system expressing the xenobiotic metabolising enzymes required for in vitro toxicity testing.

Reciprocal Id expression and myelin gene regulation in Schwann cells

Id proteins are thought to act as dominant negative antagonists of basic helix-loop-helix (bHLH) transcription factors that direct differentiation in various cell types. We found that Schwann cells express all four Id-family genes and that their transcript levels were reciprocally regulated in pairs during nerve maturation in vivo and cAMP-mediated differentiation in vitro. The rapid induction as part of the early response to axonal membranes and cytokines suggested that Id3 is involved in myelin gene repression. An inverse relationship between Id1/3 and myelin P0 expression was consistent with a role for these two Id proteins as inhibitors of differentiation, and Id1/3 proteins strongly repressed myelin gene promoter activity. Nuclear factors isolated from Schwann cells and intact sciatic nerves were found to bind three different HLH recognition sequences (E boxes) in the proximal region of the P0 promoter, and production of these DNA binding complexes was altered during differentiation. These data support the concept that Id proteins regulate myelin gene expression by controlling the formation of specific bHLH DNA binding complexes with different E-box preferences.

A role for Pak protein kinases in Schwann cell transformation

Neurofibromatosis type 1 (NF1), a common autosomal dominant disorder caused by loss of the NF1 gene, is characterized clinically by neurofibromas and more rarely by neurofibrosarcomas. Neurofibromin, the protein encoded by NF1, possesses an intrinsic GTPase accelerating activity for the Ras proto-oncogene. Through this activity, it is a negative regulator of Ras. The Pak protein kinase is a candidate for a downstream signaling protein that may mediate Ras signals because it is activated by Rac and Cdc42, two small G proteins required for Ras signaling. Here, we use Pak mutants to explore the role of Pak in Ras signaling in Schwann cells, the cells affected in NF1. Whereas an activated Pak mutant does not transform cells, dominant negative Pak mutants are potent inhibitors of Ras transformation of rat Schwann cells and of a neurofibrosarcoma cell line from an NF1 patient. Although activated Pak stimulated jun-N-terminal kinase, inhibition of Ras transformation by dominant negative Pak did not require inhibition of jun-N-terminal kinase. Instead, the Pak mutants appeared to inhibit transformation by preventing Ras activation of the ERK/mitogen-activated protein kinase cascade. These results have implications for our understanding of NF1 because a neurofibrosarcoma cell line derived from a patient with NF1 was reverted by stable expression of the Pak dominant negative mutants.

Kinase-deficient Pak1 mutants inhibit Ras transformation of Rat-1 fibroblasts

Among the mechanisms by which the Ras oncogene induces cellular transformation, Ras activates the mitogen-activated protein kinase (MAPK or ERK) cascade and a related cascade leading to activation of Jun kinase (JNK or SAPK). JNK is additionally regulated by the Ras-related G proteins Rac and Cdc42. Ras also regulates the actin cytoskeleton through an incompletely elucidated Rac-dependent mechanism. A candidate for the physiological effector for both JNK and actin regulation by Rac and Cdc42 is the serine/threonine kinase Pak (p65pak). We show here that expression of a catalytically inactive mutant Pak, Pak1(R299), inhibits Ras transformation of Rat-1 fibroblasts but not of NIH 3T3 cells. Typically, 90 to 95% fewer transformed colonies were observed in cotransfection assays with Rat-1 cells. Pak1(R299) did not inhibit transformation by the Raf oncogene, indicating that inhibition was specific for Ras. Furthermore, Rat-1 cell lines expressing Pak1(R299) were highly resistant to Ras transformation, while cells expressing wild-type Pak1 were efficiently transformed by Ras. Pak1(L83,L86,R299), a mutant that fails to bind either Rac or Cdc42, also inhibited Ras transformation. Rac and Ras activation of JNK was inhibited by Pak1(R299) but not by Pak1(L83,L86,R299). Ras activation of ERK was inhibited by both Pak1(R299) and Pak1(L83,L86,R299), while neither mutant inhibited Raf activation of ERK. These results suggest that Pak1 interacts with components essential for Ras transformation and that inhibition can be uncoupled from JNK but not ERK signaling.

