Epstein-Barr Nuclear Antigen 1 modulates replication of oriP-plasmids by impeding replication and transcription fork migration through the family of repeats

Epstein-Barr virus infection: the micro and macro worlds

Epstein‒Barr virus (EBV) is a DNA virus that belongs to the human B lymphotropic herpesvirus family and is highly prevalent in the human population. Once infected, a host can experience latent infection because EBV evades the immune system, leading to hosts harboring the virus for their lifetime. EBV is associated with many diseases and causes significant challenges to human health. This review first offers a description of the natural history of EBV infection, clarifies the interaction between EBV and the immune system, and finally focuses on several major types of diseases caused by EBV infection.

Optimising the transient expression of GABA(A) receptors in adherent HEK293 cells

Owing to their therapeutic relevance, considerable efforts are devoted to the structural characterisation of membrane proteins. Such studies are limited by the availability of high quality protein due to the difficulty of overexpression in recombinant mammalian systems. We sought to systematically optimise multiple aspects in the process of transiently transfecting HEK293 cells, to allow the rapid expression of membrane proteins, without the lengthy process of stable clone formation. We assessed the impact of medium formulation, cell line, and harvest time on the expression of GABA_A receptors, as determined by [³H]muscimol binding in cell membranes. Furthermore, transfection with the use of calcium phosphate/polyethyleneimine multishell nanoparticles was optimised, and a dual vector system utilising viral enhancing elements was designed and implemented. These efforts resulted in a 40-fold improvement in GABA_A α₁β₃ receptor expression, providing final yields of 22 fmol/cm². The findings from this work provide a guide to the optimisation of transient expression of proteins in mammalian cells and should assist in the structural characterisation of membrane proteins.

Epstein Barr Virus Infection Affects Function of Cytotoxic T Lymphocytes in Patients with Severe Aplastic Anemia

Severe aplastic anemia (SAA) is characterized by pancytopenia and failure of hematopoietic function and is caused by excessive functioning of cytotoxic T lymphocytes (CTLs). EBNA-1, a nucleoprotein of the Epstein Barr virus (EBV), can influence the proliferation and function of lymphocytes. We therefore tested the number of EBV copies in the CD8+ T cells of 27 patients with SAA and 10 healthy control subjects and observed the influences of EBNA-1 upon the CD8+ T cells of patients with SAA. The results showed that more EBV copies were found in the CD8+ T cells of patients with untreated SAA than in patients with SAA in remission or in the healthy control subjects. Their copy number was positively correlated with the expression of granzyme B and perforin, the secretion level of interferon-γ in CD8+ T cells, and the viability of CD8+ T cells, whereas no correlation was seen between the copy number and the interleukin 4 secretion level or the apoptosis rate. Meanwhile, the expression of granzyme B and perforin was reduced after EBNA-1 gene knockdown, whereas the interferon-γ secretion level and cell viability declined. Therefore, we infer that EBV infection may be a factor in the activation of CTLs and in damaging the bone marrow hematopoietic function of patients with SAA.

Epstein-Barr virus latency: current and future perspectives

EBV drives resting B cells to continuous proliferating latently infected cells. A restricted program of viral transcription contributes to latency and cell proliferation important for growth transformation. Recent interest in latency and transformation has provided new data about the roles of the EBV encoded latent proteins and non-coding RNAs. We broadly describe the transcription, epigenetic, signaling and super-enhancer functions of the latent nuclear antigens in regulating cellular transcription; the role of LMP2 in utilization of the autophagosome to control cell death, and the association between LMP1, the linear ubiquitin chain assembly complex and TRAF1 which are important for transformation. This review explores recent discoveries with new insights into therapeutic avenues for EBV related malignancies.

Establishment of a replication fork barrier following induction of DNA binding in mammalian cells

Understanding the mechanisms that lead to replication fork blocks (RFB) and the means to bypass them is important given the threat that they represent for genome stability if inappropriately handled. Here, to study this issue in mammals, we use integrated arrays of the LacO and/or TetO as a tractable system to follow in time a process in an individual cell and at a single locus. Importantly, we show that induction of the binding by LacI and TetR proteins, and not the presence of the repeats, is key to form the RFB. We find that the binding of the proteins to the arrays during replication causes a prolonged persistence of replication foci at the site. This, in turn, induces a local DNA damage repair (DDR) response, with the recruitment of proteins involved in double-strand break (DSB) repair such as TOPBP1 and 53BP1, and the phosphorylation of H2AX. Furthermore, the appearance of micronuclei and DNA bridges after mitosis is consistent with an incomplete replication. We discuss how the many DNA binding proteins encountered during replication can be dealt with and the consequences of incomplete replication. Future studies exploiting this type of system should help analyze how an RFB, along with bypass mechanisms, are controlled in order to maintain genome integrity.

Strategies for the Development of Cell Lines for Ex Vivo Gene Therapy in the Central Nervous System

Disorders of the central nervous system (CNS) as a result of trauma or ischemic or neurodegenerative processes still pose a challenge for modern medicine. Due to the complexity of the CNS, and in spite of the advances in the knowledge of its anatomy, pharmacology, and molecular and cellular biology, treatments for these diseases are still limited. The development of cell lines as a source for transplantation into the damaged CNS (cell therapy), and more recently their genetic modification to favor the expression and delivery of molecules with therapeutic potential (ex vivo gene therapy), are some of the techniques used in search of novel restorative strategies. This article reviews the different approaches that have been used and perfected during the last decade to generate cell lines and their use in experimental models of neuronal damage, and evaluates the prospects of applying these methods to treat CNS disorders.

