Multiple endogenous xenotropic and mink cell focus-forming murine leukemia virus-related transcripts are induced by polyclonal immune activators

Complete genome sequences of new xenotropic murine leukemia viruses from the senescence-accelerated mouse (SAM): molecular and phylogenetic analyses

Approximately 10% of the mouse genome is constituted by endogenous retroviruses (ERVs), and a number of mouse ERVs remain active. Many copies of endogenous murine leukemia viruses (MuLVs) are detected in the genomes of inbred mouse strains. Some of these MuLVs are transcriptionally active or produce infectious virus particles. Previously, we identified partial env sequences of new xenotropic MuLVs (X-MuLVs) from a senescence-accelerated mouse (SAM) strain. In the present study, we investigated and characterized the complete sequences of the X-MuLVs. The complete genomes and open reading frames (ORFs) of two X-MuLVs, designated xmlv15 and xmlv18 (accession nos. HQ154630 and HQ154631, respectively), were molecularly cloned from the genome of the SAM mice. We confirmed that the xmlv15 and xmlv18 sequences are distinct from all known MuLV genomes and are most similar to DG-75 MuLV. Moreover, we found that common strains of laboratory mice carry our newly identified xmlvs. Additionally, the expression levels of xmlv15-related sequences were much higher in C57BL and ICR mice than in the SAM strains without any stimulators. Our findings suggest that a specific group of endogenous MuLVs is constitutively expressed in the brain and that they may participate in normal functions and/or pathogenic conditions.

Lipopolysaccharide stress induces cell-type specific production of murine leukemia virus type-endogenous retroviral virions in primary lymphoid cells

Some murine-endogenous retroviruses, making up ∼10 % of the mouse genome, are induced during the course of experimental sepsis in which lipopolysaccharide (LPS), a pathogenic component of gram-negative bacteria, often plays a critical role. In this study, we investigated whether LPS stress induces the production of murine leukemia virus type-endogenous retrovirus (MuLV-ERV) virions in primary lymphoid cells. LPS treatment of cells (single-cell suspensions and sorted B- and T-cells) isolated from seven lymphoid organs of C57BL/6J mice resulted in a differential increase in the production of MuLV-ERV virions in most cells examined. Interestingly, among the 34 unique MuLV-ERV U3 sequences cloned from the viral genomic RNAs, the nuclear respiratory factor 1 (transcription factor) element was present only in the 20 U3 sequences that were derived from the LPS-induced MuLV-ERV U3 bands. Using the U3 sequences as a probe, 55 putative MuLV-ERV loci were mapped onto the C57BL/6J mouse genome and 15 of them retained full coding potential. Furthermore, one full-length recombinant MuLV-ERV originating from a locus on chromosome 13 was determined to be responsive to LPS stress. The findings from this study suggest that LPS stress differentially activates MuLV-ERV virion production in lymphoid organs in a cell type- and MuLV-ERV-specific manner. Further investigation is needed to define the role of MuLV-ERVs in the LPS signalling pathway(s) in general, as well as in the pathogenesis of sepsis.

Direct evidence for the expression of multiple endogenous retroviruses in the synovial compartment in rheumatoid arthritis

OBJECTIVE

Circumstantial evidence links retroviruses (RVs) with human autoimmune diseases. The aim of the present study was to obtain direct evidence of RV gene expression in rheumatoid arthritis (RA).

METHODS

Synovial samples were obtained from patients with RA, patients with osteoarthritis (OA), and normal control subjects. Reverse transcription-polymerase chain reaction (RT-PCR) was performed using synovial RNA and primers to conserved sequences in the polymerase (pol) genes of known RVs.

RESULTS

PCR products (n = 857) were cloned and sequenced. Multiple pol transcripts, many with open reading frames, were expressed in every sample. Sequences were aligned and classified into 6 families (F1-F6) that contained 33 groups of known and unknown endogenous RVs (ERVs), each distinguished by a specific, deduced peptide motif. The frequency of sequences in each family was similar between RA, OA, and normal synovial tissue, but differed significantly in RA synovial fluid cells. F1 sequences (undefined, but related to murine and primate type C RVs) were lower in frequency, F2 (ERV-9-related), F4 (HERV-K-related), and F6 (HERV-L-related) sequences were higher in frequency, and F3 (RTVL-H-related) sequences were not detected, in the RA synovial fluid cells compared with the RA synovial tissues.