Ras proteins mediate induction of uncoupling protein, IGF-I, and IGF-I receptor in rat fetal brown adipocyte cell lines

Since Ras proteins are essential intermediates of some insulin-like growth factor I (IGF-I)/insulin signaling pathways, we examined whether Ras proteins mediate the IGF-I-induced uncoupling protein expression. Additionally, the role of Ras proteins on IGF-I and IGF-I receptor expression was studied. IGF-I treatment of fetal brown adipocytes cotransfected with inducible gene constructs of SV40 large T antigen (SV40LTag) and a transforming ras gene induced uncoupling protein expression (UCP) in the absence of expression of the transfected genes. The expression of the dexamethasone-inducible transforming ras gene alone or in combination with the Zn-inducible SV40LTag mimicked the IGF-I effect inducing UCP expression and IGF-I did not induce it further. However, the expression of the Zn-inducible SV40LTag did not increase UCP expression in the absence of IGF-I. Expression of the transfected ras oncogene also induced IGF-I and IGF-I receptor mRNAs, whereas expression of SV40LTag did not increase them. Specific IGF-I binding was also specifically increased by expression of the transfected ras oncogene but was not affected by expression of the SV40LTag construct. These results indicate that Ras proteins mediate the IGF-I-induced effect on UCP expression and play a role in the expression of IGF-I and IGF-I receptor. Therefore, an IGF-I autocrine/paracrine loop might be implicated in the process of thermogenic differentiation of brown adipose tissue by a new mechanism unlike that induced by norepinephrine.

Differentiation potential of conditionally immortalized mesenchymal progenitor cells from adult marrow of a H-2Kb-tsA58 transgenic mouse

Primary cultures were initiated from marrow, spleen, and bone explants of an adult H-2Kb-tsA58 transgenic mouse (immortomouse). All cultures were initiated in immortalizing conditions, and an additional marrow culture was first incubated for 1 week in standard conditions and then switched to immortalizing conditions. Marrow cells immediately immortalized were designated the marrow immediate population (MIP); those immortalized after 1 week were termed the marrow delayed population (MDP). MIP and MDP cells both contained a mixture of fibroblastic or flattened cells, and the MIP cells contained an additional subpopulation of adipocytic (Oil Red-O positive) cells. Alkaline phosphatase expression was induced by dexamethasone (10(-7) M) in MDP cells while MIP, spleen, and bone explant cells had only a low level of expression. MDP and MIP cells differentiated into bone when combined with porous calcium phosphate ceramics and implanted subcutaneously into nude mice while bone- and spleen-derived cells did not. Clones were isolated from the MDP and MIP cell populations and tested for differentiated phenotypes. Some MIP-derived clones exhibited adipocytic characteristics while MDP-derived subclones were negative. Histologic examination of porous ceramic implanted clones showed that all of the clones had osteogenic potential. Clones exposed to either dexamethasone, human recombinant bone morphogenetic protein-2, or horse serum plus hydrocortisone showed differences in expression of adipocytic or osteogenic markers. These immortalized cultures have retained both adipocytic and osteogenic potential even after 1 year of continuous culture, and provide a model system for clonal analysis of the developmental potential of marrow-derived mesenchymal precursor cells.

Immortalization and characterization of rat microglial cells

Microglial cell lines from rat brain were established by transfer of a temperature sensitive simian virus 40 large tumour antigen by means of a retrovirus. Four weeks after infection, colonies were generated in the presence of neomycin and granulocyte-macrophage colony stimulating factor (GM-CSF), and subsequently subcloned. Both bulk cell lines and clones proliferate actively at 33 degrees C, whereas the rate of division was significantly decreased at 39 degrees C when the large T antigen is non-functional. At 39 degrees C, these cells take on the microglial phenotype as demonstrated by immunoreactivity to ED-1 (an intracellular antigen), OX-42 (complement type 3 receptor), W3/25 (CD4 homologue), OX-6 (MHC class II antigen) and OX-18 (MHC class I antigen). These cells are capable of active phagocytosis and retain these properties for 10-15 passages. Long-term culture of these lines and clones, greater than 15 passages, displayed a gradual down-regulation of all cell surface specific antigens that were not rescued by lipopolysaccharide (LPS), interferon-gamma (gamma-IFN), GM-CSF or colony-stimulating factor-1 (CSF-1). The expression of the SV-40 large T antigen was unaffected. These results demonstrate the feasibility of immortalizing short-term cell lines with the SV-40 large T antigen for their use in the characterization of microglial properties.

