BACKGROUND: Human Syncytin-1 is a placentally-expressed cell surface glycoprotein of retroviral origin. After interaction with ASCT2, its cellular receptor, Syncytin-1 triggers cell-cell fusion and formation of a multinuclear syncytiotrophoblast layer of the placenta. The ASCT2 receptor is a multi-spanning membrane protein containing a protruding extracellular part called region C, which has been suggested to be a retrovirus docking site. Precise identification of the interaction site between ASCT2 and Syncytin-1 is challenging due to the complex structure of ASCT2 protein and the background of endogenous ASCT2 gene in the mammalian genome. Chicken cells lack the endogenous background and, therefore, can be used to set up a system with surrogate expression of the ASCT2 receptor. RESULTS: We have established a retroviral heterologous chicken system for rapid and reliable assessment of ectopic human ASCT2 protein expression. Our dual-fluorescence system proved successful for large-scale screening of mutant ASCT2 proteins. Using this system, we demonstrated that progressive deletion of region C substantially decreased the amount of ASCT2 protein. In addition, we implemented quantitative assays to determine the interaction of ASCT2 with Syncytin-1 at multiple levels, which included binding of the soluble form of Syncytin-1 to ASCT2 on the cell surface and a luciferase-based assay to evaluate cell-cell fusions that were triggered by Syncytin-1. Finally, we restored the envelope function of Syncytin-1 in a replication-competent retrovirus and assessed the infection of chicken cells expressing human ASCT2 by chimeric Syncytin-1-enveloped virus. The results of the quantitative assays showed that deletion of the protruding region C did not abolish the interaction of ASCT2 with Syncytin-1. CONCLUSIONS: We present here a heterologous chicken system for effective assessment of the expression of transmembrane ASCT2 protein and its interaction with Syncytin-1. The system profits from the absence of endogenous ASCT2 background and implements the quantitative assays to determine the ASCT2-Syncytin-1 interaction at several levels. Using this system, we demonstrated that the protruding region C was essential for ASCT2 protein expression, but surprisingly, not for the interaction with Syncytin-1 glycoprotein.

• Klíčová slova
• ASCT2 (SLC1A5), Cell–cell fusion, Envelope glycoprotein, Envelope-receptor interaction, NanoLuc luciferase, Retroviral receptor, Syncytin-1,
• MeSH
• buněčné linie MeSH
• fibroblasty virologie   MeSH
• fluorescence MeSH
• genové produkty env genetika   metabolismus   MeSH
• konfokální mikroskopie MeSH
• kur domácí MeSH
• lidé MeSH
• placenta virologie   MeSH
• těhotenské proteiny genetika   metabolismus   MeSH
• těhotenství MeSH
• transportní systém ASC pro aminokyseliny genetika   metabolismus   MeSH
• vedlejší histokompatibilní antigeny genetika   metabolismus   MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• těhotenství MeSH
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• genové produkty env MeSH
• SLC1A5 protein, human MeSH Prohlížeč
• syncytin MeSH Prohlížeč
• těhotenské proteiny MeSH
• transportní systém ASC pro aminokyseliny MeSH
• vedlejší histokompatibilní antigeny MeSH

Individual groups of retroviruses and retroviral vectors differ in their integration site preference and interaction with the host genome. Hence, immediately after infection genome-wide distribution of integrated proviruses is non-random. During long-term in vitro or persistent in vivo infection, the genomic position and chromatin environment of the provirus affects its transcriptional activity. Thus, a selection of long-term stably expressed proviruses and elimination of proviruses, which have been gradually silenced by epigenetic mechanisms, helps in the identification of genomic compartments permissive for proviral transcription. We compare here the extent and time course of provirus silencing in single cell clones of the K562 human myeloid lymphoblastoma cell line that have been infected with retroviral reporter vectors derived from avian sarcoma/leukosis virus (ASLV), human immunodeficiency virus type 1 (HIV) and murine leukaemia virus (MLV). While MLV proviruses remain transcriptionally active, ASLV proviruses are prone to rapid silencing. The HIV provirus displays gradual silencing only after an extended time period in culture. The analysis of integration sites of long-term stably expressed proviruses shows a strong bias for some genomic features-especially integration close to the transcription start sites of active transcription units. Furthermore, complex analysis of histone modifications enriched at the site of integration points to the accumulation of proviruses of all three groups in gene regulatory segments, particularly close to the enhancer loci. We conclude that the proximity to active regulatory chromatin segments correlates with stable provirus expression in various retroviral species.

• Klíčová slova
• gene regulatory elements, genome-wide provirus distribution, provirus silencing, retrovirus integration,
• MeSH
• aktivace transkripce * MeSH
• Alpharetrovirus genetika   MeSH
• buněčné linie MeSH
• chromatin genetika   MeSH
• epigeneze genetická MeSH
• genetické vektory genetika   MeSH
• genový targeting MeSH
• HIV-1 genetika   MeSH
• integrace viru MeSH
• lidé MeSH
• myši MeSH
• plazmidy genetika   MeSH
• počátek transkripce MeSH
• proviry genetika   MeSH
• regulace exprese virových genů MeSH
• regulační oblasti nukleových kyselin * MeSH
• stabilita RNA MeSH
• umlčování genů MeSH
• virus myší leukemie genetika   MeSH
• zesilovače transkripce MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• myši MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• chromatin MeSH

