Interaction Between BGLF2 and BBLF1 Is Required for the Efficient Production of Infectious Epstein-Barr Virus Particles

Understanding the Neurotrophic Virus Mechanisms and Their Potential Effect on Systemic Lupus Erythematosus Development

Central nervous system (CNS) pathologies are a public health concern, with viral infections one of their principal causes. These viruses are known as neurotropic pathogens, characterized by their ability to infiltrate the CNS and thus interact with various cell populations, inducing several diseases. The immune response elicited by neurotropic viruses in the CNS is commanded mainly by microglia, which, together with other local cells, can secrete inflammatory cytokines to fight the infection. The most relevant neurotropic viruses are adenovirus (AdV), cytomegalovirus (CMV), enterovirus (EV), Epstein-Barr Virus (EBV), herpes simplex virus type 1 (HSV-1), and herpes simplex virus type 2 (HSV-2), lymphocytic choriomeningitis virus (LCMV), and the newly discovered SARS-CoV-2. Several studies have associated a viral infection with systemic lupus erythematosus (SLE) and neuropsychiatric lupus (NPSLE) manifestations. This article will review the knowledge about viral infections, CNS pathologies, and the immune response against them. Also, it allows us to understand the relevance of the different viral proteins in developing neuronal pathologies, SLE and NPSLE.

Epstein-Barr virus infectious particles initiate B cell transformation and modulate cytokine response

IMPORTANCE

The Epstein-Barr virus efficiently infects and transforms B lymphocytes. During this process, infectious viral particles transport the viral genome to the nucleus of target cells. We show here that these complex viral structures serve additional crucial roles by activating transcription of the transforming genes encoded by the virus. We show that components of the infectious particle sequentially activate proinflammatory B lymphocyte signaling pathways that, in turn, activate viral gene expression but also cause cytokine release. However, virus infection activates expression of ZFP36L1, an RNA-binding stress protein that limits the length and the intensity of the cytokine response. Thus, the infectious particles can activate viral gene expression and initiate cellular transformation at the price of a limited immune response.

Epstein-Barr Virus BBLF1 Mediates Secretory Vesicle Transport to Facilitate Mature Virion Release

Enveloped viruses undergo a complex multistep process of assembly, maturation, and release into the extracellular space utilizing host secretory machinery. Several studies of the herpesvirus subfamily have shown that secretory vesicles derived from the trans-Golgi network (TGN) or endosomes transport virions into the extracellular space. However, the regulatory mechanism underlying the release of Epstein-Barr virus, a human oncovirus, remains unclear. We demonstrate that disruption of BBLF1, a tegument component, suppressed viral release and resulted in the accumulation of viral particles on the inner side of the vesicular membrane. Organelle separation revealed the accumulation of infectious viruses in fractions containing vesicles derived from the TGN and late endosomes. Deficiency of an acidic amino acid cluster in BBLF1 reduced viral secretion. Moreover, truncational deletion of the C-terminal region of BBLF1 increased infectious virus production. These findings suggest that BBLF1 regulates the viral release pathway and reveal a new aspect of tegument protein function. IMPORTANCE Several viruses have been linked to the development of cancer in humans. Epstein-Barr virus (EBV), the first identified human oncovirus, causes a wide range of cancers. Accumulating literature has demonstrated the role of viral reactivation in tumorigenesis. Elucidating the functions of viral lytic genes induced by reactivation, and the mechanisms of lytic infection, is essential to understanding pathogenesis. Progeny viral particles synthesized during lytic infection are released outside the cell after the assembly, maturation, and release steps, leading to further infection. Through functional analysis using BBLF1-knockout viruses, we demonstrated that BBLF1 promotes viral release. The acidic amino acid cluster in BBLF1 was also important for viral release. Conversely, mutants lacking the C terminus exhibited more efficient virus production, suggesting that BBLF1 is involved in the fine-tuning of progeny release during the EBV life cycle.

Tegument proteins of Epstein-Barr virus: Diverse functions, complex networks, and oncogenesis

The tegument is the structure between the envelope and nucleocapsid of herpesvirus particles. Viral (and cellular) proteins accumulate to create the layers of the tegument. Some Epstein-Barr virus (EBV) tegument proteins are conserved widely in Herpesviridae, but others are shared only by members of the gamma-herpesvirus subfamily. As the interface to envelope and nucleocapsid, the tegument functions in virion morphogenesis and budding of the nucleocapsid during progeny production. When a virus particle enters a cell, enzymes such as kinase and deubiquitinase, and transcriptional activators are released from the virion to promote virus infection. Moreover, some EBV tegument proteins are involved in oncogenesis. Here, we summarize the roles of EBV tegument proteins, in comparison to those of other herpesviruses.