Lines of murine oligodendroglial precursor cells immortalized by an activated neu tyrosine kinase show distinct degrees of interaction with axons in vitro and in vivo

Replication-defective retroviruses expressing the t-neu oncogene, or a hybrid protein with the neu tyrosine kinase linked to the external region of the human epidermal growth factor receptor (egfr-neu), were used to establish lines of murine oligodendroglial precursor cells. Differentiation of the t-neu lines into myelin-associated glycoprotein (MAG)-positive oligodendrocytes was induced by dibutyryl cAMP, and the egfr-neu line showed limited differentiation in vitro upon withdrawal of epidermal growth factor. Cerebellar granule cell neurons expressed mitogens for the cell lines. Upon transplantation into demyelinated lesions, t-neu line cells engaged with the demyelinated axons whereas the egfr-neu line cells differentiated further and ensheathed the axons. These cell lines thus interact with neurons in vitro and in vivo and can be used as tools to define the molecules involved in different stages of neuron-glia interaction.

Comparison of the phosphorylation of myelin-associated glycoprotein in cultured oligodendrocytes and Schwann cells

Phosphorylation of the large and small isoforms of myelin-associated glycoprotein (L- and S-MAG) was investigated in primary oligodendrocyte cultures and in immortalized Schwann cells by incubating the cells with inorganic [32P]phosphate and immunoprecipitating MAG. In oligodendrocytes, both L- and S-MAG were phosphorylated, but L-MAG was much more heavily labeled. In Schwann cells, most of the phosphorylation was in S-MAG, which is the predominant isoform expressed by these cells. In both types of cells, the principal phosphorylated amino acid in MAG was serine. Radioactive phosphothreonine and phosphotyrosine were also detected in the MAG from oligodendrocytes. In Schwann cells, there was less phosphorylation of threonine and labeled phosphotyrosine was not detected. In both oligodendrocytes and Schwann cells, the phosphorylation of MAG was stimulated by phorbol ester and a calcium ionophore, but not by forskolin. The results indicate that the phosphorylation of MAG is catalyzed by protein kinase C and possibly other calcium-activated kinases in both types of myelinating cells, but not by cAMP-activated kinase. An inhibitor of tyrosine phosphatase, ammonium vanadate, increased the amount of radioactive phosphate in MAG several fold in both oligodendrocytes and Schwann cells. However, even in the presence of vanadate, the great majority of radioactivity in MAG was in phosphoserine and only a small amount was in phosphotyrosine, suggesting that tyrosine phosphorylation of other proteins may indirectly increase the phosphorylation of MAG. The current status of our understanding of MAG phosphorylation is reviewed in the context of similarities and differences between our results and other reports in the literature.

SV40 large T antigen with c-Jun down-regulates myelin P0 gene expression: a mechanism for papovaviral T antigen-mediated demyelination

Expression of myelin proteins has been shown to be altered in transgenic mice that express papovaviral large tumor (T) antigens. This paper analyzes the effect on P0 gene expression in secondary Schwann cells transfected with the SV40 T antigen gene and in Schwann cells immortalized by T antigen. In secondary Schwann cells, both T antigen and c-Jun are required for significant inhibition of the P0 promoter; expression of only one of the proteins is insufficient for repression of the P0 gene. T antigen, c-Jun (p39), and c-Jun-related protein (p47) form an immunoprecipitable complex in SV40 immortalized Schwann cell lines, and T antigen and c-Jun bind independently and as a complex to the P0 promoter. Our data suggest that the probable molecular mechanism underlying the hypomyelination observed in transgenic animals expressing T antigen may be due to the repression of the P0 gene by T antigen and c-Jun.

In vitro and in vivo characterisation of glial cells immortalised with a temperature sensitive SV40 T antigen-containing retrovirus

An oncogene-carrying replication-defective retrovirus was used to establish immortalised lines of murine glial cells. Primary cultures of early postnatal cerebellar cells were infected with a retrovirus based on the Murine Moloney Leukemia Virus containing a temperature-sensitive mutant of the Simian Virus 40 large T antigen (SV40 T) oncogene and a gene coding for resistance to the antibiotic G418. Infected cells were selected in G418 and after several in vitro passages cells expressing the O4 antigen were established as a cell line. At a later time point O4-positive single-cell clones were established. Two different types of clones were obtained: 1) "plastic" clones consisting of cells which initially had a morphological and antigenic phenotype of young glial precursor cells but which gradually lost these features, and 2) "stable" cell clones including a clone with the immunological and electrophysiological characteristics of Schwann cells. Culture of the latter cells in the presence of 1 mM dibutyryl cyclic adenosine monophosphate for a period of at least 10 days induced a change in shape and a shift in antigen expression towards a more "differentiated" maturation stage. When the SV40 T O4-positive immortalised cell line isolated on the cell sorter was transplanted into demyelinated lesions in adult rats, cells were observed ensheathing axons and forming limited amounts of PNS-type myelin. Glial cells immortalised with a temperature-sensitive mutant of the SV40 T oncogene thus retain many physiological properties of their primary culture counterparts and can be induced to undergo limited differentiation in vitro and in vivo. These cell lines, which represent immature CNS glia or Schwann cells, are providing useful tools for investigating the role of cell surface antigens involved in neuron-glial interactions.

