Regulation of the MIE Locus During HCMV Latency and Reactivation

From enhancers to genome conformation: complex transcriptional control underlies expression of a single herpesviral gene

Complex transcriptional control is a conserved feature of both eukaryotes and the viruses that infect them. Here, we illustrate this by combining high-density functional genomics, expression profiling, and viral-specific chromosome conformation capture to define with unprecedented detail the transcriptional regulation of a single gene, ORF68, from Kaposi's sarcoma-associated herpesvirus (KSHV). We first identified seven cis-regulatory regions by densely tiling the ~154 kb KSHV genome with CRISPRi. A parallel Cas9 nuclease screen indicated that three of these regions act as promoters of genes that regulate ORF68. RNA expression profiling demonstrated that three more of these regions act by either repressing or enhancing other distal viral genes involved in ORF68 transcriptional regulation. Finally, we tracked how the 3D structure of the viral genome changes during its lifecycle, revealing that these enhancing regulatory elements are physically closer to their targets when active, and that disrupting some elements caused large-scale changes to the 3D genome. These data enable us to construct a complete model revealing that the mechanistic diversity of this essential regulatory circuit matches that of human genes.

cGAS-STING-TBK1 Signaling Promotes Valproic Acid-Responsive Human Cytomegalovirus Immediate-Early Transcription during Infection of Incompletely Differentiated Myeloid Cells

Repression of human cytomegalovirus (HCMV) immediate-early (IE) gene expression is a key regulatory step in the establishment and maintenance of latent reservoirs. Viral IE transcription and protein accumulation can be elevated during latency by treatment with histone deacetylase inhibitors such as valproic acid (VPA), rendering infected cells visible to adaptive immune responses. However, the latency-associated viral protein UL138 inhibits the ability of VPA to enhance IE gene expression during infection of incompletely differentiated myeloid cells that support latency. UL138 also limits the accumulation of IFNβ transcripts by inhibiting the cGAS-STING-TBK1 DNA-sensing pathway. Here, we show that, in the absence of UL138, the cGAS-STING-TBK1 pathway promotes both IFNβ accumulation and VPA-responsive IE gene expression in incompletely differentiated myeloid cells. Inactivation of this pathway by either genetic or pharmacological inhibition phenocopied UL138 expression and reduced VPA-responsive IE transcript and protein accumulation. This work reveals a link between cytoplasmic pathogen sensing and epigenetic control of viral lytic phase transcription and suggests that manipulation of pattern recognition receptor signaling pathways could aid in the refinement of MIEP regulatory strategies to target latent viral reservoirs.

Host-encoded CTCF regulates human cytomegalovirus latency via chromatin looping

Human cytomegalovirus (HCMV) is a prevalent pathogen that establishes life-long latent infection in hematopoietic cells. While this infection is usually asymptomatic, immune dysregulation leads to viral reactivation, which can cause significant morbidity and mortality. However, the mechanisms underpinning reactivation remain incompletely understood. The HCMV major immediate early promoter (MIEP)/enhancer is a key factor in this process, as its transactivation from a repressed to active state helps drive viral gene transcription necessary for reactivation from latency. Numerous host transcription factors bind the MIE locus and recruit repressive chromatin modifiers, thus impeding virus reactivation. One such factor is CCCTC-binding protein (CTCF), a highly conserved host zinc finger protein that mediates chromatin conformation and nuclear architecture. However, the mechanisms by which CTCF contributes to HCMV latency were previously unexplored. Here, we confirm that CTCF binds two convergent sites within the MIE locus during latency in primary CD14+ monocytes, and following cellular differentiation, CTCF association is lost as the virus reactivates. While mutation of the MIE enhancer CTCF binding site does not impact viral lytic growth in fibroblasts, this mutant virus fails to maintain latency in myeloid cells. Furthermore, we show the two convergent CTCF binding sites allow looping to occur across the MIEP, supporting transcriptional repression during latency. Indeed, looping between the two sites diminishes during virus reactivation, concurrent with activation of MIE transcription. Taken together, our data reveal that three-dimensional chromatin looping aids in the regulation of HCMV latency and provides insight into promoter/enhancer regulation that may prove broadly applicable across biological systems.

