The myelin basic protein gene is expressed in differentiated blood cell lineages and in hemopoietic progenitors

Myelin basic proteins (MBP) are major constituents of the myelin sheath of oligodendrocytes and Schwann cells in the central nervous system and the peripheral nervous system, respectively. We previously showed that MBP-related transcripts are present in the bone marrow and the immune system. These mRNAs are transcribed from a region called 0', consisting of three exons, located upstream of the classical MBP exons; these three exons belong to the long MBP gene otherwise called "Golli-MBP." The most abundant of these mRNAs, now called HMBP (hemopoietic MBP), encompasses the sequence encoded by the region 0' plus exon 1 and part of intron 1 of the classic MBP gene. Antisera to recombinant HMBP proteins are immunoreactive with proteins of about 26-28 kDa in brain, thymus, and spleen. This report demonstrates that HMBP proteins are present in the vast majority (>95%) of thymic T cells, which express the corresponding transcripts, as do mature T cells from lymph nodes and spleen. HMBP mRNAs and proteins are also manifest in the majority of spleen B lymphocytes and in B cell lines. In addition to lymphoid cells, HMBP proteins are in all types of myeloid lineage cells, i.e., macrophages, dendritic cells, and granulocytes, as well as in megakaryocytes and erythroblasts. Finally, HMBP proteins are present in CD34+ bone marrow cells, and, furthermore, in highly proliferative cultures, these CD34+ cells express HMBP RNAs and proteins. Thus, MBP gene products are present both in the nervous system and in the entire hemopoietic system.

[Microglia: origin and development]

As suggested by Del Rio Ortega a long time ago, it is now widely accepted that microglia are the resident macrophages of the central nervous system. Microglia represent about 10% of the adult brain cell population. We have previously shown that the late embryonic and adult mouse brain contain potential microglial progenitors. We report here that microglial progenitors can be detected in neural folds from embryonic day 8. They originate from the yolk sac in which macrophage progenitors are found from embryonic day 7. We also report that the bulk of microglial cells (about 95%) appear during post-natal development. A major finding is that microglia arise by an intense in situ proliferation comparable to that of neural cells. Taken together, these results show that adult mouse microglia originate from cells migrating from the yolk sac and whose progeny actively proliferates in the brain during development.

No evidence for transmission disequilibrium between a new marker at the myelin basic protein locus and multiple sclerosis in French patients

The myelin basic protein (MBP) gene is a candidate locus for susceptibility to multiple sclerosis. Several groups have tested a complex (TGGA)n repeat in the 5' region of this gene for association/linkage with multiple sclerosis, with divergent results. This region of tandem repetitive sequence has been subjected to complex rearrangements, and there is a possibility that alleles of the same size have different internal structures, which reduces the interest of this marker for linkage disequilibrium studies and may at least partly explain the conflicting results obtained so far. To overcome this problem, we isolated a new polymorphic (CA)n repeat within the Golli-MBP locus. The limited number of alleles identified makes this other marker suitable for transmission disequilibrium studies. We tested this marker for linkage with multiple sclerosis, using the transmission-disequilibrium test (TDT) on a sample of 196 nuclear families in which the genotypes of both parents could be unambiguously defined. We found no evidence of transmission disequilibrium between multiple sclerosis and any of the three alleles of this marker, even when the patients were subdivided according to their HLA-DRB1*1501 status. The present data thus provide no evidence for a contribution of the MBP gene to multiple sclerosis susceptibility in French patients.

Expression of gamma-glutamyl transpeptidase in mouse perivascular astrocytes and in a protoplasmic-like astroglial cell clone

Gamma-glutamyl transpeptidase (GGT) is known to be present in the central nervous system (CNS) but its cellular localization is still subject to controversy. In this report, we have investigated, with a specific antiserum, the immunolabelling pattern of GGT in the adult mouse CNS at the light and electron microscopic (EM) levels. At the optical level, GGT immunoreactivity ensheathes the majority of vessels in the grey matter. Immunoelectron microscopy shows that labelling is essentially due to the presence of GGT in the astrocytic endfeet which surround vessels. In addition, some pericytes and periendothelial cells are also clearly labelled. We then investigated GGT activity in astroglial cell clones which may represent the in vitro counterpart of the main astroglial cell types. The striking result is that a protoplasmic-like astroglial cell clone shows a noticeable GGT activity, while, in contrast, no activity was detected in the fibrous and the Golgi-Bergmann-like astroglial clones. Taken together, these data indicate that, in the mouse CNS, GGT is essentially present in protoplasmic astrocytes.

Microglia derive from progenitors, originating from the yolk sac, and which proliferate in the brain

Microglia, the resident CNS macrophages, represent about 10% of the adult brain cell population. Although described a long time ago, their origin and developmental lineage is still debated. While del Rio-Hortega suggested that microglia originate from meningeal macrophages penetrating the brain during embryonic development, many authors claim that brain parenchymal microglia derive from circulating blood monocytes originating from bone marrow. We have previously reported that the late embryonic and adult mouse brain parenchyma contains potential microglial progenitors [F. Alliot, E. Lecain, B. Grima, B. Pessac, Microglial progenitors with a high proliferative capacity in the embryonic and the adult mouse brain, Proc. Natl. Acad. Sci. U.S.A. 88 (1991) 1541-1545]. We now report that they can be detected in the brain rudiment from embryonic day 8, after their appearance in the yolk sac and that their number increases until late gestation. We also show that microglia appear during embryonic development and that their number increases steadily during the first two postnatal weeks, when about 95% of microglia are born. Finally, the main finding of this study is that microglia is the result of in situ proliferation, as shown by the high proportion of parenchymal microglial cells that express PCNA, a marker of cell multiplication, in embryonic and postnatal brain. Taken together, our data support the hypothesis that terminally differentiated brain parenchymal microglia are derived from cells originating from the yolk sac whose progeny actively proliferates in situ during development.