A new chromosomal protein essential for mitotic spindle assembly

Assembly of the mitotic spindle, the machinery responsible for chromosomal segregation, is regulated by Cdc2 kinase and requires mitotic chromatin. However, the molecular identity of the kinase substrate and chromatin factor is unknown. Here we have cloned a human complementary DNA encoding an evolutionarily conserved chromosomal protein of relative molecular mass 47,000 (M(r) 47K) which has three consensus motifs for Cdc2 kinase-mediated phosphorylation. The protein is phosphorylated only during mitosis and is associated with polypeptides having M(r)s of 31K, 67K and 200K. Mitotic arrest is induced by antisense messenger RNA or by affinity-purified autoantibody. In the arrested cells, the chromosomes remain unsegregated and the mitotic spindle is absent. We propose that the chromosomal protein is activated by phosphorylation at the interphase/mitosis transition by Cdc2 kinase, and that the protein, alone or as a complex, is a previously unidentified Cdc2 kinase substrate and chromatin factor necessary for spindle assembly.

Transforming growth factor-beta: antisense RNA-mediated inhibition affects anchorage-independent growth, tumorigenicity and tumor-infiltrating T-cells in malignant mesothelioma

Transforming growth factor-beta (TGF-beta) is produced by a number of tumor cell types including human malignant mesothelioma (MM), but its role as a direct or indirect factor in tumorigenesis is incompletely understood. We have investigated the expression of TGF-beta isoforms by human and murine MM cells and have analysed the effects of inducible antisense RNA-mediated inhibition of TGF-beta expression on murine MM in vitro and in vivo. The results showed that (a) TGF-beta 1 and -beta 2 were produced by both human and mouse MM cells, (b) antisense RNA against either TGF-beta 1 or -beta 2 cross-inhibited both TGF-beta 1 and -beta 2 expression, (c) inhibition of TGF-beta expression reduced the anchorage-independent growth of MM cells in vitro and the tumorigenicity of MM cells in vivo, and (d) inhibition of TGF-beta expression led to increased T lymphocyte infiltration into tumors. The data suggest that TGF-beta has multiple tumor-enhancing effects in MM.

Characterization of the cis-acting elements of the mouse myelin P2 promoter

Myelin P2 is a basic protein of an apparent molecular weight of 14,800. Expression of P2 has been found largely in the cytosol of Schwann cells in the peripheral nervous system. Although the function of P2 is unknown, its striking homology to a family of fatty acid binding proteins has led to the idea that P2 may function as a fatty acid transport molecule. To investigate the DNA elements that control the expression of P2, sequences 5' to the coding region were cloned upstream of the cat reporter gene. A series of 3' and 5' promoter mutants was constructed and their activity determined following transfection into secondary Schwann cells and the MT4H1 Schwann cell line. Using this strategy, we have identified a 217 bp silencer region and a 142 bp positive regulatory region. In addition, we have localized the 5' flanking sequences in the promoter that are responsive to cAMP induction and to the transcription factor CCAAT/enhancer binding protein (C/EBP).

