Rozprawy doktorskie na temat „Retrovirus Induced Cell-Cell Fusion”

Oncolytic measles virus is a promising cancer therapeutic in clinical trials which possesses multiple characteristics that are advantageous over traditional therapies. Currently, clinical oncolytic measles virus vectors are unmodified or express reporter transgenes that benefit its therapeutic efficacy. The next phase in its development will see genetically engineered vectors encoding transgenes that enhance its antineoplastic effects. To this end, preclinical research has focused on studying novel transgenes which favour viral replication, cytotoxicity, and the anti-cancer immune response. We sought to encode artificial micoRNAs targeting RIG-I as a strategy to interfere with innate immunity. Silencing RIG-I with multiple siRNAs yielded one which promotes measles virus syncytia formation through a mechanism that appears to be independent of RIG-I. The mechanism caused by the siRNA leads to enhanced measles virus cell-cell fusion and has peculiar characteristics which are not fully understood.

Human immunodeficiency virus type 1 (HIV-1) is a human retrovirus of the lentivirus subgroup which primarily infects T cells and macrophages, and causes acquired immune deficiency syndrome (AIDS). Since its emergence in the early 1980s, HIV-1 has caused a global pandemic which is still responsible for over one million deaths per year, primarily in sub-Saharan Africa. HIV-1 has been the subject of intense study for over three decades, which has resulted not only in major advances in cell biology, but also in numerous drug treatments that effectively control the infection. However, cessation of treatment always results in reemergence of the infection due to the ability of HIV-1 (and other lentiviruses) to establish a persistent quiescent infection known as latency. The elimination of latently-infected cells is the primary goal of current research towards a cure for HIV-1, alongside efforts to develop vaccines, which have thus far been fruitless. The spread of HIV-1 to susceptible target cells (which express the receptor CD4 and a co-receptor; CXCR4 or CCR5) can take place when antigen-presenting cells, such as dendritic cells, capture virus particles and then pass them on to target cells, without themselves becoming infected. Alternatively, productively infected T cells or macrophages can spread HIV-1 either by shedding virus particles to the milieu, which are then stochastically acquired by target cells, or through transient contacts between infected and uninfected cells known as virological synapses (VSs). VS-mediated cell-to-cell transmission is thought to be highly efficient due to the release of virus directly onto (or very near to) a target cell, and some evidence suggests that the VS is a privileged site which allows the virus to evade neutralizing antibodies and drugs. However, and most importantly, it is of central interest to us because the same transient cell adhesions that facilitate virus transfer can also result in the fusion of the two cells to form a syncytium, due to the presence of the viral fusogen Env and its receptor and co-receptor on either side of the VS. While T cell syncytia can be found in vivo, they remain small, and it appears that the majority of VSs resolve without fusion. The regulation of HIV-1-induced cell-cell fusion and the fate of those syncytia are the focus of the work presented here. A family of host transmembrane proteins, the tetraspanins, which regulate cell-cell fusion in other contexts (e.g. the fusion of myoblasts to form and maintain myotubes), were found to inhibit HIV-1-induced cell-cell fusion. Our investigations have further characterized this regulation, concluding that tetraspanins allow cells to reach the fusion intermediate known as hemifusion before their ability to repress fusion takes effect. In parallel, because syncytia are nevertheless found both in infected individuals and in a humanized mouse model for HIV-1, we also became interested in whether small T cell-based syncytia were able to participate in HIV-1 spread by transmitting virus to target cells. Using a simple three dimensional in vitro culture system which closely recapitulates those in situ observations, we found that small syncytia can contact target cells and transmit virus without fusing with them. Overall, these studies further our understanding of HIV-1-induced syncytia and reveal a previously unrecognized role for these entities as active participants in HIV-1 spread.