Comparative Analysis of the Humoral Immune Response to the EBV Proteome across EBV-Related Malignancies

BACKGROUND

Epstein-Barr virus (EBV) is linked to multiple cancers, including classical Hodgkin lymphoma (cHL), endemic Burkitt lymphoma (eBL), nasopharyngeal carcinoma (NPC), and extranodal natural killer/T-cell lymphoma (NKTCL).

METHODS

Anti-EBV IgG and IgA antibody responses targeting 202 sequences from 86 EBV proteins were measured using the same EBV whole proteome array across four case-control studies investigating EBV-positive cHL, eBL, NPC, and NKTCL (407 cases/620 controls). We grouped EBV-targeted antibodies into pathways by immunoglobulin type (IgA and IgG) and life-cycle stage (latent, immediate early lytic, early lytic, late lytic, and glycoprotein) and evaluated their association with each cancer type. In an additional analysis, we focused on the subset of 46 individual antibodies representing the top candidates for each cancer and compared their associations across the four cancer types using multivariable linear regression models.

RESULTS

IgA antibody responses targeting all EBV life-cycle stages were associated with NPC but limited to anti-early lytic stage for cHL. NPC and eBL were associated with IgG antibodies across the viral life cycle; cHL with antibodies in the early lytic, late lytic and glycoprotein stages; and NKTCL with antibodies in the latent, immediate early lytic and early lytic phases. EBNA3A, BBLF1, BDLF4, and BLRF2 IgG antibodies were associated with all cancer types.

CONCLUSIONS

Our observed similarities and differences across four EBV-associated cancers may inform EBV-related oncogenesis.

IMPACT

Understanding the comparative humoral immune response across EBV-related cancers may aid in identifying shared etiologic roles of EBV proteins and inform unique pathogenic processes for each cancer.

Subcellular Distribution of BALF2 and the Role of Rab1 in the Formation of Epstein-Barr Virus Cytoplasmic Assembly Compartment and Virion Release

Epstein-Barr virus (EBV) replicates its genome in the nucleus and undergoes tegumentation and envelopment in the cytoplasm. We are interested in how the single-stranded DNA binding protein BALF2, which executes its function and distributes predominantly in the nucleus, is packaged into the tegument of virions. At the mid-stage of virus replication in epithelial TW01-EBV cells, a small pool of BALF2 colocalizes with tegument protein BBLF1, BGLF4 protein kinase, and the cis-Golgi marker GM130 at the perinuclear viral assembly compartment (AC). A possible nuclear localization signal (NLS) between amino acids 1100 and 1128 (C29), which contains positive charged amino acid 1113RRKRR1117, is able to promote yellow fluorescent protein (YFP)-LacZ into the nucleus. In addition, BALF2 interacts with the nucleocapsid-associated protein BVRF1, suggesting that BALF2 may be transported into the cytoplasm with nucleocapsids in a nuclear egress complex (NEC)-dependent manner. A group of proteins involved in intracellular transport were identified to interact with BALF2 in a proteomic analysis. Among them, the small GTPase Rab1A functioning in bi-directional trafficking at the ER-Golgi interface is also a tegument component. In reactivated TW01-EBV cells, BALF2 colocalizes with Rab1A in the cytoplasmic AC. Expression of dominant-negative GFP-Rab1A(N124I) diminished the accumulation of BALF2 in the AC, coupling with attenuation of gp350/220 glycosylation. Virion release was significantly downregulated by expressing dominant-negative GFP-Rab1A(N124I). Overall, the subcellular distribution of BALF2 is regulated through its complex interaction with various proteins. Rab1 activity is required for proper gp350/220 glycosylation and the maturation of EBV. IMPORTANCE Upon EBV lytic reactivation, the virus-encoded DNA replication machinery functions in the nucleus, while the newly synthesized DNA is encapsidated and transported to the cytoplasm for final virus assembly. The single-stranded DNA binding protein BALF2 executing functions within the nucleus was also identified in the tegument layer of mature virions. Here, we studied the functional domain of BALF2 that contributes to the nuclear targeting and used a proteomic approach to identify novel BALF2-interacting cellular proteins that may contribute to virion morphogenesis. The GTPase Rab1, a master regulator of anterograde and retrograde endoplasmic reticulum (ER)-Golgi trafficking, colocalizes with BALF2 in the juxtanuclear concave region at the midstage of EBV reactivation. Rab1 activity is required for BALF2 targeting to the cytoplasmic assembly compartment (AC) and for gp350/220 targeting to cis-Golgi for proper glycosylation and virion release. Our study hints that EBV hijacks the bi-directional ER-Golgi trafficking machinery to complete virus assembly.