CONCLUSION

Multiple ERVs are expressed in normal and diseased synovial compartments, but specific transcripts can be differentially expressed in RA.

Expression of endogenous retroviruses in blood mononuclear cells and brain tissue from multiple sclerosis patients

OBJECTIVES

To compare the expression of endogenous retroviruses in MS patients and controls.

MATERIAL AND METHODS

Peripheral blood mononuclear cells were obtained from 22 MS patients, a corresponding number of matched healthy donors and five patients with other central nervous system disease. Also brain specimens from MS patients and controls were obtained. Transcripts of various endogenous retroviruses in these samples were detected by RNA-PCR.

RESULTS

Several endogenous retroviral sequences were transcribed in peripheral blood mononuclear cells and brain tissue from MS patients as well as controls. A composite transcript of an endogenous retrovirus and a zinc finger sequence was more frequently found in healthy donors than in MS patients.

CONCLUSION

Some endogenous retroviruses are normally transcribed in white blood cells and brain tissue. The significance of those findings, which concerned the composite transcripts of the zinc finger sequence and its associated endogenous retrovirus is uncertain.

The potential roles of endogenous retroviruses in autoimmunity

Endogenous retroviruses (ERVs) are estimated to comprise up to 1% of human DNA. While the genome of many ERVs is interrupted by termination codons, deletions or frame shift mutations, some ERVs are transcriptionally active and recent studies reveal protein expression or particle formation by human ERVs. ERVs have been implicated as aetiological agents of autoimmune disease, because of their structural and sequence similarities to exogenous retroviruses associated with immune dysregulation and their tissue-specific or differentiation-dependent expression. In fact, retrovirus-like particles distinct from those of known exogenous retroviruses and immune responses to ERV proteins have been observed in autoimmune disease. Quantitatively or structurally aberrant expression of normally cryptic ERVs, induced by environmental or endogenous factors, could initiate autoimmunity through direct or indirect mechanisms. ERVs may lead to immune dysregulation as insertional mutagens or cis-regulatory elements of cellular genes involved in immune function. ERVs may also encode elements like tax in human T-lymphotrophic virus type I (HTLV-I) or tat in human immunodeficiency virus-I (HIV-I) that are capable of transactivating cellular genes. More directly, human ERV gene products themselves may be immunologically active, by analogy with the superantigen activity in the long terminal repeat (LTR) of mouse mammary tumour viruses (MMTV) and the non-specific immunosuppressive activity in mammalian type C retrovirus env protein. Alternatively, increased expression of an ERV protein, or expression of a novel ERV protein not expressed in the thymus during acquisition of immune tolerance, may lead to its perception as a neoantigen. Paraneoplastic syndromes raise the possibility that novel ERV-encoded epitopes expressed by a tumour elicit immunity to cross-reactive epitopes in normal tissues. Recombination events between different but related ERVs, to whose products the host is immunologically tolerant, may also generate new antigenic determinants. Frequently reported humoral immunity to exogenous retrovirus proteins in autoimmune disease could be elicited by cross-reactive ERV proteins. A review of the evidence implicating ERVs in immune dysfunction leads to the conclusion that direct molecular studies are likely to establish a pathogenic role for ERVs in autoimmune disease.

Glucocorticoid-mediated immunomodulation: hydrocortisone enhances immunosuppressive endogenous retroviral protein (p15E) expression in mouse immune cells

To define glucocorticoid (GC)-regulated genes contributing to the anti-inflammatory and immunosuppressive effects of GC, previous work from our laboratory revealed up-regulation of transcripts from endogenous type B mouse mammary tumour virus (Mtv) and type C murine leukaemia virus (Emv) loci by high dose GC treatment of P388D1 macrophage-like cells. This study demonstrates enhancement of expression from Mtv and Emv loci in P388D1 cells by more physiological hydrocortisone concentrations (1 microM), and shows direct transcriptional mode of regulation by blocking GC-mediated signal transduction at different levels. Furthermore, we found up-regulation of Emv mRNA steady-state levels in murine lymphoid lineage cells (T-like EL4 and BW5147 cells; B-like X63 cells) upon GC treatment. The Emv transcripts shown by us to be GC-up-regulated encode for the transmembrane envelope protein TM/p15E which is highly conserved in several retroviruses. TM/p15E and the p15E-like products found in humans exert immunosuppressive effects in different test systems. Thus, our findings raise the possibility that immunomodulation by GC might be mediated in part by enhanced expression of p15E(-like) products.