Respiratory syncytial virus (RSV) infections are the major contributor to acute lower respiratory syndrome in newborns. Infections generally result in hospitalization and sometimes in death. A vaccine is not available yet, despite decades of research. Vaccine development is hampered in consequence of a failed vaccine trial in the 1960s which entailed fatal outcomes. An alternative to direct vaccination of children is maternal vaccination for passive immunization of babies before birth. RSV infects every person repeatedly throughout life which implies that pregnant woman carry RSV-directed memory B cell (MBC) repertoires. A successful maternal vaccine would therefore elicit high titer RSV-neutralizing IgGs by reactivation of pre-existing MBCs which would protect newborns during the first months of life. RSV has two neutralizing antigens, of which the fusion protein (RSV F) is the most promising vaccine candidate. RSV F mediates the fusion process of viral and cellular membranes, wherefore it exists in a pre-fusion (pre F) or post-fusion (post F) conformation. Even though there is more interest in the pre F conformation as immunogen, the post F conformation may be equally considered as successful vaccine antigen. The pre F conformation is very metastable and readily switches to the highly stable post F conformation, which implies that a post F-based vaccine would be more cost-efficient to produce. More importantly, several neutralizing epitopes are preserved on the post F conformation and a substantial amount of RSV F-directed MBCs induced by natural infection are actually pre/post F cross-reactive. Since most of the pre/post F cross-reactive MBCs were previously shown to have higher affinities for the post F conformation, a post F-based vaccine may be ideal for reactivation and clonal expansion of MBCs which express neutralizing B cell receptors (BCRs). Every antigen, also RSV post F protein, has its own signatures within BCR repertoires because of preferential selection of BCR characteristics for B cell clonal expansions. In order to understand how RSV F protein shapes BCR repertoires, RSV F-directed BCRs were isolated from a healthy blood donor and three vaccinees who received an RSV post F vaccine. BCR repertoire analysis confirmed the assumption that the pre and post F protein have their own signatures within RSV F-directed BCR repertoires. The different characteristics indicated longer affinity maturation of pre F-reactive MBCs. Furthermore, estimation of clonal relatedness between the pre and post F-binding BCR repertoires from the healthy donor provided indications that a substantial number of the isolated BCRs are actually pre/post F cross-reactive, which confirmed a previous study. Analysis of the BCR repertoires isolated from the RSV post F-vaccinees showed that the vaccine induced a biased MBC response with preferential BCR characteristics. There were several implications that the post F-vaccine expanded primarily pre/post F cross-reactive MBCs. In contrast to the high variability of pre/post F cross-reactive BCRs induced by natural infection, the vaccine-induced MBC response indicated a skewed selection of VH4 gene family-encoded BCRs for clonal expansion and affinity maturation. More importantly, estimation of clonal relatedness revealed convergent MBC responses between the three analyzed subjects, while several MBC lineages shared stereotypic characteristics with pre F-binding BCRs or RSV-neutralizing antibodies. Some of the supposedly pre/post F cross-binding or neutralizing BCR sequences were expressed as mAbs and functionally characterized. RSV pre/post F cross-binding and neutralization activities could be demonstrated for all of the expressed mAbs. This project demonstrated the potential of ‘clonal’ grouping as novel reverse approach to screen BCR repertoires for functional antibodies.