CMV-encoded GPCRs in infection, disease, and pathogenesis

G protein coupled receptors (GPCRs) are seven-transmembrane domain proteins that modulate cellular processes in response to external stimuli. These receptors represent the largest family of membrane proteins, and in mammals, their signaling regulates important physiological functions, such as vision, taste, and olfaction. Many organisms, including yeast, slime molds, and viruses encode GPCRs. Cytomegaloviruses (CMVs) are large, betaherpesviruses, that encode viral GPCRs (vGPCRs). Human CMV (HCMV) encodes four vGPCRs, including UL33, UL78, US27, and US28. Each of these vGPCRs, as well as their rodent and primate orthologues, have been investigated for their contributions to viral infection and disease. Herein, we discuss how the CMV vGPCRs function during lytic and latent infection, as well as our understanding of how they impact viral pathogenesis.

c-Jun signaling during initial HSV-1 infection modulates latency to enhance later reactivation in addition to directly promoting the progression to full reactivation

Herpes simplex virus-1 (HSV-1) establishes a latent infection in peripheral neurons and periodically reactivates to permit transmission, which can result in clinical manifestations. Viral transactivators required for lytic infection are largely absent during latent infection, and therefore, HSV-1 relies on the co-option of neuronal host signaling pathways to initiate its gene expression. The activation of the neuronal c-Jun N-terminal kinase (JNK) cell stress pathway is central to initiating biphasic reactivation in response to multiple stimuli. However, how host factors work with JNK to stimulate the initial wave of gene expression (known as Phase I) or the progression to full Phase II reactivation remains unclear. Here, we found that c-Jun, the primary target downstream of neuronal JNK cell stress signaling, functions during reactivation but not during the JNK-mediated initiation of Phase I gene expression. Instead, c-Jun was required to transition from Phase I to full HSV-1 reactivation and was detected in viral replication compartments of reactivating neurons. Interestingly, we also identified a role for both c-Jun and enhanced neuronal stress during initial neuronal infection in promoting a more reactivation-competent form of HSV-1 latency. Therefore, c-Jun functions at multiple stages during the HSV latent infection of neurons to promote reactivation but not during the initial JNK-dependent Phase I. Importantly, by demonstrating that initial infection conditions can contribute to later reactivation abilities, this study highlights the potential for latently infected neurons to maintain a molecular scar of previous exposure to neuronal stressors.IMPORTANCEThe molecular mechanisms that regulate the reactivation of herpes simplex virus-1 (HSV-1) from latent infection are unknown. The host transcription and pioneer factor c-Jun is the main target of the JNK cell stress pathway that is known to be important in exit of HSV from latency. Surprisingly, we found that c-Jun does not act with JNK during exit from latency but instead promotes the transition to full reactivation. Moreover, c-Jun and enhanced neuronal stress during initial neuronal infection promoted a more reactivation-competent form of HSV-1 latency. c-Jun, therefore, functions at multiple stages during HSV-1 latent infection of neurons to promote reactivation. Importantly, this study contributes to a growing body of evidence that de novo HSV-1 infection conditions can modulate latent infection and impact future reactivation events, raising important questions on the clinical impact of stress during initial HSV-1 acquisition on future reactivation events and consequences.

Epigenetic Restriction Factors (eRFs) in Virus Infection

The ongoing arms race between viruses and their hosts is constantly evolving. One of the ways in which cells defend themselves against invading viruses is by using restriction factors (RFs), which are cell-intrinsic antiviral mechanisms that block viral replication and transcription. Recent research has identified a specific group of RFs that belong to the cellular epigenetic machinery and are able to restrict the gene expression of certain viruses. These RFs can be referred to as epigenetic restriction factors or eRFs. In this review, eRFs have been classified into two categories. The first category includes eRFs that target viral chromatin. So far, the identified eRFs in this category include the PML-NBs, the KRAB/KAP1 complex, IFI16, and the HUSH complex. The second category includes eRFs that target viral RNA or, more specifically, the viral epitranscriptome. These epitranscriptomic eRFs have been further classified into two types: those that edit RNA bases-adenosine deaminase acting on RNA (ADAR) and pseudouridine synthases (PUS), and those that covalently modify viral RNA-the N6-methyladenosine (m6A) writers, readers, and erasers. We delve into the molecular machinery of eRFs, their role in limiting various viruses, and the mechanisms by which viruses have evolved to counteract them. We also examine the crosstalk between different eRFs, including the common effectors that connect them. Finally, we explore the potential for new discoveries in the realm of epigenetic networks that restrict viral gene expression, as well as the future research directions in this area.

c-Jun Signaling During Initial HSV-1 Infection Modulates Latency to Enhance Later Reactivation in addition to Directly Promoting the Progression to Full Reactivation

Herpes simplex virus-1 (HSV-1) establishes a latent infection in peripheral neurons and can periodically reactivate to permit transmission and clinical manifestations. Viral transactivators required for lytic infection are largely absent during latent infection and therefore HSV-1 relies on the co-option of neuronal host signaling pathways to initiate its gene expression. Activation of the neuronal c-Jun N-terminal kinase (JNK) cell stress pathway is central to initiating biphasic reactivation in response to multiple stimuli. However, how host factors work with JNK to stimulate the initial wave of gene expression (known as Phase I) or the progression to full, Phase II reactivation remains unclear. Here, we found that c-Jun, the primary target downstream of neuronal JNK cell stress signaling, functions during reactivation but not during the JNK-mediated initiation of Phase I gene expression. Instead, c-Jun was required for the transition from Phase I to full HSV-1 reactivation and was detected in viral replication compartments of reactivating neurons. Interestingly, we also identified a role for both c-Jun and enhanced neuronal stress during initial neuronal infection in promoting a more reactivation-competent form of HSV-1 latency. Therefore, c-Jun functions at multiple stages during HSV latent infection of neurons to promote reactivation. Importantly, by demonstrating that initial infection conditions can contribute to later reactivation abilities, this study highlights the potential for latently infected neurons to maintain a molecular scar of previous exposure to neuronal stressors.