Expression of endogenous retroviruses in blood mononuclear cells and brain tissue from multiple sclerosis patients

The aim of the present study was to examine whether there is an abnormal expression of certain endogenous retroviruses in MS patients. For this purpose samples of peripheral blood mononuclear cells were obtained from 22 MS patients, a corresponding number of age and sex-matched healthy donors and five patients with other diseases affecting the central nervous system. In addition, brain specimens of macroscopic normal white and gray matter from four MS patients and a similar number of controls were included in the study. Using an enzymatic amplification technique, we found expression of the endogenous retroviral sequences, HRES-1, HERV-K10 and ERV3 in most samples of peripheral blood mononuclear cells from MS patients and controls without obvious differences between these two groups. In contrast, composite transcripts of ERV3 and a zinc finger sequence were more frequently detected in healthy donors than in MS patients. At present, the possible significance of this is uncertain. The retroviral element 4-1 was not transcribed or only transcribed at a very low level in peripheral blood cells of controls and MS patients. Transcripts of various endogenous retroviruses were also detected in the brain samples, but a different pattern was not apparent in the MS group as compared with controls. Aspects concerning a possible association between endogenous retroviruses and autoimmunity are considered.

Antisense oligonucleotide complementary to endogenous retroviral MCF env gene inhibits both BFU-E and CFU-S colony formation in mice

A possible biologic activity of endogenously expressed env sequence of retroviral mink cell focus-forming virus (MCF) genome for hematopoietic colony formation was studied in mice. Antisense 20-mer complementary to MCF env sequence was used to detect the result of blockage of this gene translation on the potency of marrow cells to form colonies of erythroid (BFU-E), myeloid granulocyte-macrophage (CFU-GM), and stem cell (day 11 CFU-S) hematopoietic compartments. A large relative decrease in BFU-E number was found in bone marrow cell cultures preincubated with antisense oligonucleotide during 4 h, whereas CFU-GM colonies remained unaffected. A marked reduction of CFU-S colony formation was also registered under antisense oligomer influence. Following a decreased proliferation of erythroid progenitors, we suggest the mechanism by which antisense oligonucleotide could cause the loss of colony formation. Taken together, these data allow to propose that the expression of this gene is naturally significant for hematopoietic progenitor activity exerting some property of env gene products to regulate the growth of erythroid and multilineage hematopoietic precursors.

Endogenous retroviral gp70 genes of the murine lymphoma L5178Y: analysis of restriction fragment polymorphism upon induction of drug-mediated immunogenicity

Highly immunogenic ("xenogenized") tumor variants appear after treatment of murine lymphoma L5178Y with the triazene derivative DTIC, this phenomenon being associated with the appearance of structurally abnormal gp70 env proteins in the cell variants. In the present study, we have isolated and sequenced several PCR-amplified gp70 cDNA genes from L5178Y cells. One of the resulting clones was used as a probe in Southern and Northern analysis of parental and xenogenized cells. The results indicated that xenogenization of tumor cells is associated with changes in retroviral env sequences detectable at the genomic level.

Do endogenous retroviruses have etiological implications in inflammatory and degenerative nervous system diseases?

Vertebrates carry large numbers of endogenous retroviruses (ERVs) and related sequences in their genomes. These retroviral elements are inherited as Mendelian traits. Generally, ERVs are defective without the ability of being expressed as viral particles. However, ERV sequences often have a potential for expression of at least some proteins. So far, the possible biological significance of ERVs is not clear. Nonetheless, there are observations suggesting a connection between ERVs and various diseases. This is the case with murine lupus and a spinal cord disease of certain mouse strains. In the present review, we discuss possible mechanisms by which ERVs could contribute to the development of human degenerative and inflammatory nervous system diseases, including direct effects on nervous system cells and immune cells. Interactions between ERVs and infectious viruses are also discussed. Finally, we review a possible retroviral etiology of multiple sclerosis.