Pericytes and periendothelial cells of brain parenchyma vessels co-express aminopeptidase N, aminopeptidase A, and nestin

Within the parenchyma of the CNS, the endothelium of all vessels is surrounded by a layer of cells, pericytes in capillaries and periendothelial or intima smooth muscle cells in other vessels. The origin of these cell types, their relationship, and their role are unclear. However, it has been recently shown that genetically engineered mice that lack pericytes develop microaneurysms at late gestation and die before birth (Lindahl et al. [1997] Science 277:242-245). The goal of this study was to identify in situ molecular markers that would be common to pericytes and periendothelial cells of adult mouse brain. Immunocytochemistry experiments were carried out at the optical and electron-microscopic levels on mouse brain sections with antibodies specific for aminopeptidase N, aminopeptidase A, and the intermediate filament nestin. The results of our experiments show that in all brain parenchyma vessels of all sizes, pericytes and periendothelial cells are immunoreactive for aminopeptidase N, essentially at the plasma membrane level, and are also labeled by nestin specific antibodies, which decorate typical intermediate filaments. In addition, brain pericytes and periendothelial cells are also immunoreactive to monoclonal antibodies to aminopeptidase A. In contrast, pericytes and periendothelial cells do not express microglial markers. Taken together these data show that pericytes and periendothelial intima smooth muscle cells share common markers, suggesting a common origin or function, and are distinct from microglia.

Brain parenchyma vessels and the angiotensin system

It is now recognized that the brain contains an autonomous angiotensin (AG) system, including the aminopeptidases A and N required for angiotensin metabolism. Using immunohistochemical techniques, we show that capillary pericytes and periendothelial cells of other vessels express aminopeptidase A (APA) and aminopeptidase N (APN) at their plasma membrane in adult mouse brain parenchyma. We therefore investigated the localization of angiotensin II(III), known as putative substrates for these enzymes, as well as that of their precursor angiotensin I. We report here the presence of immunoreactivity to angiotensin I and II(III) around most brain vessels. Angiotensins are present at the plasma membrane of brain parenchymal cells, presumably perivascular astrocytes which are also immunoreactive to AT1-receptor antibodies. The very close relationship between AGII(III) and their metabolizing enzymes APA and APN suggests a specific functional role for brain perivascular angiotensins.

Ly-6C is expressed in brain vessels endothelial cells but not in microglia of the mouse

The Ly-6C antigen is expressed in various cell types of the immune system, including macrophages. Using the monoclonal antibody ER-MP20 which specifically recognises Ly-6C, we have investigated whether brain parenchymatous microglia express Ly-6C in vivo as well as in vitro. In brain sections from developing and adult C57/BI mice, all vessels were strongly immunolabelled. Electron microscopic immunohistochemistry showed that the endothelial cells are the cell type expressing Ly-6C. In contrast, we never observed immunoreactivity on microglia; however, microglial cells proliferating in vitro were strongly ER-MP20 positive. These data show that Ly-6C is not a marker for microglia in vivo.

Retinal dysplasia in mice lacking p56lck

The product of the proto-oncogene p56lck is a non-receptor tyrosine kinase member of the Src family. It is found in T cells (Marth et al., 1985, 1988) and in the mouse brain (Omri et al., 1996; Van Tan et al., 1996). In this report, we describe experiments showing that Lck is present in the mouse retina neurons. Lck gene expression was identified after isolating and sequencing the specific 5' and 3' part of the cDNA obtained by RT-PCR. In adult retina Lck immunoreactivity was most abundant in photoreceptor cells and within the outer plexiform layers. Staining was also observed in the inner nuclear and plexiform layers. In transgenic mice, the disruption of the Lck gene had serious consequences on the organization of the retina causing retinal dysplasia. These mice have partial retinal detachment with infolding and rosette formation in the photoreceptor sheet. These retinal abnormalities observed in Lck deficient mice lead to the loss of normal architecture of the photoreceptor and the inner nuclear layers, and provide an important role of Lck protein in the retina development. The lack of the Lck protein produces a spectrum of retinal pathology that resembles human retinopathy of prematurity (ROP).

Myelin basic protein-related proteins in mouse brain and immune tissues

Brain and immune system tissues express myelin basic protein (MBP) mRNAs that contain novel exons upstream of those of the classic MBPs. We have generated antisera against a recombinant protein that includes the deduced sequence for one of the predicted species of the new MBP-related protein, hemopoietic HMBPR1. We report here the presence of multiple proteins reactive to the antisera in a range of tissues. The principal finding is that the antisera recognize a family of proteins of approximately 25 kDa that are restricted to brain, thymus, and spleen, the only tissues in which the new MBP-related transcripts are present. These antisera also detect other proteins of apparent molecular mass consistent with other isoforms of predicted MBP-related proteins. The expression of MBP-related proteins in immune system tissues may be important for self-tolerance to CNS MBPs and the initiation of immune-associated demyelinating diseases.

Nectinepsin: a new extracellular matrix protein of the pexin family. Characterization of a novel cDNA encoding a protein with an RGD cell binding motif

We report the isolation and characterization of a novel cDNA from quail neuroretina encoding a putative protein named nectinepsin. The nectinepsin cDNA identifies a major 2.2-kilobase mRNA that is detected from ED 5 in neuroretina and is increasingly abundant during embryonic development. A nectinepsin mRNA is also found in quail liver, brain, and intestine and in mouse retina. The deduced nectinepsin amino acid sequence contains the RGD cell binding motif of integrin ligands. Furthermore, nectinepsin shares substantial homologies with vitronectin and structural protein similarities with most of the matricial metalloproteases. However, the presence of a specific sequence and the lack of heparin and collagen binding domains of the vitronectin indicate that nectinepsin is a new extracellular matrix protein. Furthermore, genomic Southern blot studies suggest that nectinepsin and vitronectin are encoded by different genes. Western blot analysis with an anti-human vitronectin antiserum revealed, in addition to the 65- and 70-kDa vitronectin bands, an immunoreactive protein of about 54 kDa in all tissues containing nectinepsin mRNA. It seems likely that the form of vitronectin found in chick egg yolk plasma by Nagano et al. ((1992) J. Biol. Chem. 267, 24863-24870) is the protein that corresponds to the nectinepsin cDNA. This new protein could be an important molecule involved in the early steps of the development.