The cells from the inner cell mass (ICM) of an early embryo have the potential to differentiate into all the different cell types present in an adult organism. Cells from the ICM can be isolated and cultured in vitro, becoming embryonic stem cells (ESCs). ESCs have several properties that make them unique: they are unspecialized, can self-renew indefinitely in culture, and given the appropriate cues can differentiate into cells from all three germ layers (ecto-, meso-, and endoderm), including the germline, both in vivo and in vitro. Induced pluripotent stem cells (iPSCs) can be generated from adult, terminally differentiated somatic cells by transient exogenous expression of four transcription factors (Oct4, Sox2, Klf4, and cMyc; OSKM) present normally in ESCs. It has been shown that iPSCs are equivalent to ESCs in terms of morphology, gene expression, epigenetic signatures, in vitro proliferation capacity, and in vitro and in vivo differentiation potential. However, unlike ESCs, iPSCs can be obtained from a specific individual without the need for embryos. This makes them a promising source of pluripotent cells for regenerative medicine, tissue engineering, drug discovery, and disease modelling; additionally, in livestock species such as the bovine, they also have applications in genetic selection, production of transgenic animals for agricultural and biomedical purposes, and species conservancy. Nevertheless, ESC and iPSC lines that meet all pluripotency criteria have, to date, only been successfully produced in mice, rats, humans, and non-human primates. In the first part of this dissertation, we attempted reprogramming of three types of bovine somatic cells: fetal fibroblasts (bFFs), adult fibroblasts (bAFs), and bone marrow-derived mesenchymal stem cells (bMSCs), using six different culture conditions adapted from recent work in mice and humans. Using basic mouse reprogramming conditions, we did not succeed in inducing formation of ESC-like colonies in bovine somatic cells. The combination of 2i/LIF plus ALK5 inhibitor II and ascorbic acid, induced formation of colony-like structures with flat morphology, that occasionally produced trophoblast-like structures. These trophoblast-like vesicles did not appear when an inhibitor of Rho-associated, coiled-coil containing protein kinase 1 (ROCK) was included in the medium. We screened for expression of exogenous OSKM vector with RT-PCR and found upregulation of OSKM vector 24h after Dox was added to the medium; however, expression was sharply decreased on day 2 after Dox induction, and was not detectable after day 3. In a separate experiment, we induced reprogramming of bFF and bAFs using medium supplemented with 50% of medium conditioned by co-culture with the bovine trophoblast CT1 line. These cells expressed both OCT4 and the OSKM vector 24h after Dox induction. However, similar to our previous observations, both markers decreased expression until no signal was detected after day 3. In summary, we were unable to produce fully reprogrammed bovine iPSCs using mouse and human protocols, and the exact cause of our lack of success is unclear. It is possible that a different method of transgene expression could play a role in reprogramming. However, these ideas would be driven by a rather empirical reasoning, extrapolating findings from other species, and not contributing in our understanding of the particular differences of pluripotecy in ungulates. Our inability to produce bovine iPSCs, combined with the only partial reprogramming observed by others, justifies the need for in depth study of bovine pluripotency mechanisms, before meaningful attempts to reprogram bovine somatic cells to plutipotency are made. Therefore, we focused on getting a better understanding of bovine nuclear reprogramming. This would allow us to rationally target the specific requirements of potential bovine pluripotent cells. Cell fusion is a process that involves fusion of the membrane of two or more cells to form a multinucleated cell. Fusion of a somatic cell to an ESC is known to induce expression of pluripotency markers in the somatic nucleus. In the second part of this dissertation, we hypothesized that fusion of bFFs to mouse ESCs (mESCs) would induce expression of pluripotency markers in the bFF nucleus. We first optimized a cell fusion protocol based on the use of polyethylene glycol (PEG), and obtained up to 11.02% of multinucleated cells in bFFs. Next, we established a method to specifically select for multinucleated cells originated from the fusion of mESCs with bFFs (heterokaryons), using indirect immunofluorescence. With this in place, flow cytometry was used to select 200 heterokaryons which were further analyzed using RNA-seq. We found changes in bovine gene expression patterns between bFFs and heterokaryons obtained 24h after fusion. Focusing on the bovine transcriptome, heterokaryons presented upregulation of early pluripotency markers OCT4 and KLF4, as well as hypoxia response genes, contrasted with downregulation of cell cycle inhibitors such as SST. The cytokine IL6, known to increase survival of early embryos in vitro, was upregulated in heterokaryons, although its role and mechanism of action is still unclear. This indicates that the heterokaryon cell fusion model recapitulates several of the events of early reprogramming, and can therefore be used for further study of pluripotency in the bovine. The cell fusion model presented here can be used as a tool to characterize early changes in bovine somatic nuclear reprogramming, and to study the effect of different reprogramming conditions on the bovine transcriptome.
Ph. D.

The c-Jun NH2-terminal kinase (JNK) group of kinases include ten members that are created by alternative splicing of transcripts derived from Jnk1, Jnk2 and Jnk3 genes. The JNK1 and JNK2 protein kinases are ubiquitously expressed while JNK3 is expressed in a limited number of tissues. The JNK signaling pathway is implicated in multiple physiological processes including cell transformation. There is growing evidence that JNK signaling is involved in oncogenesis. Nevertheless, the role that JNK plays in malignant transformation is still unclear. The aim of this thesis is to examine the role of JNK in malignant transformation. For this purpose, I used the Bcr/Abl oncogene as a transforming agent. Bcr/Abl is a leukemogenic oncogene that is created by reciprocal translocation between chromosome 9 and 22. The translocation breakpoint is variable and several different Bcr/Abl isoforms have been identified such as Bcr/AblP185 and Bcr/AblP210, whose expression is associated with different types of leukemia. Bcr/Abl activates the JNK signaling pathway in hematopoietic cells and increases AP-1 transcription activity. Furthermore, dominant negative approaches demonstrate that inhibition of c-Jun or JNK prevents Bcr/ Abl-induced cell transformation in vitro. These data implicate the JNK signaling pathway in Bcr/Abl transformation although the role that JNK might have in this process is unclear. Thus, I examined the importance of JNK signaling in Bcr/Abl-induced lymphoid or myeloid transformation. For this purpose I compared Bcr/AblP185- and Bcr/AblP210- induced transformation of wild-type and JNK1-deficient cells using three approaches: in vitro, in vivo and ex vivo. The results obtained with the in vitro approach suggest that both Bcr/AblP185 and Bcr/AblP210 require JNK activity to induce lymphoid transformation. While JNK1-deficiency inhibits Bcr/AblP210 oncogenic potential in lymphoid cells both in vitro and in vivo, pharmacological inhibition of JNK activity (JNK1 and/or JNK2) blocked Bcr/AblP185 induced malignant proliferation in vitro. The differential requirement for JNK observed in the two Bcr/Abl isoforms can be ascribed to the presence in Bcr/AblP210 of the Dbl domain which can activate the JNK pathway in vitro. In the case of Bcr/AblP210, JNK1 is critical for the survival of the ex vivo derived transformed lymphoblasts upon growth factor removal. This result correlates with the fact that mice reconstituted with Bcr/AblP210 transformed Jnk1-l- bone marrow showed normal malignant lymphoid expansion in the bone marrow yet they had reduced numbers of lymphoblast in the bloodstream and lacked peripheral organ infiltration. Thus JNK1 is essential for the survival of the transformed lymphoblast outside the bone marrow microenvironment in Bcr/AblP210induced lymphoid leukemia. Interestingly, while JNK1 is essential for lymphoid transformation, it is dispensable for the proliferation of transformed myeloblasts. Taken together these results indicate that the JNK signaling pathway plays an essential role in the survival of Bcr/AblP210 lymphoblasts and that JNK-deficiency decreases the leukomogenic potential of Bcr/AblP210 in vivo. Thus, cell survival mediated by JNK may contribute to the pathogenesis of proliferative diseases.