Myelin-associated glycoprotein is phosphorylated by protein kinase C

The myelin-associated glycoprotein (MAG) is a neural recognition molecule involved in heterophilic interactions between myelin-forming cells and neurons. To characterize the molecular mechanisms underlying post-translational modifications which may be instrumental in signal transduction following the recognition event, we have studied the stimuli leading to modification of 32P-orthophosphate incorporation into MAG in cultures of oligodendrocytes or transformed differentiated Schwann cells. Here we show that in oligodendrocytes both the 67 and 72 kD isoforms of MAG were phosphorylated exclusively on serine, while in the transformed Schwann cells only the 67 kD isoform was found to be present and phosphorylated. The phorbol ester phorbol-12-myristoyl-13-acetate (PMA) did not affect biosynthesis of the protein backbone, but enhanced incorporation of phosphate by a factor of 2-3, indicating the involvement of protein kinase C. Exclusive phosphorylation of serine residues was also observed, when purified MAG was incubated with protein kinase C in the presence of [gamma-32P]ATP. In searching for the physiological stimuli which may trigger phosphorylation of MAG, cultures of oligodendrocytes were exposed to extracellular signals, such as coculture with dorsal root ganglion and spinal cord neurons carrying the MAG receptor, to membrane fractions of these neurons, monoclonal MAG antibody 513 binding to the recognition site of MAG, or platelet-derived growth factor. None of these additives modified the phosphorylation of MAG. These observations point to the possibility that phosphorylation of MAG is controlled by yet unknown intracellular cues rather than by extracellular signals interacting with cell surface receptors of oligodendrocytes.

Modulation of the neurofibromatosis type 1 gene product, neurofibromin, during Schwann cell differentiation

Neurofibromin, the product of the neurofibromatosis type 1 (NF1) gene, is a approximately 250 kDa protein expressed predominantly in cortical neurons and oligodendrocytes in the central nervous system (CNS) and sensory neurons and Schwann cells in the peripheral nervous system (PNS). To gain insight into the biological role of neurofibromin in Schwann cells, the modulation of NF1 gene expression in a Schwann cell line (MT4H1) stimulated to either proliferate or differentiate in response to agents that elevate intracellular cAMP was examined. Untreated cells and cells exposed to mitogenic doses of forskolin (1-10 microM) or 8-bromo-cAMP (0.1 mM) expressed low levels of NF1 mRNA and the protein was barely detectable. High doses of forskolin (100 microM) or 8-bromo-cAMP (1 mM) induced the expression of both myelin P0 protein and neurofibromin with an identical time course. Although NF1 mRNA levels peaked within 1-6 hr, the rise in neurofibromin was not apparent until 24-48 hr and peaked 72 hr after treatment. P0 and neurofibromin were also coinduced by cell-cell contact in high density, untreated cultures. Moreover, differentiation initiated by either cAMP stimulation or high density culture conditions was associated with predominant expression of the type 2 NF1 mRNA isoform. In contrast, type 1 NF1 mRNA isoform expression was observed in untreated Schwann cells or those stimulated with mitogenic doses of forskolin or 8-bromo-cAMP. A switch from the type 1 neurofibromin that can efficiently downregulate p21-ras to the type 2 isoform with reduced activity may facilitate a p21-ras signaling pathway associated with Schwann cell differentiation.

In vitro and in vivo analysis of a rat bipotential O-2A progenitor cell line containing the temperature-sensitive mutant gene of the SV40 large T antigen

Primary cultures from neonatal optic nerve contain pluripotential O-2A progenitor cells that are capable of differentiating into oligodendrocytes, type-2 astrocytes or adult O-2A progenitors (O-2Aadult). Since primary optic nerve cultures contain a mixture of glial cell types of which only a small number are O-2A progenitors, experiments on cell lineage and differentiation carried out using these cultures are both intrinsically limited and difficult to interpret. Ideally, cells from a clonal cell population would provide the optimal starting material for biological studies. In this paper we describe the creation of an O-2A progenitor cell line using a retrovirus carrying a temperature-sensitive mutant SV40 large T antigen gene. This cell line has provided sufficient numbers of cells to allow analysis of their in vitro properties and their behaviour following transplantation into an in vivo environment. At the non-permissive temperature (39 degrees C), these cells differentiate into oligodendrocytes and type-2 astrocytes in a similar fashion to O-2A progenitor cells from primary cultures (O-2Aprim). When grown in media containing platelet-derived growth factor and basic fibroblast growth factor, the cell numbers can be expanded in culture without differentiating, consistent with the behaviour of O-2Aprim progenitor cells. By exploiting this property, it has been possible to culture large numbers of O-2A progenitors for in vivo analysis. In this study we have shown that transplantation of this O-2A cell line into glia-free areas in adult rat spinal cord results in differentiation of a proportion of cells into oligodendrocytes which are capable of myelinating axons. Furthermore, differentiation of O-2A cells into astrocytes was also observed, indicating that the bipotentiality of these cells in vitro can also be demonstrated in vivo.