The Lck tyrosine kinase is expressed in brain neurons

The lck gene product, p56lck, is a member of the src-related family of protein tyrosine kinases. It is known as lymphocyte specific and involved in thymocyte development and in the immune response mediated by the T cell receptor. We report that the lck gene is also expressed in adult mouse CNS and that brain p56lck is similar to the thymus protein. In situ hybridization and immunohistochemistry show that the lck gene is expressed in neurons throughout the brain in distinct regions, including hippocampus and cerebellum. In primary cultures from fetal mouse brain, neuronal cells are immunoreactive to Lck antiserum. This suggests that the lck gene product might be involved in a new signal transduction pathway in mouse brain.

A spontaneously immortalized mouse microglial cell line expressing CD4

We have derived a microglial clone, named C8-B4, from the 8-day mouse cerebellum organ culture which gave rise to distinct astroglial cell lines as previously reported. Indeed, the C8-B4 clone expresses classical microglial markers (MAC1, F4/80, 2-4G2) and appears to be derived from a committed microglial precursor since it does not express differentiation antigens present during the early stage of the monocytic lineage. This microglial clone expresses two characteristics not previously reported for microglial cell lines: it synthesizes the CD4 molecule and produces and releases large amounts of glutamate.

Retinal engineering: engrafted neural cell lines locate in appropriate layers

A major question in central nervous system development, including the neuroretina, is whether migrating cells express cues to find their way and settle at specific locations. We have transplanted quail neuroretinal cell lines QNR/D, a putative amacrine or ganglion cell, and QNR/K2, a putative Müller cell into chicken embryo eyes. Implanted QNR/D cells migrate only to the retinal ganglion and amacrine cell layers and project neurites in the plane of retina; in contrast, QNR/K2 cells migrate through the ganglion and amacrine layers, locate in the inner nuclear layer, and project processes across the retina. These data show that QNR/D and QNR/K2 cell lines represent distinct neural cell types, suggesting that migrating neural cells express distinct address cues. Furthermore, our results raise the possibility that immortalized cell lines can be used for replacement of specific cell types and for the transport of genes to given locations in neuroretina.

A novel glial fibrillary acidic protein mRNA lacking exon 1

Glial fibrillary acidic protein (GFAP) is the major intermediate filament protein in the mature astrocytes. We have assayed for the presence of GFAP mRNA gene in mouse tissues outside the nervous system. Nuclease S1 protection experiments show that RNAs lacking exon 1 are transcribed in bone marrow. From a mouse bone marrow cDNA library we isolated GFAP cDNAs which start in the 3' part of intron 1 and contain all the downstream GFAP exons. The new GFAP mRNAs, which we call GFAP gamma mRNAs, are already present in the brain at embryonic day 15 and in adult forebrain and cerebellum. Their presence in astrocytic cell lines suggests that astrocytes may be the site of in vivo expression of these mRNAs. In addition we have detected GFAP gamma mRNAs in mouse spleen. Furthermore in human an analogous GFAP mRNA containing the 3' part of intron 1 and lacking the exon 1 is also present in adult brain. These results suggest a new regulation of the GFAP gene expression.

Identification of new human myelin basic protein transcripts in the immune and central nervous systems

Five new myelin basic protein (MBP) transcripts were identified which each have preferential sites of expression in adult human brain and immune system. They contain a novel 5' coding region which presents extensive sequence similarity to the mouse 0' region. One of these ribonucleic acid (RNA) species, HMBPR1, is found essentially, if not only, in haemopoietic and immune cells. Two alternatively spliced transcripts called MBP2a and c are only expressed in the central nervous system (CNS). In addition, the two other transcripts are expressed in both immune and nervous systems. Thus, the MBP locus can generate multiple forms of RNA, whose start sites and splicing depend on the tissue in which they are expressed. The presence of an MBP transcript specifically expressed in the adult human immune system suggests previously unsuspected functions related to the pathogenesis of multiple sclerosis.

CD4 expression in neurons of the central nervous system

CD4 is a member of the Ig gene super family expressed on the surface of many thymocytes and of a subset of T lymphocytes. Human CD4 is the receptor for HIV envelope glycoprotein gp120. Human and mouse CD4 transcripts are expressed in human and mouse central nervous system (CNS), but no corresponding proteins have been reported yet. We have analyzed mRNA expression and carried out immunological experiments on adult mouse brain with probes specific for the long and short CD4 transcripts and with antibodies monospecific for mouse CD4. The main result of these experiments is that the full length CD4 transcript and the CD4 protein are expressed coordinately in neurons throughout the adult mouse brain. CD4 immunoreactivity is also present in brain small vessel walls, ependymal cells, and choroid plexus. The brain mouse CD4 protein is indistinguishable from the thymus protein. In addition, we show that neuronal cells in primary cultures from human fetal CNS are immunoreactive to human CD4 mAbs.