Carbon tetrachloride induction of rapid changes in liver nuclear protein factors capable of sequence-specific binding to regulatory elements in the long terminal repeat of polytropic-class endogenous murine leukemia virus-related proviruses

Treatment of mice with hepatic carcinogens, including CCl4, has been shown to rapidly enhance the transcription of endogenous murine leukemia virus-related proviral sequences in the liver. To understand the mechanism for this transcriptional stimulation, we used nuclear protein preparations from mouse livers to perform DNase I protection analyses and identified nuclear protein binding on approximately 20 individual sequences within the regulatory regions of the long terminal repeat (LTR) of a polytropic-class endogenous provirus clone. From 3 to 144 h after treatment with CCl4, the livers of FVB/N mice were analyzed for specific nuclear protein binding to the LTR DNA. Three to nine hours after CCl4 treatment, decreased protection was seen at potential regulatory cis-elements throughout the LTR, including specific sites within the putative negative regulatory element (located 5' of the consensus enhancer sequences) and the 3' terminal portion of the polytropic class-specific enhancer-like inserted sequence element and around the CCAA(C/T) box in the promoter region. In addition, by 3-6 h after treatment, a transient increase in protection activity for the transcription initiation site occurred. The loss of cis-element protection expanded to other binding sites and became most marked by 48 h after treatment. As the regenerating liver recovered, the nuclear protein binding activities for these LTR sequences also recovered, but protection at the TATAA and transcription initiation sites remained deprotected at 144 h after treatment. Nuclear protein protection of other sites, particularly in the conserved LTR enhancer sequences, was minimally affected by CCl4 treatment. Three nuclear protein binding sites that showed rapid CCl4-induced kinetic changes were homologous to the consensus sequence for the binding of the transcription factor families MEF-2, HNF-1, and C/EBP. The complex kinetic changes in factors that may contribute to the rapid and transient induction of endogenous retroviral gene expression in the liver after CCl4 exposure are discussed.

Expression of antigen reactive with a monoclonal antibody to HTLV-1 P19 in salivary glands in Sjögren's syndrome

To examine the possible involvement of retroviruses in Sjögren's syndrome (SS), labial salivary gland sections from 99 individuals were probed with three MoAbs to core (gag) proteins of human T cell leukaemia virus-1 (HTLV-1) and two MoAbs to HIV-1. Sections from 31% of 39 patients with primary SS (pSS) contained an epithelial cytoplasmic protein reactive with a MoAb (197) to the p19 group specific antigen (gag) of HTLV-1. The antigen was also detected in samples from 24% of 17 patients with rheumatoid arthritis (RA) and SS, 21% of 14 patients with sicca symptoms and 12.5% of 16 patients with other connective tissue diseases. It was not found in the salivary glands of 13 normal controls. A second MoAb to p19 gag, a MoAb to the p24 gag of HTLV-1 and MoAbs to HIV-1 p17 and p24 gags gave negative reactions. Serum antibodies to HTLV-1 were negative, confirming that the antigen was not part of HTLV-1. The antigen showed properties consistent with an endogenous retrovirus in that it was absent in healthy tissues or resting cells but inducible by stimulation with phytohaemagglutinin (PHA) or interferon-gamma (IFN-gamma). It appeared to be distinct from the endogenous retroviral sequence HRES-1. These data suggest the presence of an endogenous retrovirus in salivary gland epithelium which could contribute to the chronic inflammation of SS.

Activation of endogenous retroviral transcription in SV40-transformed mouse cells

We have previously isolated a number of cDNA clones that correspond to mRNAs present at higher levels in SV40-transformed cells than in the untransformed parental cells (Scott, M.R.D., Westphal, K.-H. and Rigby, P.W.J. (1983) Cell 34, 557-567). We have now determined the nucleotide sequence of the archetypal Set 2 clone, pAG59, and can thus identify it as corresponding to the env gene of the endogenous, ecotropic C-type retrovirus of Balb/c mice, Emv-1. We have shown that in the subset of SV40-transformed cells that express the provirus both of the proteins encoded by env, gp70 and p15E, are synthesised and that the former is displayed on the cell surface. We discuss the significance of these observations for the biology of SV40 transformation.