Increased P0 glycoprotein gene expression in primary and transfected rat Schwann cells after treatment with axolemma-enriched fraction

To elucidate the role of axonal plasma membrane factors in the differentiation of Schwann cells, we investigated the effect of an axolemma-enriched fraction (AEF) isolated from myelinated CNS tissue on the expression of P0 glycoprotein, the major glycoprotein in peripheral myelin, in primary rat Schwann cells (PSC) isolated from sciatic nerve, as well as in a transfected rat Schwann cell line (TSC). AEF increased PO-mRNA levels in PSC and TSC in a concentration-dependent manner, producing a maximal induction of nearly twofold after 48 hr of treatment. A similar induction of P0 mRNA was elicited in TSC by the cAMP-activating agents 8-bromo-cAMP and forskolin, which have been shown to induce myelin proteins in PSC. In addition to inducing P0 mRNA, AEF and forskolin also increased the amount of P0 protein in TSC, as indicated by increased P0-immunoreactive staining. However, in TSC, axolemma caused no increase in expression of CAT linked to a P0 promoter while forskolin caused a marked increase in the expression from the P0 promoter. These results suggest that AEF, in contrast to forskolin, does not regulate P0-mRNA expression at the level of transcriptional activity. These in vitro systems may be useful for the study of axolemmal factors that induce Schwann cell differentiation.

Expression of myelin protein genes and other myelin components in an oligodendrocytic cell line conditionally immortalized with a temperature-sensitive retrovirus

We have conditionally immortalized oligodendrocytes isolated from normal and shiverer primary mouse brain cultures through the use of the retroviral vector ZIPSVtsA58. This vector encodes an immortalizing thermolabile simian virus 40 large T antigen (Tag) and allows for clonal selection by conferring neomycin (G418) resistance. We isolated 14 shiverer and 10 normal lines that expressed the early oligodendrocyte marker 2',3'-cyclic nucleotide 3'-phosphodiesterase mRNA. These cell lines grew continuously at the permissive temperature (34 degrees C) and displayed Tag nuclear immunostaining. On shifting to nonpermissive temperatures (39 degrees C), the cells showed rapid arrested cell growth and loss of Tag staining. One line (N20.1) engineered from normal oligodendrocytes also expressed myelin basic protein (MBP) and proteolipid protein (PLP) mRNAs, genes normally expressed by mature, differentiated oligodendrocytes. No differences in any of the myelin-specific protein mRNA levels were observed in N20.1 cells grown at 39 degrees C for > 9 days compared with cells maintained at 34 degrees C. Immunocytochemical staining revealed N20.1 cells to be positive for the oligodendrocyte surface markers--galactocerebroside, A007, and A2B5. However, MBP and PLP polypeptides could not be detected by western blot or immunocytochemical staining at either the permissive or nonpermissive temperature. Cell-free protein synthesis experiments indicated that the MBP mRNAs isolated from N20.1 cells were translatable and directed the synthesis of the 17-, 18.5-, and 21.5-kDa MBP isoforms. Analysis of the PLP/DM20 gene splice products by polymerase chain reaction indicated that the expression of DM20 mRNA predominated over that of PLP mRNA in this cell line. Because the cell line expressed the MBP and PLP genes, it represents a "mature" oligodendrocyte, but the splicing patterns of these genes indicate that it is at an early stage of "maturation." This cell line has now been passaged > 40 times with fidelity of phenotype and genotype.

Role of the large hepatitis B virus envelope protein in infectivity of the hepatitis delta virion

The hepatitis delta virus (HDV) is coated with large (L), middle (M), and small (S) envelope proteins encoded by coinfecting hepatitis B virus (HBV). To study the role of the HBV envelope proteins in the assembly and infectivity of HDV, we produced three types of recombinant particles in Huh7 cells by transfection with HBV DNA and HDV cDNA: (i) particles with an envelope containing the S HBV envelope protein only, (ii) particles with an envelope containing S and M proteins, and (iii) particles with an envelope containing S, M, and L proteins. Although the resulting S-, SM-, and SML-HDV particles contained both hepatitis delta antigen and HDV RNA, only particles coated with all three envelope proteins (SML) showed evidence of infectivity in an in vitro culture system susceptible to HDV infection. We concluded that the L HBV envelope protein, and more specifically the pre-S1 domain, is important for infectivity of HDV particles and that the M protein, which has been reported to bear a site for binding to polymerized albumin in the pre-S2 domain, is not sufficient for infectivity. Our data also show that the helper HBV is not required for initiation of HDV infection. The mechanism by which the L protein may affect HDV infectivity is discussed herein.