A novel cDNA corresponding to transcripts expressed in retina post-mitotic neurons

The long term objective of this study is to isolate genes specifically expressed at the onset of neuronal cell cycle withdrawal. As an experimental paradigm we have used a quail neuroretinal cell clone (clone K2) immortalized by a thermosensitive mutant of Rous Sarcoma Virus. K2 cells proliferate at 36 degrees C but stop synthesizing DNA after a shift to 41.5 degrees C. We have constructed a cDNA library from K2 cells transferred to 41.5 degrees C and autosubtracted with RNAs from K2 cells maintained at 36 degrees C. This strategy has led to the isolation of cDNAs which recognize mRNAs expressed in quail neuroretina (NR) during development. We report here one of these cDNAs, cDNA QN1, that hybridizes with transcripts expressed in retina neurons, in parallel with their withdrawal from the cell cycle. QN1 ORF codes for a 138 kDa polypeptide corresponding to the protein observed in Western blot analysis. A role of QN1 product(s) on neuronal quiescence is suggested by the positive effect of an antisense oligonucleotide on DNA synthesis of K2 cells.

A new family of transcripts of the myelin basic protein gene: expression in brain and in immune system

A cDNA clone (MBP2) corresponding to a novel mouse myelin basic protein (MBP) mRNA has been isolated from an adult mouse bone marrow cDNA library. It contains the MBP exons 1a-7 except exon 5. Using PCR experiments we have determined that this MBP2 mRNA belongs to a new MBP mRNA family initiated upstream from exon 1b. Their 5' end extends into exon 1a and/or the region 0' previously described. These mRNAs are generated by alternative splicing of the primary transcript involving excision of exon 1a, 1b, 2, 5, or 6. Thus, these new mRNAs are produced from a promoter(s) located upstream from the major promoter 1b. They are expressed in brain (at least from embryonic day 15), in bone marrow, and in other hemolymphopoietic tissues, particularly in macrophage cells. As their expression is not restricted to myelinating cells, the function of these novel MBP mRNAs and putative proteins might not be related to myelination.

Serum factors and v-src control two complementary mitogenic pathways in quail neuroretinal cells in culture

Quail neuroretinal cells (QNR cells) from 7-day-old embryos do not proliferate even in the presence of 8% fetal calf serum. After infection by the Rous sarcoma virus (RSV) they proliferate actively and exhibit a transformed phenotype; this effect is mediated by the oncoprotein pp60v-src. Secondary cultures infected by the thermosensitive strain tsNY68 of RSV are blocked in G0 either by thermal inactivation of pp60v-src at 41.5 degrees C or by serum deprivation at the permissive temperature (36.5 degrees C). Cell division is reinduced either by pp60v-src thermal renaturation or by subsequent serum addition. Our results indicate that v-src and serum control two synergic pathways leading to G0/G1 transition in QNR cells. In order to characterize genes related to the mitogenic and transforming effects of v-src in nerve cells, we have constructed a cDNA library from QNR cells transformed by tsNY68. We report the properties of five molecular clones isolated by differential screening of this library. Unlike immediate-early genes like c-fos, they are induced in mid and late G1. Four of them correspond to unknown mRNAs and the last one codes for nucleolin. This set of v-src-regulated genes is likely to code for functions deficient in terminally differentiated QNR cells and necessary for the progression in G1.

A novel transcript overlapping the myelin basic protein gene

Myelin basic protein (MBP) is a major constituent of myelin synthesized by oligodendrocytes and Schwann cells. We have investigated the expression of mouse MBP RNAs outside the nervous system. Nuclease protection experiments indicate that RNAs containing exon 1 and not the six downstream exons of the MBP gene are transcribed in various hemopoietic tissues. We have isolated a hemopoietic MBP-related (HMBPR) cDNA clone from a mouse bone marrow cDNA library screened with an MBP cDNA probe. This clone contains exons 1a and 1b and a part of intron 1 of the MBP gene. An additional 5' region, encoded by at least three unidentified exons, lies upstream of exon 1a. The HMBPR clone corresponds to a 5-kb RNA expressed in bone marrow, spleen, thymus, and macrophagic cells. This transcript is expressed at a similar level in brain, although at a lower level than the classical 2-kb mRNA. These data indicate that a new transcript, overlapping the MBP transcription unit and controlled by a distinct promoter, is expressed in hemopoietic tissues. This RNA might encode a 21-kDa protein sharing a common domain with MBP.

Isolation of a novel cDNA corresponding to a transcript expressed in the choroid plexus and leptomeninges

The CNS is composed of neurons and glial cells (i.e., astrocytes, oligodendrocytes, and microglia). The brain communicates with the blood circulation through choroid plexus and meninges as well as through the blood-brain barrier. To identify transcripts specifically expressed in a distinct brain cell type, we have previously constructed a subtracted cDNA library from the poly(A)+ RNAs of a velate protoplasmic-like astrocytic cell line, designated D19. This library was screened in order to isolate transcripts over-expressed in this astroglial cell line versus another astroglial cell line. Of the six recombinants which have been isolated, three sequences have not been described. Their sizes ranged from 100 to 200 bp and they hybridized to mRNAs expressed in vivo inside and outside the CNS. We have constructed a size-selected D19 cDNA library in order to obtain the full length cDNAs corresponding to the three undescribed sequences. We report here the isolation of a 1.6 kb cDNA corresponding to one of these recombinants, named p14. Its sequence has not yet been described and its deduced amino acid sequence codes for a 43 kDa protein with a putative signal peptide. In situ hybridization shows that this transcript is expressed at a high level in choroid plexus and leptomeninges and also in perivascular cells in the adult mouse brain. It is also expressed in cell subsets of kidney and gonads as well as in perivascular cells in skeletal and cardiac muscles. These localizations suggest that the encoded protein might be involved in transport processes and hormonally controlled.