Virological events leading to spontaneous AKR thymomas

The spontaneous leukemias of AKR mice are caused by mink cell focus-forming (MCF) viruses. These viruses are generated by recombination between several endogenous murine retroviruses. The virological events leading to the generation of the leukemogenic agent were investigated by using an oligonucleotide specific for the U3 region of the leukemogenic virus and env-reactive oligonucleotide probes specific for the different classes of endogenous murine leukemia virus. It was shown that (i) the leukemogenic MCF virus is formed by recombination between at least three different endogenous sequences; (ii) the U3 donor for the leukemogenic virus is the inducible xenotropic virus Bxv-1; (iii) all spontaneous tumors contain viruses with duplicated enhancer regions in their long terminal repeats; (iv) enhancer duplication is a somatic event, since Bxv-1 contains only one copy; (v) the first recombinant virus detectable in mass populations of thymocytes by Southern hybridization analysis contains all structural features of the ultimate leukemogenic virus; and (vi) the multiple novel viruses in a given tumor represent progeny of the same unique recombination events. On the basis of these results, an analysis of the virological events leading to AKR thymomas is presented.

Uptake of oligodeoxyribonucleotides by lymphoid cells is heterogeneous and inducible

Oligonucleotide uptake was studied in cultured murine spleen and lymph node cells using internally radiolabeled and fluorescein-5-isothiocyanate (FITC)-labeled oligonucleotides. Lymphoid subpopulations were distinguished by flow cytometry and staining with antibodies to cell-surface molecules. Approximately 5% of fresh lymphoid cells take up substantial amounts of oligonucleotide. The percentage of B cells that take up oligonucleotide increased fivefold if cells were cultured for at least 24 hr prior to incubation with labeled oligonucleotides, and increased 10-fold if cells were precultured for 48 hr. T-cell uptake changed very little in culture. Cultured CD4+ and CD8+ T cells had similar oligonucleotide uptake that was less than one-third of that in cultured B cells, but CD4-CD8- T cells had a higher percentage of cells taking up oligonucleotide than did B cells. T- or B-cell mitogens caused markedly increased oligonucleotide uptake in T or B cells, respectively. Oligonucleotide uptake could be inhibited only partially with competitor DNA. To distinguish between cell membrane-bound and intracellular oligonucleotide, cells were washed in acid glycine buffer (which removes most surface oligonucleotide). This demonstrated that most of the oligonucleotide was intracellular. We conclude that oligonucleotide uptake is quite heterogeneous among cultured cells, and that this uptake is inducible by mitogens. These data may be important for the design and interpretation of in vitro experiments, and for the planning of in vivo therapy with antisense oligonucleotides.

Origin of pathogenic determinants of recombinant murine leukemia viruses: analysis of Bxv-1-related xenotropic viruses from CWD mice

The acquisition of U3 region sequences derived from the endogenous xenotropic provirus Bxv-1 appears to be an important step in the generation of leukemogenic recombinant viruses in AKR, HRS, C58, and some CWD mice. We report here that each of three CWD lymphomas produced infectious xenotropic murine leukemia virus related to Bxv-1. In Southern blot experiments, these proviruses hybridized to probes that were specific for the xenotropic envelope and Bxv-1 U3 region sequences. Nucleotide sequence analysis of a cloned CWD xenotropic provirus, CWM-S-5X, revealed that the envelope gene was closely related to but distinct from those of other known xenotropic viruses. In addition, the U3 region of CWM-S-5X contained a viral enhancer sequence that was identical to that found in MCF 247, a recombinant AKR virus that is thought to contain the Bxv-1 enhancer. Finally, restriction enzyme sites in the CWM-S-5X provirus were analogous to those reported within Bxv-1. These results establish that the virus progeny of Bxv-1 have the potential to donate pathogenic enhancer sequences to recombinant polytropic murine leukemia viruses. Interestingly, the three CWD polytropic viruses that were isolated from the same tumor cells that produced the Bxv-1-like viruses had not incorporated Bxv-1 sequences into the U3 region.