Les cardiomyocytes dérivés des cellules souches pluripotentes humaines induites (hiPSC-CMs) représentent des modèles in vitro prometteurs pour plusieurs applications scientifiques et thérapeutiques allant de la modélisation de pathologies à la découverte de médicaments et de la toxicologie prédictive à la médecine régénérative. Malgré les nombreux progrès dans ce domaine, les protocoles de différenciation actuels ne permettent pas d’atteindre le stade de maturité que l’on retrouve chez le myocarde adulte de l’Homme. En effet, certaines caractéristiques majeures des hiPSC-CMs demeurent similaires à celles de cardiomyocytes fœtaux telles que l’expression de plusieurs gènes cardiaques, l’électrophysiologie ou leur fonction contractile. En effet, des analyses transcriptomiques réalisés au sein de notre laboratoire à Sanofi ont révélé que les gènes KCNJ2 et CASQ2, impliqués respectivement dans l’électrophysiologie et la gestion du calcium, étaient sous-exprimés dans les hiPSC-CMs en comparaison aux cardiomyocytes adultes. Cette thèse avait pour objectif d’améliorer la maturation des hiPSC-CMs en utilisant des technologies d’édition du génome. Ainsi, nous avons généré des lignées stables de hiPSC-CMs qui expriment de manière inductible KCNJ2 ou CASQ2 ou les deux gènes simultanément puis nous avons examiné leurs phénotypes fonctionnels et électrophysiologiques par le biais de méthodes d’analyses complémentaires. A la suite à l’induction de l’expression de KCNJ2 et CASQ2 par la doxycycline, les hiPSC-CMs montraient des bénéfices phénotypiques tels que la diminution drastique de la fréquence des battements spontanés, une hyperpolarisation du potentiel de repos membranaire, la diminution de la durée du potentiel d’action et l’amélioration du flux de calcium transitoire. En plus de ces bénéfices attendus, l’expression concomitante de ces deux gènes a amélioré la pente de la pointe du potentiel de champ extracellulaire associée au courant sodique ainsi que la gestion du calcium. Nous avons ensuite évalué le bénéfice de l’expression de ces transgènes sur la toxicologie prédictive en testant des molécules agonistes ou antagonistes de canaux ioniques utilisées classiquement dans le cadre des essais précliniques de toxicité cardiaque. Nous avons notamment observé plus d’arythmies induites par l’E4031 avec les hiPSC-CMs exprimant conjointement KCNJ2 et CASQ2 par rapport aux cardiomyocytes contrôles. Ainsi, les hiPSC-CMs exprimant simultanément KCNJ2 et CASQ2 présentent un phénotype plus mature que les hiPSC-CMs natifs et de tels cardiomyocytes édités génétiquement peuvent être utiles pour l’évaluation de la toxicité cardiaque de nouveaux médicaments candidats
Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are a very promising model for several scientific and therapeutic applications ranging from disease modeling to drug discovery, and from predictive toxicology to regenerative medicine. Despite numerous efforts, current protocols do not yet lead to a maturation phenotype equivalent to adult human myocardium. Indeed, key features of hiPSC-CMs remaining closer to fetal stages of development, such as gene expression, electrophysiology and function. Transcriptome analysis performed at Sanofi have confirmed these findings at the genome-wide level. Indeed, KCNJ2 and CASQ2 which are implicated in the two major physiological characteristics of cardiac cells, their electrophysiological behavior and calcium handling, respectively, were expressed at very low levels in hiPSC-CMs in comparison with adult cardiomyocytes. This thesis aimed to improve the maturation of hiPSC-CMs by using genome editing technologies. We generated stable hiPSC-CMs with inducible expression of KCNJ2, or CASQ2 or both genes (KCNJ2-CASQ2 hiPSC-CMs) and studied their functional and electrophysiological phenotype by several complementary methods. Upon doxycycline induction of KCNJ2 and CASQ2, KCNJ2-CASQ2 hiPSC-CMs displayed phenotypic benefits expected from previous studies of each maturation gene, including a drastic reduction of spontaneous beating, hyperpolarized resting membrane potential, shortened action potential duration and enhanced calcium transients. In addition, co-expression of the two genes enhanced Na+ spike slope of extracellular field potential and Ca2+ handling. We tested four reference drugs and observed signatures of known cardiac effects in KCNJ2-CASQ2 hiPSC-CMs, including arrhythmias induced by QT prolonging drug (E-4031), which were more easily detected than in control hiPSC-CMs. Therefore, KCNJ2-CASQ2 hiPSC-CMs exhibited a more mature phenotype than hiPSC-CMs and such genetically engineered hiPSC-CMs could be useful for testing cardiac toxicity of novel candidate drugs