Transcription of a quail gene expressed in embryonic retinal cells is shut off sharply at hatching

The avian neuroretina (NR) is part of the central nervous system and is composed of photoreceptors, neuronal cells, and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors that differentiate after terminal mitosis and become organized in cell strata. Genes that are specifically expressed at the various stages of retinal development are presently unknown. We have isolated a quail (Coturnix coturnix japonica) cDNA clone, named QR1, encoding a 676-amino acid protein whose carboxyl-terminal portion shows significant similarity to those of the extracellular glycoprotein osteonectin/SPARC/BM40 and of the recently described SC1 protein. The QR1 cDNA identifies a mRNA detected in NR but not in other embryonic tissues examined. The levels of this mRNA are markedly reduced when nondividing NR cells are induced to proliferate by the v-src oncogene. QR1 expression in NR is limited to the middle portion of the inner nuclear layer, a localization that essentially corresponds to that of Müller cells. Transcription of QR1 takes place only during the late phase of retinal development and is shut off sharply at hatching. Signals that regulate this unique pattern of expression appear to originate within the NR, since the QR1 mRNA is transcribed in cultured NR cells and is shut off also in vitro at a time coinciding with hatching.

Alpha isoform of smooth muscle actin is expressed in astrocytes in vitro and in vivo

We have previously reported that astroglial cell lines derived from spontaneously immortalized mouse cerebellar cultures as well as primary astrocyte cultures express the mRNA of the alpha isoform of smooth muscle actin. In this report, we have used an antiserum specific for the alpha smooth muscle actin protein to investigate the presence and the pattern of expression of alpha smooth muscle actin protein at the cellular level with immunocytochemical methods. The results show that an anti-smooth muscle vessels alpha actin antiserum labels a typical actin network in the D19 astroglial cell clone and in flat astrocytes of primary cultures derived from various CNS regions of embryonic and postnatal mice. Furthermore, this antiserum labels distinct populations of astrocytes in the adult mouse brain, in particular in the corpus callosum and the fornix. However, in the corpus callosum, astrocytic processes are strongly labeled by anti-SMV alpha actin antibodies only in parasagittal planes. Thus, alpha smooth muscle actin represents a new marker for subsets of astrocytes.

Isolation of cDNAs from a mouse astroglial cell line by a subtracted cDNA library

Astrocytes belong to the glial cell population and represent a major subclass of the CNS. Although different subtypes of astrocytes have been described according to their morphological characteristics, biochemical markers of each subtype of astrocytes are not yet available. We have thus undertaken to compare gene expression pattern of different astroglial subtypes. In this study we have taken advantage of two astroglial cell clones derived from 8 day postnatal mouse cerebellar explants and which might be the in vitro equivalents of the velate protoplasmic (D19) and of the Golgi-Bergmann (C8S) astrocytes (Alliot and Pessac, Brain Res., 306: 283-291, 1984). We have constructed a subtracted cDNA library derived from cytoplasmic poly(A)+ RNAs of the D19 cell line. This library was enriched 12-fold for D19 specific sequences by subtractive hybridization with an excess of cytoplasmic poly(A)+ RNAs purified from the C8S astroglial clone. This subtracted library was differentially screened with cDNA probes derived from D19 and C8S cell lines; both probes were subtracted with C8S poly(A)+ RNAs. Eight cDNA clones corresponding to transcripts overexpressed in D19 were selected. Three cDNAs encode for smooth muscle actin, one for fibronectin and one for polyadenylate binding protein. The three other gene products have not been previously reported. The in vivo distribution pattern of these sequences in various mouse adult tissues shows that all these transcripts are expressed in the cerebellum and/or in the brain.

Spontaneous glutamate release by a "fibrous"-like cerebellar astroglial cell clone

To investigate the role of astrocytes in the metabolism of glutamate, the neurotransmitter of the granule cells of the cerebellar cortex, we have analyzed various parameters related to the synthesis of glutamate in astroglial cell clones that may be the in vitro counterparts of the cerebellar astrocytes. The "fibrous"-like clone spontaneously released large quantities of glutamate, even in the absence of glutamine in the culture medium, but did not release alanine. In contrast, the "Golgi-Bergmann"-like cells released alanine but not glutamate, whereas the "velate-protoplasmic"-like astrocytes released little glutamate and alanine. However, the glutamate oxaloacetate transaminase and glutamate pyruvate transaminase activities of the three astroglial cell lines, measured in the direction of glutamate synthesis, were comparable. In addition, the "velate protoplasmic" and "Golgi-Bergmann" clones did not consume glutamine present at 2 mM in the culture medium. These data suggest that the different types of in vivo cerebellar astrocytes may have distinct roles regarding glutamate-glutamine metabolism.

Activation and transduction of c-mil sequences in chicken neuroretina cells induced to proliferate by infection with avian lymphomatosis virus

We report that nondividing neuroretina cells from chicken embryos can be induced to proliferate following infection with Rous-associated virus type 1 (RAV-1), an avian lymphomatosis retrovirus lacking transforming genes. Multiplication of RAV-1-infected neuroretina cells is observed after a long latency period and takes place initially in a small number of cells. We also show that serial virus passaging onto fresh neuroretina cultures leads to the generation of novel mitogenic viruses containing the mil oncogene. DNA analysis indicated that RAV-1 is the only provirus detected in cells infected at virus passage 1, whereas neuroretina cells infected at subsequent virus passages harbor mil-containing proviruses. Three viruses, designated IC1, IC2, and IC3, were molecularly cloned. Restriction mapping indicated that in each virus, truncated c-mil sequences were inserted within different portions of the RAV-1 genome. In addition, IC1 and IC2 viruses have transduced novel sequences that belong to the 3' noncoding portion of the c-mil locus. All three viruses induce neuroretina cell multiplication and direct the synthesis of mil-specific proteins. Proliferation of neuroretina cells infected at passage 1 of RAV-1 was not associated with any detectable rearrangement of c-mil, when a v-mil probe was used. However, these cells expressed high levels of an aberrant 2.8-kilobase mRNA hybridizing to mil but not to a long terminal repeat probe. Therefore, transcriptional activation of a portion of c-mil could represent the initial events induced by RAV-1 infection and lead to retroviral transduction of activated c-mil sequences.