Analysis of thymic endogenous retroviral expression in murine lupus. Genetic and immune studies

Inbred mouse genomes contain two subclasses of proviruses related to mink cell focus-forming (MCF) retroviruses: polytropic (Pmv), and modified polytropic (Mpmv). To determine whether one of these subclasses is associated with murine lupus, oligonucleotide probes specific for Pmv or Mpmv sequences were used in Northern analyses. Thymus 8.4 kb Mpmv RNA was expressed in five of five lupus-prone strains and crosses and this expression was not affected by genes that retard or accelerate development of lupus. Two of four leukemia-prone strains expressed low levels of such thymic transcripts, but none of 11 control strains did. 8.4 kb Mpmv RNA expression was not induced in thymuses of control mice by the lpr/lpr or gld/gld genotypes (which cause polyclonal immune activation) nor by treatment with mitogens. In contrast to Mpmv, thymic 8.4 kb Pmv expression was poorly associated with autoimmunity: it was easily detected in nearly all strains, and was increased by polyclonal activation in control mice. These studies indicate that the organ-specific thymic 8.4 kb Mpmv expression (a) is characteristic of several genetic backgrounds which predispose to murine lupus, (b) precedes and does not correlate with disease development, (c) is not due to polyclonal activation, and (d) is regulated independently of 8.4 kb Pmv expression.

Preferential nuclear compartmentalization of endogenous mink cell focus-forming-related retroviral transcripts

Endogenous mink cell focus-forming (MCF)-like retroviral sequences in the murine genome are stable, inherited sequences analogous to other chromosomal genes. As such, it is thought that they are transcribed and translated in a manner analogous to other genes. However, when the SL12.4 CD4-, CD8- thymoma cell line was studied for nuclear/cytoplasmic distribution of endogenous MCF-related transcripts, there was a nuclear predominance. The great majority of full-length 8.4-kb endogenous MCF-related transcripts were nuclear. Even the smaller, spliced 3.0-kb transcripts were at least as prominent in the nucleus as the cytoplasm, whereas cellular RNA was 80% cytoplasmic and other cellular transcripts were represented in the cytoplasm to a much greater extent than the nucleus. Size cannot fully account for the nuclear presence of MCF-related endogenous transcripts, because the 3.0-kb MCF transcripts occurred in the nucleus to a much greater relative extent than 3.8-kb c-myb transcripts. These studies point to retroviral-like structures of these transcripts as influencing their intracellular compartmentalization.

Alignment of U3 region sequences of mammalian type C viruses: identification of highly conserved motifs and implications for enhancer design

We aligned published sequences for the U3 region of 35 type C mammalian retroviruses. The alignment reveals that certain sequence motifs within the U3 region are strikingly conserved. A number of these motifs correspond to previously identified sites. In particular, we found that the enhancer region of most of the viruses examined contains a binding site for leukemia virus factor b, a viral corelike element, the consensus motif for nuclear factor 1, and the glucocorticoid response element. Most viruses containing more than one copy of enhancer sequences include these binding sites in both copies of the repeat. We consider this set of binding sites to constitute a framework for the enhancers of this set of viruses. Other highly conserved motifs in the U3 region include the retrovirus inverted repeat sequence, a negative regulatory element, and the CCAAT and TATA boxes. In addition, we identified two novel motifs in the promoter region that were exceptionally highly conserved but have not been previously described.

Truncated gag products encoded by Gv-1-responsive endogenous retrovirus loci

The conversion of endogenous or exogenous murine retroviruses to a leukemogenic phenotype involves recombination with retroviral sequences present in host genomic DNA. In the 129 Gix+ inbred strain, these endogenous sequences are replication defective but still express retroviral proteins under the apparent transcriptional control of the Gv-1 regulatory locus. To study the protein-coding potential of Gv-1-regulated endogenous retroviral loci, we used oligonucleotide probes directed to env deletion breakpoints identified in previously characterized cDNA clones. Four endogenous retroviral loci were isolated from a library of 129 Gix+ genomic DNA with these probes. Three loci cloned with the env deletion probe del env-1 had virtually identical proviral inserts by restriction analysis. A unique locus was identified and cloned with the del env-2 probe, which must therefore represent a Gv-1-responsive element. Restriction enzyme and nucleotide sequence analyses indicated that the del env-1 and del env-2 loci represented members of the polytropic and modified polytropic classes of endogenous retrovirus, respectively. Despite this divergence, members of both classes contained identical deletions of 19 nucleotides within p30gag and of 1,474 nucleotides from p10gag into the reverse transcriptase-coding region of pol, suggesting that a recombination event had occurred between these proviral sequences prior to insertion within the genome. The del env-1 and del env-2 loci retained coding capacity for truncated gag polyproteins, confirmed by in vitro translation and immunoprecipitation of the protein products. Nucleotide sequence comparison of the untranslated leader (L) regions of the del env-1 and del env-2 loci to a replication-competent ecotropic virus indicated regions that might be important to dispersion of these endogenous retroviral elements throughout the host genome.