Transformation of Bowes melanoma cells with SV40 T antigen

A serum-dependent and two serum-independent variants of the Bowes melanoma cell line, RPMI7272, were transfected with plasmids containing a geneticin-resistance (neo) gene transcribed by the HSV thymidine kinase promoter and an SV40 T antigen gene under control of the mouse metallothionein I promoter. T-antigen increased the cloning efficiency of the serum-dependent cell line in soft-agar more than 50-fold, but cloning efficiency of serum-independent lines was not increased. Trypsinization of serum-independent lines required 100 times lower concentrations of trypsin than serum-dependent cells. Human metal-inducible T-antigen-producing (HMT) melanoma cells supported replication of transfected plasmids containing an SV40 origin of replication. Transient expression of interferon or plasminogen activator from such plasmids was 40-fold higher than in untransformed melanoma cells and could be enhanced 30-fold more by stimulation of transcription of the T antigen gene with cadmium chloride. HMT cells can be grown in suspension and thus may represent an attractive alternative to monkey kidney COS cells.

Simian virus 40 mutants with amino-acid substitutions near the amino terminus of large T antigen

A series of amino-acid substitution mutants has been made with changes in the region of simian virus 40 large tumor antigen (T antigen) that is shared with the small tumor antigen (t antigen). Both single and multiple amino-acid replacements were obtained using the heteroduplex deletion loop method and sodium bisulfite as the mutagen. The mutants could be divided into five phenotypic classes on the basis of their biological properties: a) mutants whose changes did not affect their ability to propagate on permissive monkey cells, nor to transform nonpermissive rodent cells; b) mutants that were not viable, replicated their DNA to 5% or less of wild type, but were positive for transformation; c) mutants that were not viable, replicated their DNA to 5% or less of wild type, and were defective for transformation; and d) mutants that completely lost all three activities coordinately. In addition, one mutant with changes in this region, 5002, replicated its DNA to about 50% of wild type, had an impaired transformation activity, and produced virions at a level of about 4% that of wild type.

Down-regulation of myelin-associated glycoprotein on Schwann cells by interferon-gamma and tumor necrosis factor-alpha affects neurite outgrowth

To investigate the influence of inflammatory cytokines on the potential of peripheral nerves to regenerate, we analyzed the effect of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on the ability of immortalized Schwann cells to mediate outgrowth of neurites from primary DRG neurons. We found that IFN-gamma and TNF-alpha synergistically inhibited the neurite outgrowth-promoting properties of the Schwann cells by specifically down-regulating myelin-associated glycoprotein (MAG) at the levels of mRNA and cell surface protein by approximately 60%. Antibodies to MAg inhibited the outgrowth of neurites on Schwann cells to the same extent as treatment with the two cytokines. Since MAG appears to be involved in both neurite outgrowth and myelination, our findings may provide evidence for a mechanism, by which inflammatory cytokines interfere with Schwann cell-neuron interactions.

Cell cycle regulation of thymidine kinase: residues near the carboxyl terminus are essential for the specific degradation of the enzyme at mitosis

The level of human thymidine kinase (TK) polypeptide is subject to cell cycle regulation. The enzyme is barely detectable in G1 phase but increases 10- to 20-fold by M phase. The low level of human TK in G1 phase is due primarily to the specific degradation of the protein during cell division. Substitution of heterologous promoters, removal of the introns, and deletion of all of the 3' untranslated region from the human TK gene do not affect cell cycle regulation of the enzyme. However, deletion of the carboxyl-terminal 40 amino acids or fusion of beta-galactosidase to the carboxyl terminus of human TK completely abolishes cell cycle regulation and stabilizes the protein throughout the cell cycle. These alterations do not significantly alter the specific enzymatic activity of TK. Changing the carboxyl terminus or deletion of the last 10 amino acids does not alter cell cycle regulation. These data demonstrate that residues near the carboxyl terminus of TK are essential for the cell cycle phase-specific degradation of the enzyme.

Cell cycle regulation of thymidine kinase: residues near the carboxyl terminus are essential for the specific degradation of the enzyme at mitosis

The level of human thymidine kinase (TK) polypeptide is subject to cell cycle regulation. The enzyme is barely detectable in G1 phase but increases 10- to 20-fold by M phase. The low level of human TK in G1 phase is due primarily to the specific degradation of the protein during cell division. Substitution of heterologous promoters, removal of the introns, and deletion of all of the 3' untranslated region from the human TK gene do not affect cell cycle regulation of the enzyme. However, deletion of the carboxyl-terminal 40 amino acids or fusion of beta-galactosidase to the carboxyl terminus of human TK completely abolishes cell cycle regulation and stabilizes the protein throughout the cell cycle. These alterations do not significantly alter the specific enzymatic activity of TK. Changing the carboxyl terminus or deletion of the last 10 amino acids does not alter cell cycle regulation. These data demonstrate that residues near the carboxyl terminus of TK are essential for the cell cycle phase-specific degradation of the enzyme.