Role of astroglial cell clones in the survival and differentiation of cerebellar embryonic neurons

To investigate the role of astrocytes in the survival and differentiation of cerebellar neurons during development, we have used astroglial cell clones, derived from 8-day postnatal cerebellar explants and which might be the in vitro equivalents of the 3 main types of cerebellar astrocytes, the Golgi epithelial cells and their Bergmann processes, the velate protoplasmic and the fibrous astrocytes (F. Alliot and B. Pessac, Brain Res., 306 (1984) 283-291). Nearly all single cells, dissociated from 15-day embryonic mouse cerebella and seeded at low density, adhered to layers of each of the cerebellar astroglial cell clones as well as to other glial lines or artificial substrates. However, the cerebellar embryonic neurons survived well only on monolayers of either the 'Golgi-Bergmann'-like or the 'velate protoplasmic'-like clones. On these layers, 60-80% of the neurons were still present after 5 days of co-culture, while only less than 5% survived on the other types of substrates. The differentiation pattern of the neurons surviving on the 'Golgi-Bergmann' and the 'velate protoplasmic' astroglial clones was studied with markers of postmitotic granule cells, the major neuronal population in adult cerebellum. The velate protoplasmic-like clone was the only one able to support the coordinate acquisition by most surviving neurons of the phenotypic characteristics of granule cells, i.e. a distinct morphology, a specific epitope binding the monoclonal antibody 7-8 D2 and immunoreactivity to glutamate. These data show a broad heterogeneity in the capacity of astroglial cell clones to support embryonic cerebellar neurons. In addition, they indicate that neuronal survival per se is not sufficient for the acquisition of a differentiated neuronal phenotype.

A novel oncogene related to c-mil is transduced in chicken neuroretina cells induced to proliferate by infection with an avian lymphomatosis virus

Non-dividing neuroretina cells from chicken embryos are induced to proliferate after a long latency, following infection with Rous associated virus type 1, an avian retrovirus which does not carry a transforming gene. We have isolated from these proliferating cells an acutely mitogenic retrovirus, designated IC10, which contains a novel oncogene. Nucleotide sequencing showed that the IC10 virus has transduced 1101 nucleotides of cellular origin inserted between the gag and env genes of RAV-1. This oncogene, designated v-Rmil, is 70.1% homologous to v-mil. v-Rmil encodes a protein of 40,976 daltons sharing 83.8% homology with the catalytic domain of the v-mil protein. Divergence with the v-mil gene product is observed at the NH2- and COOH-terminal portions of the v-Rmil protein. Restriction analysis of normal chicken DNA indicated that v-Rmil is derived from a cellular gene distinct from c-mil. The c-Rmil gene is transcribed through a major mRNA, greater than 10 kb in length, that is detected at much higher levels in neuroretinas, as compared to other embryonic tissues.

Glutamate dehydrogenase activity is markedly higher in a "Golgi-Bergmann"-like glial clone than in other astroglial cell lines

Glutamate appears to be the neurotransmitter of granule cells, the major neuronal population of the cerebellar cortex. To determine the role of astroglial cells in the synthesis of glutamate, we have measured the specific activity of glutamate dehydrogenase (GDH) in clonal cell lines that might be the in vitro equivalents of the different cerebellum astroglial cell types. In conditions where GDH operates in the direction of glutamate synthesis, the specific activity of GDH measured in the "Golgi-Bergmann"-like clone was 4-6 times higher than in the "velate protoplasmic"- or "fibrous-like" astrocytic clones. These data correlate well with the intense immunoreactivity to GDH in Golgi-Bergmann astrocytes in vivo that has been recently reported.

Free amino acid content of astroglial cell clones derived from 8-day postnatal mouse cerebella

We have measured the free amino acid content of three distinct astroglial cell clones derived from permanent lines obtained after "spontaneous immortalization" of 8-day postnatal mouse cerebellar cultures; these clones show characteristics similar to the Golgi Bergmann glia cells, the fibrous astrocytes, and the velate protoplasmic astrocytes, i.e., the three main types of cerebellar astrocytes. The relative concentrations of amino acids that are thought to act as neurotransmitters were compared in confluent cultures of the different astroglial clones. The most striking result was a high concentration of glycine (20% of free amino acids), even in astroglial cells cultured in a glycine-free medium, a finding suggesting that glycine is synthesized by the astroglial clones. Furthermore, no gamma-aminobutyric acid (GABA) was detected. In contrast, a "neuron-like" clone derived from the same cerebellar culture contained GABA, whereas its glycine content was much lower than that of the astroglial clones. The present results, together with our previous finding of glycine synthesis in an astrocytic clone derived from 14-day postnatal mouse cerebella transformed by simian virus 40, indicate that a high glycine content may be characteristic of many cerebellar astroglial types.

Differentiation of retrovirus-infected avian neuroretina cells

The effects of oncogenic retroviruses on the expression of differentiation markers were studied in monolayer cultures of chick and quail embryo neuroretinas. Transformation by Rous sarcoma virus (RSV) did not affect the appearance of synapses, and the expression of glutamic acid decarboxylase was stimulated by pp60v-src, the product of the src gene. Quail embryo neuroretina cells transformed by Mill Hill 2 (which contains the two oncogenes v-mil and v-myc) were induced to proliferate into permanent cultures that synthesized crystallins and produced lentoid bodies. In contrast, transformation with a temperature-sensitive mutant of RSV reversibly blocked the production of crystallins and lentoid bodies. These data show that given cellular genes can respond differently to distinct oncogenes.

The presence of glutamic acid decarboxylase and gamma-aminobutyric acid immunoreactivity in photoreceptors of hatching quail retina

The presence of glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) was investigated in neuroretina sections from hatching quail embryos by immunocytochemistry. The photoreceptors were found to be intensely immunoreactive to anti-GAD antiserum and to two distinct anti-GABA antisera.