Most retroviruses preferentially integrate into certain genomic locations and, as a result, their genome-wide integration patterns are non-random. We investigate the epigenetic landscape of integrated retroviral vectors and correlate it with the long-term stability of proviral transcription. Retroviral vectors derived from the avian sarcoma/leukosis virus expressing the GFP reporter were used to transduce the human myeloid lymphoblastoma cell line K562. Because of efficient silencing of avian retrovirus in mammalian cells, only ∼3% of established clones displayed stable proviral expression. We analyzed the vector integration sites in non-selected cells and in clones selected for the GFP expression. This selection led to overrepresentation of proviruses integrated in active transcription units, with particular accumulation in promoter-proximal areas. In parallel, we investigated the integration of vectors equipped with an anti-silencing CpG island core sequence. Such modification increased the frequency of stably expressing proviruses by one order. The modified vectors are also overrepresented in active transcription units, but stably expressed in distal parts of transcriptional units further away from promoters with marked accumulation in enhancers. These results suggest that integrated retroviruses subject to gradual epigenetic silencing during long-term cultivation. Among most genomic compartments, however, active promoters and enhancers protect the adjacent retroviruses from transcriptional silencing.

• MeSH
• Alpharetrovirus genetika   MeSH
• buněčné linie MeSH
• buňky K562 MeSH
• CpG ostrůvky genetika   MeSH
• epigeneze genetická MeSH
• genetická transkripce * MeSH
• genetické vektory genetika   MeSH
• integrace viru genetika   MeSH
• lidé MeSH
• promotorové oblasti (genetika) genetika   MeSH
• proviry genetika   MeSH
• umlčování genů MeSH
• zesilovače transkripce genetika   MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH

Porcine endogenous retroviruses (PERV) represent a major safety concern in pig-to-human xenotransplantation. To date, no PERV infection of a xenograft recipient has been recorded; however, PERVs are transmissible to human cells in vitro. Some recombinants of the A and C PERV subgroups are particularly efficient in infection and replication in human cells. Transcription of PERVs has been described in most pig cells, but their sequence and insertion polymorphism in the pig genome impede identification of transcriptionally active or silenced proviral copies. Furthermore, little is known about the epigenetic regulation of PERV transcription. Here, we report on the transcriptional suppression of PERV by DNA methylation in vitro and describe heavy methylation in the majority of PERV 5' long terminal repeats (LTR) in porcine tissues. In contrast, we have detected sparsely methylated or nonmethylated proviruses in the porcine PK15 cells, which express human cell-tropic PERVs. We also demonstrate the resistance of PERV DNA methylation to inhibitors of methylation and deacetylation. Finally, we show that the high permissiveness of various human cell lines to PERV infection coincides with the inability to efficiently silence the PERV proviruses by 5'LTR methylation. In conclusion, we suggest that DNA methylation is involved in PERV regulation, and that only a minor fraction of proviruses are responsible for the PERV RNA expression and porcine cell infectivity.

• MeSH
• DNA virů genetika   MeSH
• endogenní retroviry genetika   MeSH
• epigeneze genetická * MeSH
• koncové repetice genetika   MeSH
• kultivované buňky MeSH
• kvantitativní polymerázová řetězová reakce MeSH
• ledviny metabolismus   virologie   MeSH
• lidé MeSH
• messenger RNA genetika   MeSH
• metylace DNA * MeSH
• miniaturní prasata genetika   virologie   MeSH
• nemoci prasat genetika   přenos   virologie   MeSH
• polymerázová řetězová reakce s reverzní transkripcí MeSH
• prasata MeSH
• proviry genetika   MeSH
• replikace viru * MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• DNA virů MeSH
• messenger RNA MeSH

The autonomous transcription of integrated retroviruses strongly depends on genetic and epigenetic effects of the chromatin at the site of integration. These effects are mostly suppressive and proviral activity can be finally silenced by mechanisms, such as DNA methylation and histone modifications. To address the role of the integration site at the whole-genome-scale, we performed clonal analysis of provirus silencing with an avian leucosis/sarcoma virus-based reporter vector and correlated the transcriptional silencing with the epigenomic landscape of respective integrations. We demonstrate efficient provirus silencing in human HCT116 cell line, which is strongly but not absolutely dependent on the de novo DNA methyltransferase activity, particularly of Dnmt3b. Proviruses integrated close to the transcription start sites of active genes into the regions enriched in H3K4 trimethylation display long-term stability of expression and are resistant to the transcriptional silencing after over-expression of Dnmt3a or Dnmt3b. In contrast, proviruses in the intergenic regions tend to spontaneous transcriptional silencing even in Dnmt3a(-/-) Dnmt3b(-/-) cells. The silencing of proviruses within genes is accompanied with DNA methylation of long terminal repeats, whereas silencing in intergenic regions is DNA methylation-independent. These findings indicate that the epigenomic features of integration sites are crucial for their permissivity to the proviral expression.

• MeSH
• Alpharetrovirus genetika   MeSH
• DNA methyltransferasa 3A MeSH
• DNA-(cytosin-5-)methyltransferasa genetika   metabolismus   MeSH
• DNA-methyltransferasa 3B MeSH
• epigeneze genetická * MeSH
• genetická transkripce MeSH
• integrace viru * MeSH
• lidé MeSH
• metylace DNA * MeSH
• nádorové buněčné linie MeSH
• proviry genetika   MeSH
• umlčování genů * MeSH
• Check Tag
• lidé MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• DNA methyltransferasa 3A MeSH
• DNA-(cytosin-5-)methyltransferasa MeSH
• DNMT3A protein, human MeSH Prohlížeč

The past 15 years opened new avenues for retrovirus and retroelement research. Not surprisingly, they stemmed from essential knowledge collected in the past, which remains the ground of the present and therefore should be remembered. However, a short supplement of new break-through discoveries and ideas should be recollected. Using selected examples of recent works, I tried to extend and supplement my original article published in Folia Biologica (1996).

• Publikační typ
• časopisecké články MeSH