Suppression of cGAS- and RIG-I-mediated innate immune signaling by Epstein-Barr virus deubiquitinase BPLF1

Epstein-Barr virus (EBV) has developed effective strategies to evade host innate immune responses. Here we reported on mitigation of type I interferon (IFN) production by EBV deubiquitinase (DUB) BPLF1 through cGAS-STING and RIG-I-MAVS pathways. The two naturally occurring forms of BPLF1 exerted potent suppressive effect on cGAS-STING-, RIG-I- and TBK1-induced IFN production. The observed suppression was reversed when DUB domain of BPLF1 was rendered catalytically inactive. The DUB activity of BPLF1 also facilitated EBV infection by counteracting cGAS-STING- and TBK1-mediated antiviral defense. BPLF1 associated with STING to act as an effective DUB targeting its K63-, K48- and K27-linked ubiquitin moieties. BPLF1 also catalyzed removal of K63- and K48-linked ubiquitin chains on TBK1 kinase. The DUB activity of BPLF1 was required for its suppression of TBK1-induced IRF3 dimerization. Importantly, in cells stably carrying EBV genome that encodes a catalytically inactive BPLF1, the virus failed to suppress type I IFN production upon activation of cGAS and STING. This study demonstrated IFN antagonism of BPLF1 mediated through DUB-dependent deubiquitination of STING and TBK1 leading to suppression of cGAS-STING and RIG-I-MAVS signaling.

Features and Functions of the Conserved Herpesvirus Tegument Protein UL11 and Its Binding Partners

The herpesvirus UL11 protein is encoded by the UL11 gene and is a membrane-anchored protein with multiple functions. In the last stage of viral replication, UL11 participates in the secondary envelopment process. It also plays a key role in primary envelopment, the transportation of newly assembled viral particles through cytoplasmic vesicles, and virion egress from the cell. UL11 is an important accessory protein and sometimes cooperates with other proteins that participate in virus-induced cell fusion. Cell fusion is necessary for cell-to-cell transmissions. This review summarizes the latest literature and discusses the roles of UL11 in viral assembly, primary and secondary envelopment, and cell-to-cell transmission to obtain a better understanding of the UL11 protein in the life cycle of herpesviruses and to serve as a reference for studying other viruses. Additionally, some recently discovered characteristics of UL11 are summarized.

Epstein-Barr Virus BGLF2 commandeers RISC to interfere with cellular miRNA function

The Epstein-Barr virus (EBV) BGLF2 protein is a tegument protein with multiple effects on the cellular environment, including induction of SUMOylation of cellular proteins. Using affinity-purification coupled to mass-spectrometry, we identified the miRNA-Induced Silencing Complex (RISC), essential for miRNA function, as a top interactor of BGLF2. We confirmed BGLF2 interaction with the Ago2 and TNRC6 components of RISC in multiple cell lines and their co-localization in cytoplasmic bodies that also contain the stress granule marker G3BP1. In addition, BGLF2 expression led to the loss of processing bodies in multiple cell types, suggesting disruption of RISC function in mRNA regulation. Consistent with this observation, BGLF2 disrupted Ago2 association with multiple miRNAs. Using let-7 miRNAs as a model, we tested the hypothesis that BGLF2 interfered with the function of RISC in miRNA-mediated mRNA silencing. Using multiple reporter constructs with 3’UTRs containing let-7a regulated sites, we showed that BGLF2 inhibited let-7a miRNA activity dependent on these 3’UTRs, including those from SUMO transcripts which are known to be regulated by let-7 miRNAs. In keeping with these results, we showed that BGLF2 increased the cellular level of unconjugated SUMO proteins without affecting the level of SUMO transcripts. Such an increase in free SUMO is known to drive SUMOylation and would account for the effect of BGLF2 in inducing SUMOylation. We further showed that BGLF2 expression inhibited the loading of let-7 miRNAs into Ago2 proteins, and conversely, that lytic infection with EBV lacking BGLF2 resulted in increased interaction of let-7a and SUMO transcripts with Ago2, relative to WT EBV infection. Therefore, we have identified a novel role for BGLF2 as a miRNA regulator and shown that one outcome of this activity is the dysregulation of SUMO transcripts that leads to increased levels of free SUMO proteins and SUMOylation.