Crystallin gene expression and lentoid body formation in quail embryo neuroretina cultures transformed by the oncogenic retrovirus Mill Hill 2 or Rous sarcoma virus

The lens-specific proteins alpha and delta crystallins and lentoid bodies, structures that follow a differentiation pathway similar to that of the lens, regularly appear after 4 to 5 weeks in quail embryo neuroretina monolayer cultures. We have investigated the effects of the avian oncogenic retroviruses Mill Hill 2 and Rous sarcoma virus on this process. Quail embryo neuroretina cells transformed by Mill Hill 2 virus were established into permanent cultures that synthesized alpha and delta crystallins and contained stem cells for the production of lentoid bodies. In contrast, transformation with the Rous sarcoma virus mutant tsNY-68 blocked the appearance of mRNA crystallins, but cytoplasmic alpha and delta crystallin mRNA and alpha crystallin appeared 44 h after a shift to the nonpermissive temperature. However, delta crystallins and lentoid bodies were only present after 7 days. The crystallins of transformed quail neuroretina cultures were immunologically indistinguishable from those of quail lenses and of normal quail embryo neuroretina cultures.

Astrocytic cerebellar cell clones synthesize the beta' isoforms of the beta-tubulin protein family

We have analysed the isotubulin pattern of three astrocytic cell clones, derived from spontaneously established permanent cell cultures originating from 8-day postnatal mice cerebellar explants, in comparison with that of primary astroglial cultures from embryonic brain and cerebellum. These astrocytic clones, which may represent the different astroglial cell types of mouse cerebellum, did not produce the alpha- and beta-acidic isoforms, these being found only in cells of neuronal lineage. However, the three astrocytic clones, but not the primary astroglial cultures, did synthesize the beta'-tubulin isoforms; in addition quantitative analysis of the beta' proteins showed a positive correlation between the ability of the cells to extend processes and their synthesis of the beta' isoforms. These data suggest that the presence of beta'-tubulin is not specific for neuronal cells but may be related to the ability of cells from the nervous system to extend processes.

Induction of proliferation or transformation of neuroretina cells by the mil and myc viral oncogenes

The genome of the avian retrovirus MH2 contains, in addition to the v-myc oncogene shared with three other avian retroviruses (MC29, CMII and OK-10), a second cell-derived oncogene, v-mil (refs 1-3). Like the three other viruses, which contain only v-myc, MH2 induces mainly liver and kidney carcinomas in fowl and transforms fibroblasts and macrophages in vitro. However, MH2 and MC29 differ in their biological properties when assayed on cultures of chicken embryo neuroretina (NR) cells. Indeed, NR cells, which normally do not multiply in vitro, are induced to proliferate and become transformed upon infection with MH2, whereas infection with MC29 has no apparent effect on these cells. To analyse the functions of the two oncogenes of MH2, we isolated spontaneous and in vitro-constructed mutants of this virus and investigated their effects on NR cell multiplication and transformation. We report here that expression of v-mil is sufficient to induce NR cell proliferation, although it does not result in cell transformation. In addition, viruses expressing only the v-myc oncogene fail to induce any detectable change in NR cells. However, cooperation of the two oncogenes is required to achieve transformation of NR cells by MH2.

Glutamic acid decarboxylase activity is stimulated in quail retina neuronal cells transformed by Rous sarcoma virus and is regulated by pp60v-src

Rous sarcoma virus (RSV) stimulates in quail embryo neuro-retina (NR) cultures the specific activity of glutamic acid decarboxylase (GAD), the enzyme responsible for the synthesis of gamma-aminobutyric acid, a major inhibitory neurotransmitter in NR and in central nervous system. In quail embryo NR cultures transformed by ts NY-68, a thermodependent transformation-defective mutant of RSV, stimulation of GAD activity is regulated by pp60v-src, the product of the src gene of RSV. Fibroblasts and myoblasts have a very low GAD activity that is not stimulated after transformation by RSV. Neuronal clones, previously derived from ts NY-68-transformed established NR cell lines, have a high GAD activity which is regulated by pp60v-src, while other clones have a low GAD activity apparently not regulated by pp60v-src. These data indicate that pp60v-src selectively activates the expression of GAD in distinct neuronal cells of quail embryo NR cultures transformed by RSV. GAD activity is also stimulated in NR cells infected with viruses containing v-mil.

Quail neural crest cells transformed by Rous sarcoma virus can be established into differentiating permanent cell cultures

Quail neural crest cells derived from the truncal neural primordium, infected in vitro by Rous sarcoma virus (RSV) in January 1978, were induced to multiply and have been established into permanent cultures. These cultures contain cells that differentiate into melanocytes, neuron-like cells and flat cells. About 50% of these different cell types are tetanus-toxin positive. Electrophysiological studies have shown that some cells can generate action potentials similar to those reported in quail neural crest primary cultures. Taken together these data show that the RSV-transformed quail neural crest permanent cultures are composed of stem cells which can differentiate into cell types specific for neural crest.

Astrocytic cell clones derived from established cultures of 8-day postnatal mouse cerebella

Clonal permanent cell lines with astrocytic properties have been established from explant cultures of 8-day postnatal mouse cerebella after in vitro spontaneous transformation, i.e. without the addition of carcinogens or oncogenic viruses. The cell lines were derived in a multistage process. Slowly proliferating foci with several morphologies appeared 4 months after initiation of the cultures and became progressively enriched by cells with a homogeneous appearance. These cells could be established into permanent cell lines from which many clones were obtained. Some of these cloned cell lines bound anti-GFAP sera and therefore appeared to be astrocytic. According to their morphology, 3 separate types of these GFAP-positive clones could be distinguished. Type I and II cells had small somata; type I had several short processes, while type II had two processes, one of which was very thin and long (greater than 200 microns). Type III cells had large flat somata and no processes. The three types of clonal cell lines were labeled by monoclonal antibodies which bind to astrocytes in vivo. In particular, three monoclonal antibodies (BSP-3, M2 and M3) bound only to type II cells in a distinct pattern. Type I and II astrocytes are pseudodiploid and type III, heteroploid. The properties of these different clonal cell lines are very stable. We have thus obtained permanently established clonal cultures of mouse cerebellum astrocyte-like cells, which might be the in vitro counterparts of fibrous (type I), or velamentous (type III) astrocytes and of Golgi epithelial cells (type II).