MORC3 represses the HCMV major immediate early promoter in myeloid cells in the absence of PML nuclear bodies

Human cytomegalovirus (HCMV) can undergo either a latent or a lytic infection in cells of the myeloid lineage. Whilst the molecular mechanisms which determine the outcome of infection are far from clear, it is well established that a key factor is the differential regulation of the major immediate early promoter (MIEP) responsible for driving lytic immediate early gene expression. Using a myelomonocytic cell line stably transduced with a GFP reporter under the control of the MIEP, which recapitulates MIEP regulation in the context of virus infection, we have used an unbiased CRISPR-Cas9 sub-genomic, epigenetic library screen to identify novel cellular factors involved in MIEP repression during establishment and maintenance of latency in myeloid cells. One such cellular factor identified was MORC3. Consistent with MORC3 being a robust repressor of the MIEP, we show that THP1 cells devoid of MORC3 fail to establish latency. We also show that MORC3 is induced during latent infection, recruited to the MIEP and forms MORC3 nuclear bodies (MORC3-NBs) which, interestingly, co-localize with viral genomes. Finally, we show that the latency-associated functions of MORC3 are regulated by the deSUMOylase activity of the viral latency-associated LUNA protein likely to prevent untimely HCMV reactivation.

Virion-associated US28 rapidly modulates Akt activity to suppress HCMV lytic replication in monocytes

Establishing a non-productive quiescent/silent infection within monocytes is essential for spread of human cytomegalovirus (HCMV). Yet, how HCMV establishes a quiescent infection in monocytes remains unclear. US28 is a viral G protein-coupled receptor (GPCR) essential for silent infections within cells of the myeloid lineage. We found virion-associated US28 was rapidly delivered to monocytes, while de novo synthesized US28 was delayed for several days. A recombinant mutant virus lacking US28 (US28Δ) was unable to establish a quiescent infection, resulting in a fully productive lytic replication cycle. Mechanistically, viral entry of US28Δ phosphorylated Akt at both serine 473 (S473) and threonine 308 (T308), which contrasted with the site-specific phosphorylation of Akt at S473 following WT infection. Preventing Akt bi-phosphorylation prevented lytic replication of US28Δ, and ectopic expression of a constitutively phosphorylated Akt variant triggered lytic replication of WT infection. Our data demonstrate that virion-delivered US28 fine-tunes Akt activity to permit HCMV infection to enter a quiescent state following primary infection of monocytes.

The Human Cytomegalovirus Latency-Associated Gene Product Latency Unique Natural Antigen Regulates Latent Gene Expression

Human cytomegalovirus (HCMV) infection can lead to either lytic or latent infection, which is dependent on the regulation of the viral major immediate early promoter (MIEP). Suppression of the MIEP is a pre-requisite for latency and is driven by repressive epigenetic modifications at the MIEP during latent infection. However, other viral genes are expressed during latency and this is correlated with activatory epigenetic modifications at latent gene promoters. Yet the molecular basis of the differential regulation of latent and lytic gene expression by epigenetics is unclear. LUNA, a latent viral transcript, has been suggested to be important for HCMV latency and has also been shown to be important for efficient reactivation likely through its known deSUMOylase activity. Intriguingly, we and others have also observed that LUNA enhances latency-associated expression of the viral UL138 gene. Here, we show that in the absence of LUNA, the expression of multiple latency-associated transcripts is reduced during latent infection, which is correlated with a lack of activatory marks at their promoters. Interestingly, we also show that LUNA interacts with the hematopoietic transcription factor GATA-2, which has previously been shown to bind to a number of latency-associated gene promoters, and that this interaction is dependent on the deSUMOylase domain of LUNA. Finally, we show that the deSUMOylase activity of LUNA is required for the establishment and/or maintenance of an open chromatin configuration around latency-associated gene promoters. As such, LUNA plays a key role in efficient latency-associated viral gene expression and carriage of viral genome during latent carriage.