Epstein-Barr virus (EBV) infects most people worldwide, persists for life and is associated with several kinds of cancer. In order to undergo efficient lytic infection, EBV must manipulate multiple cellular pathways. BGLF2 is an EBV lytic protein known to modulate several cellular processes including increasing the modification of cellular proteins with the Small Ubiquitin-Like Modifier (SUMO), a process referred to as SUMOylation. Here we show for the first time that BGLF2 interacts with a cellular complex (RISC) required for miRNA function and interferes with the function of some cellular miRNAs by sequestering this complex. One of the consequences of this effect is the increased expression of SUMO proteins, due to inhibition of the miRNAs that normally downregulate their expression. The resulting increase in SUMO proteins drives SUMOylation, providing a mechanism for the previously reported BGLF2-induced SUMOylation of cellular proteins. In addition, the discovery of BGLF2 as a miRNA regulator suggests that this EBV protein can control many cellular pathways by interfering with cellular miRNAs that normally regulate them.

Disruption of the Interaction between ORF33 and the Conserved Carboxyl-Terminus of ORF45 Abolishes Progeny Virion Production of Kaposi Sarcoma-Associated Herpesvirus

The Open Reading Frame 45 (ORF45) of Kaposi sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus-specific, immediate-early, tegument protein required for efficient viral replication and virion production. We have previously shown that ORF45 interacts with the conserved herpesviral protein ORF33 through the highly conserved C-terminal 19 amino acids (C19) of ORF45. Because the deletion of C19 abolished ORF33 accumulation and viral production, we reasoned that this interaction could be critical for viral production and explored as an antiviral target for gammaherpesviruses. In work described in this article, we characterize this interaction in further detail, first by revealing that this interaction is conserved among gammaherpesviruses, then by identifying residues in C19 critical for its interaction with and stabilization of ORF33. More importantly, we show that disruption of the interaction, either by mutating key residues (W403A or W405A) in C19 or by using competing cell penetration peptide TAT-C19, dramatically reduce the yield of KSHV progeny viruses. Our results not only reveal critical roles of this interaction to viral production but also provide a proof of concept for targeting the ORF33-ORF45 interaction as a novel antiviral strategy against KSHV and other gammaherpesviruses.

Suppression of JAK-STAT signaling by Epstein-Barr virus tegument protein BGLF2 through recruitment of SHP1 phosphatase and promotion of STAT2 degradation

Some lytic proteins encoded by Epstein-Barr virus (EBV) suppress host interferon (IFN) signaling to facilitate viral replication. In this study we sought to identify and characterize EBV proteins antagonizing IFN signaling. The induction of IFN-stimulated genes (ISGs) by IFN-β was effectively suppressed by EBV. A functional screen was therefore performed to identify IFN-antagonizing proteins encoded by EBV. EBV tegument protein BGLF2 was identified as a potent suppressor of JAK-STAT signaling. This activity was found to be independent of its stimulatory effect on p38 and JNK pathways. Association of BGLF2 with STAT2 resulted in more pronounced K48-linked polyubiquitination and proteasomal degradation of the latter. Mechanistically, BGLF2 promoted the recruitment of SHP1 phosphatase to STAT1 to inhibit its tyrosine phosphorylation. In addition, BGLF2 associated with cullin 1 E3 ubiquitin ligase to facilitate its recruitment to STAT2. Consequently, BGLF2 suppressed ISG induction by IFN-β. Furthermore, BGLF2 also suppressed type II and type III IFN signaling, although the suppressive effect on type II IFN response was milder. When pre-treated with IFN-β, host cells became less susceptible to primary infection of EBV. This phenotype was reversed when expression of BGLF2 was enforced. Finally, genetic disruption of BGLF2 in EBV led to more pronounced induction of ISGs. Taken together, our study unveils the roles of BGLF2 not only in the subversion of innate IFN response but also in lytic infection and reactivation of EBV. Importance Epstein-Barr virus (EBV) is an oncogenic virus associated with the development of lymphoid and epithelial malignancies. EBV has to subvert interferon-mediated host antiviral response to replicate and cause diseases. It is therefore of great interest to identify and characterize interferon-antagonizing proteins produced by EBV. In this study we perform a screen to search for EBV proteins that suppress the action of interferons. We further show that BGLF2 protein of EBV is particularly strong in this suppression. This is achieved by inhibiting two key proteins STAT1 and STAT2 that mediate the antiviral activity of interferons. BGLF2 recruits a host enzyme to remove the phosphate group from STAT1 thereby inactivating its activity. BGLF2 also redirects STAT2 for degradation. A recombinant virus in which BGLF2 gene has been disrupted can activate host interferon response more robustly. Our findings reveal a novel mechanism by which EBV BGLF2 protein suppresses interferon signaling.