Cells with neuronal properties in permanent cultures of quail embryo neuroretinas infected with Rous sarcoma virus

Neuroretina cells from 7-day quail embryos infected 'in vitro' with the mutant ts NY-68 of Rous sarcoma virus, were established into permanent cultures. An initial stage of cellular proliferation was followed by a period of minimal multiplication. After recovery from this crisis, cell proliferation resumed. About 30% of the cells had binding sites for tetanus toxin and the monoclonal antibody A2B5 which seem to be specific for neurons, and an ultrastructural study suggested the presence of neurons and Müller (astroglial) cells. The specific activity of glutamic acid decarboxylase, the enzyme responsible for the synthesis of the neurotransmitter gamma-aminobutyric acid was high (10-30 nmol CO2/h/mg of protein) and electrophysiology showed that some cells had 'active' membranes. After about 18 months in culture, approximately 20% of the cells were able to respond to electrical stimulation by producing action potentials which were inhibited by 10(-7) M tetrodotoxin. These electrophysiological properties are stable: they have been repeatedly found at regular intervals throughout a 20 months period. Furthermore, a clone in which all tested cells are excitable, has been derived from the mass culture. Quail embryo neuroretina cells with typical neuronal properties can thus be established into permanent cultures after infection with Rous sarcoma virus.

A glycine-enriched astrocytic cell clone derived from mouse cerebella transformed in vitro by simian virus-40

Measurements were made of the amino acid content of a cellular clone (K55) derived from mouse cerebellar cultures transformed in vitro by simian virus-40 (Alliot and Pessac, 1981) and that appears to be astroglial. Both the total amount of amino acids as well as the percentage of glycine in K55 cells were higher than in the mixed cultures from which they are derived. Further, glycine accumulates in the culture medium of K55 cells, but not in the medium of the parental mixed cell culture (C14), thereby suggesting that glycine is synthesized and released by K55 cells.

A neuronal clone derived from a Rous sarcoma virus-transformed quail embryo neuroretina established culture

Neuroretina (NR) is an evagination of the central nervous system (CNS) which is composed of photoreceptors, glial (Müller) cells and horizontal, bipolar, amacrine and ganglion neuronal cells. We describe here the usefulness of Rous sarcoma virus (RSV) in the establishment of a neuronal clone from quail embryo neuroretina. When primary cultures of chick and quail embryo neuroretina cells are transformed by RSV, neuronal markers such as ribbon synapses, choline acetyltransferase (CAT) and glutamic acid decarboxylase (GAD) specific activity are present. These RSV-transformed primary cultures can be established into permanent cell lines from which neuronal clones have been isolated. One of them, clone QNR/D, can generate tetrodotoxin(TTX)-inhibitable action potentials on electrical stimulation, has a high GAD activity and binds monoclonal antibodies raised against chick embryo neuroretina. The presence of these neuronal markers suggests that the QNR/D clone is derived from cells of the amacrine or ganglionic lineage. This is the first time that a neuronal cell clone of defined origin has been obtained from the CNS. The neuronal markers of the QNR/D clone are expressed at both the permissive and the non-permissive temperatures for transformation.

[Immunoreactive neuropeptides in cell lines derived from nervous and non nervous tissues]

Immunoreactive neuropeptides have been found in cells of permanent lines originating from nervous (cerebellum, retina) and non nervous (mesenchyme, kidney, bladder, colon, lung, melanocyte) tissues. The pattern of neuropeptides varied from one cell type to another, but all the cell lines studied contained immunoreactive alpha-endorphin and methionin-enkephalin.

Expression of neuronal markers in chick and quail embryo neuroretina cultures infected with Rous sarcoma virus

Cultures of neuroretina (NR) cells from 7-day chick and quail embryos were infected with ts NY-68, a thermosensitive mutant of Rous sarcoma virus (RSV) which transformed NR cells at 36 degrees C. The following differentiation markers for neurones were studied: tetanus toxin-binding sites at the cell surfaces, presence of synapses, and the specific activity of the enzymes choline acetyltransferase (CAT) and glutamic acid decarboxylase (GAD). Appearance of synapses and expression of CAT were similar in control and transformed cultures. Tetanus toxin-binding cells were observed in transformed primary cultures and also in quail NR subcultures. GAD-specific activity was markedly stimulated in chick and quail primary cultures transformed by ts NY-68 and further increased in subcultures of ts NY-68-transformed quail NR cells. Stimulation of GAD activity is controlled by the transforming (src) gene of RSV since it was not observed in cultures infected with RAV-1, a leukosis virus which lacks the src gene. These data show that infection of chick and quail NR cultures with RSV results in the transformation of cells with neuronal markers.

[Demonstration of neuropeptides in cultured neuroretinal cells of chick embryo]

Immunoreactive neuropeptides have been found in cultures of neuroretina cells obtained from 7 day Chick embryos. Methionine-enkephalin and thyroliberin (TRH) were present in a large number of neurons, while somatostatin, beta-melanotropin (MSH) and alpha-endorphin were occasionally seen in a few neurons. In cultures devoid of neurons and probably made of Müller (astroglial) cells, methionine-enkephalin, alpha-MSH and TRH were each detected in 60 to 80% of the cells. Immunoreactive methionine enkephalin was also localized in Müller cells in retina sections of 10 and 20 day old embryos.