Negative control region at the 5' end of murine leukemia virus long terminal repeats

Using in vitro protein binding and in vivo functional studies, we have identified novel regulatory sequences near the 5' end of murine leukemia virus (MuLV) long terminal repeats (LTRs). These sequences are highly conserved in all MuLV LTRs as well as in feline leukemia virus and gibbon ape leukemia virus LTRs. In this upstream conserved region (UCR), gel retardation assays detected two overlapping but distinct binding sites (UCR-U and UCR-L) for nuclear proteins (UCRF-U and UCRF-L). Three lines of evidence suggest a negative regulatory role for the UCR in viral transcription: (i) an inverse correlation was found between MuLV transcripts and nuclear proteins binding the UCR in the spleens of five different mouse strains; (ii) in vivo treatment of NFS mice with lipopolysaccharide resulted in the induction of splenic viral transcripts and the concomitant disappearance of UCR-binding proteins; and (iii) in mouse L cells transfected with an MuLV LTR linked to the chloramphenicol acetyltransferase (CAT) gene, cotransfected UCR oligonucleotides increased CAT expression, presumably by competing for inhibitory trans-acting factors.

Expression of an endogenous retroviral transcript is associated with murine lupus

We have been investigating whether murine lupus is associated with endogenous type C retroviral expression. We used Northern blot analyses and oligonucleotide probes, which are able to distinguish the envelope genes of the xenotropic and mink cell focus-forming (MCF) classes of type C retroviruses. Xenotropic expression in the spleen varied markedly among inbred mouse strains; although all strains expressed a 1.8-kb transcript, only one-half expressed one or more larger transcripts (8.4, 7.2, and/or 3.0 kb). Autoimmune disease did not correlate with expression of any of the xenotropic transcripts. Xenotropic and MCF transcripts were expressed independently among the mouse strains studied. Splenic RNA from all strains contained 7.2-, 3.0-, and 1.8-kb MCF transcripts. Some strains also expressed 8.4-kb MCF splenic RNA. There was a strong association between murine lupus and expression of 8.4-kb MCF transcripts: 6 of 6 lupus-prone strains, but only 2 of 11 nonautoimmune strains, had detectable 8.4-kb MCF RNA. The xid and Yaa mutations had minimal effects on expression of 8.4-kb MCF-related transcripts, despite their major and opposite effects on disease. Moreover, New Zealand black mice highly expressed this RNA from day 1 of life, before disease development. The data suggest that expression of 8.4-kb MCF endogenous retroviral transcripts is a primary feature of murine lupus and is not secondary to disease expression.

Negative control region at the 5' end of murine leukemia virus long terminal repeats

Using in vitro protein binding and in vivo functional studies, we have identified novel regulatory sequences near the 5' end of murine leukemia virus (MuLV) long terminal repeats (LTRs). These sequences are highly conserved in all MuLV LTRs as well as in feline leukemia virus and gibbon ape leukemia virus LTRs. In this upstream conserved region (UCR), gel retardation assays detected two overlapping but distinct binding sites (UCR-U and UCR-L) for nuclear proteins (UCRF-U and UCRF-L). Three lines of evidence suggest a negative regulatory role for the UCR in viral transcription: (i) an inverse correlation was found between MuLV transcripts and nuclear proteins binding the UCR in the spleens of five different mouse strains; (ii) in vivo treatment of NFS mice with lipopolysaccharide resulted in the induction of splenic viral transcripts and the concomitant disappearance of UCR-binding proteins; and (iii) in mouse L cells transfected with an MuLV LTR linked to the chloramphenicol acetyltransferase (CAT) gene, cotransfected UCR oligonucleotides increased CAT expression, presumably by competing for inhibitory trans-acting factors.