碩士
國立清華大學
動力機械工程學系
103
Cell fusion is a critical course for all sort of biomedical applications including cell reprogramming, hybridoma formation, cancer immunotherapy, and tissue regeneration. It can be realized by using biological, chemical, or physical methods. However, efficiency and yields are limited by unstable cell contact and random cell pairings in traditional methods. Hence, improving cell contact and cell pairing are the two key factors to enhance efficiency and yields of cell fusion. This study therefore reported a new approach called optically-induced cell fusion (OICF) which integrates cell-pairing microstructures and optically-induced, localized electrical field to achieve precise cell fusion with high yields and high efficiency. By projecting light patterns on a photoconductive film (hydrogen-rich amorphous silicon, a-Si: H) coated on an indium-tin-oxide (ITO) glass while an alternating-current (AC) electric field was applied on the top and bottom ITO glasses, “virtual” electrodes would be constructed accordingly. In fact, this method could be used on several biomedical applications, including cell manipulation, cell separation, cell lysis and electroporation. Therefore, a locally enhanced electric field would be induced and the pairing cells could be precisely fused by the virtual electrodes. In this study, 57% cell paring rate and 87% fusion efficiency were achieved. Therefore, OICF is a promising method to succeed in cell fusion with high efficiency and high yields.

This thesis investigates the use of engineered DNA to program fusion and aggregation behaviors of artificial cells, mimicking biological cells and their important functions. To achieve this goal, we construct synthetic cells from engineered lipids and DNA to recognize and process intercellular signals.