V-myc immortalization of early rat neural crest cells yields a clonal cell line which generates both glial and adrenergic progenitor cells

We describe the isolation and characterization of an immortal cell line derived by infection of rat neural crest cells with a v-myc-containing replication-defective retrovirus. This clonal cell line, called NCM-1, contains a majority cell population with antigenic and morphologic properties that suggest it may represent a peripheral glial progenitor. In conditioned or in serum-free medium, these NGF receptor-positive cells differentiate to an elongated, bipolar morphology resembling that of primary Schwann cells. This morphologic differentiation is prevented by TGF-beta 1, which also acts as a mitogen for the cells. The NCM-1 line is also able to generate clonal derivatives which have extinguished expression of most or all glial markers. Once generated, such cells are stable and do not revert to the glial phenotype. At least some of these cells have acquired sympathoadrenal progenitor-like properties, as shown by their capacity to coexpress tyrosine hydroxylase (TH) and neurofilament (NF) in response to basic FGF and dexamethasone. These data imply that the NCM-1 line contains self-renewing cells with the potential to generate precursors in at least two of the sublineages that normally develop from the neural crest. This in turn suggests that the process of immortalization may preserve at least some of the developmental properties characteristic of multipotential neural crest cells. NCM-1 cells may prove useful for the study of neural crest cell lineage segregation, Schwann cell differentiation, and the mechanisms controlling the initial induction of TH and NF gene expression.

Production of Schwann cell lines using a regulated oncogene

The process of myelination in the central and peripheral nervous systems has been well characterized morphologically by a variety of techniques. It is evident from these studies that, in the peripheral nervous system myelin formation is a multistep process. Clearly, a 1:1 relationship must be established with the axon, which is followed by formation of the basal lamina and eventually myelin. Because immortalized Schwann cell lines obtained using SV40 T antigen under the control of an inducible promoter have many properties of untransfected Schwann cells in culture, including their ability to form myelin in vitro, these cells will enable us to dissect more easily the process of myelination. Having successfully immortalized rat Schwann cells without affecting their ability to differentiate fully, we are applying this approach to generate analogous cell lines from the peripheral nerves of other species such as mouse and human. Unlike rat Schwann cells, there are no known mitogens for human and mouse Schwann cells, making it impossible to expand these cell populations. The ability to produce large numbers of human Schwann cells from nerve biopsy and to analyze their biochemical properties would be of enormous value in identifying the cellular abnormalities that result in demyelinating disease. Likewise, there are several mutant mouse strains with defects in myelin formation, and cell lines from these animals would facilitate our understanding of the process leading to dysmyelination.

Transforming activity of E5a protein of human papillomavirus type 6 in NIH 3T3 and C127 cells

Human papillomavirus type 6 (HPV-6) is the etiologic agent of genital warts and recurrent respiratory papillomatosis. We are investigating the mechanism by which this virus stimulates cell proliferation during infection. In this paper, we report that the E5a gene of HPV-6c, an independent isolate of HPV-11, is capable of transforming NIH 3T3 cells. The E5a open reading frame (ORF) was expressed under the control of the mouse metallothionein promoter in the expression vector pMt.neo.1, which also contains the gene for G418 resistance. Transfected cells were selected for G418 resistance and analyzed for a transformed phenotype. The transformed NIH 3T3 cells overgrew a confluent monolayer, had an accelerated generation time, and were anchorage independent. In contrast, E5a did not induce foci in C127 cells, but C127 cells expressing E5a did form small colonies in suspension. The presence of the 12-kilodalton E5a gene product in the transformed NIH 3T3 cells was shown by immunoprecipitation and was localized predominantly to nuclei by an immunoperoxidase assay. A mutation in the E5a ORF was engineered to terminate translation. This mutant was defective for transformation, demonstrating that translation of the E5a ORF is required for biological activity. This is the first demonstration of a transforming oncogene in HPV-6, and the differential activity of E5a in these two cell lines should facilitate future investigations on the mechanism of transformation.