Molecular and cellular characterization of an AT-hook protein from Leishmania

AT-rich DNA, and the proteins that bind it (AT-hook proteins), modulate chromosome structure and function in most eukaryotes. Unlike other trypanosomatids, the genome of Leishmania species is unusually GC-rich, and the regulation of Leishmania chromosome structure, replication, partitioning is not fully understood. Because AT-hook proteins modulate these functions in other eukaryotes, we examined whether AT-hook proteins are encoded in the Leishmania genome, to test their potential functions. Several Leishmania ORFs predicted to be AT-hook proteins were identified using in silico approaches based on sequences shared between eukaryotic AT-hook proteins. We have used biochemical, molecular and cellular techniques to characterize the L. amazonensis ortholog of the L. major protein LmjF06.0720, a potential AT-hook protein that is highly conserved in Leishmania species. Using a novel fusion between the AT-hook domain encoded by LmjF06.0720 and a herpesviral protein, we have demonstrated that LmjF06.0720 functions as an AT-hook protein in mammalian cells. Further, as observed for mammalian and viral AT-hook proteins, the AT-hook domains of LmjF06.0720 bind specific regions of condensed mammalian metaphase chromosomes, and support the licensed replication of DNA in mammalian cells. LmjF06.0720 is nuclear in Leishmania, and this localization is disrupted upon exposure to drugs that displace AT-hook proteins from AT-rich DNA. Coincidentally, these drugs dramatically alter the cellular physiology of Leishmania promastigotes. Finally, we have devised a novel peptido-mimetic agent derived from the sequence of LmjF06.0720 that blocks the proliferation of Leishmania promastigotes, and lowers amastigote parasitic burden in infected macrophages. Our results indicate that AT-hook proteins are critical for the normal biology of Leishmania. In addition, we have described a simple technique to examine the function of Leishmania chromatin-binding proteins in a eukaryotic context amenable to studying chromosome structure and function. Lastly, we demonstrate the therapeutic potential of compounds directed against AT-hook proteins in Leishmania.

Ultrafine anaphase bridges, broken DNA and illegitimate recombination induced by a replication fork barrier

Most DNA double-strand breaks (DSBs) in S- and G2-phase cells are repaired accurately by Rad51-dependent sister chromatid recombination. However, a minority give rise to gross chromosome rearrangements (GCRs), which can result in disease/death. What determines whether a DSB is repaired accurately or inaccurately is currently unclear. We provide evidence that suggests that perturbing replication by a non-programmed protein-DNA replication fork barrier results in the persistence of replication intermediates (most likely regions of unreplicated DNA) into mitosis, which results in anaphase bridge formation and ultimately to DNA breakage. However, unlike previously characterised replication-associated DSBs, these breaks are repaired mainly by Rad51-independent processes such as single-strand annealing, and are therefore prone to generate GCRs. These data highlight how a replication-associated DSB can be predisposed to give rise to genome rearrangements in eukaryotes.

The replisome pausing factor Timeless is required for episomal maintenance of latent Epstein-Barr virus

The Epstein-Barr virus (EBV) genome is maintained as an extrachromosomal episome during latent infection of B lymphocytes. Episomal maintenance is conferred by the interaction of the EBV-encoded nuclear antigen 1 (EBNA1) with a tandem array of high-affinity binding sites, referred to as the family of repeats (FR), located within the viral origin of plasmid replication (OriP). How this nucleoprotein array confers episomal maintenance is not completely understood. Previous studies have shown that DNA replication forks pause and terminate with high frequency at OriP. We now show that cellular DNA replication fork pausing and protection factors Timeless (Tim) and Tipin (Timeless-interacting protein) accumulate at OriP during S phase of the cell cycle. Depletion of Tim inhibits OriP-dependent DNA replication and causes a complete loss of the closed-circular form of EBV episomes in latently infected B lymphocytes. Tim depletion also led to the accumulation of double-strand breaks at the OriP region. These findings demonstrate that Tim is essential for sustaining the episomal forms of EBV DNA in latently infected cells and suggest that DNA replication fork protection is integrally linked to the mechanism of plasmid maintenance.

Telomeric repeat mutagenicity in human somatic cells is modulated by repeat orientation and G-quadruplex stability

Telomeres consisting of tandem guanine-rich repeats can form secondary DNA structures called G-quadruplexes that represent potential targets for DNA repair enzymes. While G-quadruplexes interfere with DNA synthesis in vitro, the impact of G-quadruplex formation on telomeric repeat replication in human cells is not clear. We investigated the mutagenicity of telomeric repeats as a function of G-quadruplex folding opportunity and thermal stability using a shuttle vector mutagenesis assay. Since single-stranded DNA during lagging strand replication increases the opportunity for G-quadruplex folding, we tested vectors with G-rich sequences on the lagging versus the leading strand. Contrary to our prediction, vectors containing human [TTAGGG]₁₀ repeats with a G-rich lagging strand were significantly less mutagenic than vectors with a G-rich leading strand, after replication in normal human cells. We show by UV melting experiments that G-quadruplexes from ciliates [TTGGGG]₄ and [TTTTGGGG]₄ are thermally more stable compared to human [TTAGGG]₄. Consistent with this, replication of vectors with ciliate [TTGGGG]₁₀ repeats yielded a 3-fold higher mutant rate compared to the human [TTAGGG]₁₀ vectors. Furthermore, we observed significantly more mutagenic events in the ciliate repeats compared to the human repeats. Our data demonstrate that increased G-quadruplex opportunity (repeat orientation) in human telomeric repeats decreased mutagenicity, while increased thermal stability of telomeric G-quadruplexes was associated with increased mutagenicity.