博士
國立臺灣大學
生物化學暨分子生物學研究所
99
Part I Fungal allergens are associated with the development of asthma, and some have been characterized as proteases. Here, we established an animal model of allergic airway inflammation in response to continuous exposure to proteolytically active Pen c 13, a major allergen secreted by Penicillium citrinum. In functional analyses, Pen c 13 exposure led to increased airway hyperresponsiveness, significant inflammatory cell infiltration, mucus overproduction, and collagen deposition in the lung, dramatically elevated serum levels of total IgE and Pen c 13-specific IgE and IgG1, and increased production of the Th2 cytokines IL-4, IL-5, and IL-13 by splenocytes stimulated in vitro with Pen c 13. To examine the mechanisms, we performed two-dimensional fluorescence difference gel electrophoresis (2-D DIGE) analysis combined with nano-LC-MS-MS, followed by Ingenuity Pathways Analysis (IPA) to map significant functional networks. The highest-scoring network that associated with acute allergic pulmonary eosinophilia and cell movement in the Functions and Diseases analysis was selected for further dissection. Additionally, canonical pathways, including actin cytoskeleton, leukocyte extravasation, integrin, NRF2-mediated oxidative stress response, FAK, tight junction, and acute phase response, were also highlighted. Using IPA tools to identify potential targets, galectin-3 and laminin might be involved in novel pathogenic mechanisms. Finally, we focused on junctional proteins, because, in addition to opening of the epithelial barrier by environmental proteases possibly being the initial step in the development of asthma, these proteins are also associates with actin rearrangement. Taken together, Pen c 13 exposure causes junctional structure alterations and actin cytoskeletal rearrangements, resulting in increased permeability and airway structural changes. These effects probably change the lung microenvironment and foster the allergic sensitization. Part II Specific immunotherapy (SIT) that is in use at present involves the administration of allergen extracts to patients leading to the clinical tolerance of the allergens and cure for allergic symptoms. However, the risk of therapy-induced side effects limits its broad application. Recent studies have revealed that the epitope complexity of allergen extracts can be recreated using recombinant allergens, and hypoallergenic derivatives of these can be engineered to increase treatment safety. In present study, we developed the nonanaphylactic peptides derived from B cell epitopes of Pen c 13, an immunodominant human allergen secreted by Penicillium citrinum identified as an alkaline serine protease, to be a generally applicable strategy for the therapy of allergy. To find linear epitopes on Pen c 13, mapping of allergenic epitopes was performed by cleaved peptides which cover most of the protein sequence. The results showed that at least ten different linear IgE-binding epitopes located throughout the Pen c 13. Of these, peptide S16 (A148-E166) and S22 (A243-K274) were recognized by sera from 90% and 100% of the patients tested. In addition, the specificity of IgE binding was confirmed by ELISA inhibition assays. The peptide S22 was selected for dissection of its IgE-binding ability, and therefore we exerted molecular modeling and B-cell epitope predicted server to predict six most possible residues involved in IgE binding. Furthermore, the peptide S22 was split into two parts which comprise N-terminal (A243-A260) or C-terminal (T261-K274) part fused to GST. The result of the serum screening showed that the majority of IgE-binding ability resides indeed in its C-terminal fragment. Final, six most possible residues within C-terminus of the peptide S22 were substituted for alanine individual by point mutations; one of the mutants of Pen c 13 (T261-K274), K274A, had dramatically reduced IgE reactivity and may be designed hypoallergenic forms of the allergen, which develop a safe and efficient therapeutic strategy for treating human allergic diseases in the future.

Most viruses can infect only a reduced range of organisms and an effective replication is possible only in selected hosts. These hosts are called permissive for the virus. Molecular principles of a nonpermissiveness and viral mechanisms of overcoming replication obstacles are still not clearly elucidated. This thesis discusses the molecular causes of the cellular nonpermissiveness against a model retrovirus - Rous sarcoma virus. The research is conducted on duck cells which are semipermis- sive to the subgroup C of Rous sarcoma virus. The virus can enter those cells, but it is not able to produce enough infectious viral progeny. Two blocks of the viral replication cycle in the duck cells are described in the thesis. The first one is the probably not optimal cellular receptor recognition. The second one is in the late phase of the replication cycle when the viral proteins are synthesized. The amount of the envelope glyco- protein coding mRNA is reduced due to the altered splicing ratios, and the virions produced from the duck cells are less infectious. This block is recessive and can be partially omitted by cell fusions with permissive chicken cells; therefore, the block is not caused by specific restriction fac- tors in sensu stricto. Additionally, the influence of mutations in duck adapted Rous...

One of the important tasks of virology and immunology is to explore the species- and cell-barriers preventing virus horizontal transmission and reveal the ways how viruses overcome these barriers and "adapt" to different species. This work is based on a well- established retroviral model - avian Rous sarcoma virus (RSV) and studies virus replication blocks in mammalian cells at both pre- and post-integration level. Interaction of the viral envelope glycoprotein (Env) with a specific cellular receptor mediates virus entry into cells. Although mammalian orthologues of specific chicken receptors do not support RSV entry, it was observed that some RSV strains are able to enter mammalian cells. Several RSV-transformed rodent cells lines were described and analysis of provirus H20- RSV in one these cells lines (hamster H-20 tumor cell line) showed multiple mutations including two crucial amino acid substitutions in different regions of Env. Substitutions D32G and L378S confer virus transmission to hamster, human and also chicken cells lacking the appropriate receptor. Altered conformation of H20-RSV Env is similar to a receptor-primed (activated) state of Env. This observation indicates that virus can circumvent the need of original cell receptor because of spontaneous Env activation caused by single...