Chromatin control of human cytomegalovirus infection

Human cytomegalovirus (HCMV) is a betaherpesvirus that establishes lifelong infection in its host and can cause severe comorbidities in individuals with suppressed or compromised immune systems. The lifecycle of HCMV consists of lytic and latent phases, largely dependent upon the cell type infected and whether transcription from the major immediate early locus can ensue. Control of this locus, which acts as a critical "switch" region from where the lytic gene expression cascade originates, as well as regulation of the additional ~235 kilobases of virus genome, occurs through chromatinization with cellular histone proteins after infection. Upon infection of a host cell, an initial intrinsic antiviral response represses gene expression from the incoming genome, which is relieved in permissive cells by viral and host factors in concert. Latency is established in a subset of hematopoietic cells, during which viral transcription is largely repressed while the genome is maintained. As these latently infected cells differentiate, the cellular milieu and epigenetic modifications change, giving rise to the initial stages of virus reactivation from latency. Thus, throughout the cycle of infection, chromatinization, chromatin modifiers, and the recruitment of specific transcription factors influence the expression of genes from the HCMV genome. In this review, we discuss epigenetic regulation of the HCMV genome during the different phases of infection, with an emphasis on recent reports that add to our current perspective.

Hematopoietic stem cells and betaherpesvirus latency

The human betaherpesviruses including human cytomegalovirus (HCMV), human herpesvirus (HHV)-6a and HHV-6b, and HHV-7 infect and establish latency in CD34+ hematopoietic stem and progenitor cells (HPCs). The diverse repertoire of HPCs in humans and the complex interactions between these viruses and host HPCs regulate the viral lifecycle, including latency. Precise manipulation of host and viral factors contribute to preferential maintenance of the viral genome, increased host cell survival, and specific manipulation of the cellular environment including suppression of neighboring cells and immune control. The dynamic control of these processes by the virus regulate inter- and intra-host signals critical to the establishment of chronic infection. Regulation occurs through direct viral protein interactions and cellular signaling, miRNA regulation, and viral mimics of cellular receptors and ligands, all leading to control of cell proliferation, survival, and differentiation. Hematopoietic stem cells have unique biological properties and the tandem control of virus and host make this a unique environment for chronic herpesvirus infection in the bone marrow. This review highlights the elegant complexities of the betaherpesvirus latency and HPC virus-host interactions.

Rethinking human cytomegalovirus latency reservoir

Human cytomegalovirus (HCMV) is a prevalent herpesvirus, infecting the majority of the human population. Like other herpesviruses, it causes lifelong infection through the establishment of latency. Although reactivation from latency can cause significant morbidity and mortality in immunocompromised hosts, our understanding of HCMV latency and how it is maintained remains limited. Here, we discuss the characterized latency reservoir in hematopoietic cells in the bone marrow and the gaps in our knowledge of mechanisms that facilitate HCMV genome maintenance in dividing cells. We further review clinical evidence that strongly suggests the tissue origin of HCMV reactivation, and we outline similarities to murine cytomegalovirus where latency in tissue-resident cells has been demonstrated. Overall, we think these observations call for a rethinking of HCMV latency reservoirs and point to potential sources of HCMV latency that reside in tissues.

NFκB and Cyclic AMP Response Element Sites Mediate the Valproic Acid and UL138 Responsiveness of the Human Cytomegalovirus Major Immediate Early Enhancer and Promoter

The major immediate early enhancer and promoter (MIEP) of human cytomegalovirus (HCMV) drives the transcription of the immediate early one (IE1) and IE2 genes, whose encoded proteins stimulate productive, lytic replication. The MIEP is activated by the virally encoded and tegument-delivered pp71 protein at the start of de novo lytic infections of fully differentiated cells. Conversely, the MIEP is silenced at the start of de novo latent infections within incompletely differentiated myeloid cells in part because tegument-delivered pp71 is sequestered in the cytoplasm in these cells, but also by viral factors that repress transcription from this locus, including the UL138 protein. During both modes of infection, MIEP activity can be increased by the histone deacetylase inhibitor valproic acid (VPA); however, UL138 inhibits the VPA-responsiveness of the MIEP. Here, we show that two families of cellular transcription factors, NF-κB and cAMP response element-binding protein (CREB), together control the VPA-mediated activation and UL138-mediated repression of the HCMV MIEP. IMPORTANCE Artificial regulation of the HCMV MIEP, either activation or repression, is an attractive potential means to target the latent reservoirs of virus for which there is currently no available intervention. The MIEP could be repressed to prevent latency reactivation or induced to drive the virus into the lytic stage that is visible to the immune system and inhibited by multiple small-molecule antiviral drugs. Understanding how the MIEP is regulated is a critical part of designing and implementing either strategy. Our revelation here that NF-κB and CREB control the responsiveness of the MIEP to the viral UL138 protein and the FDA-approved drug VPA could help in the formulation and execution of promoter regulatory strategies against latent HCMV.

Cytomegalovirus US28 regulates cellular EphA2 to maintain viral latency

Cytomegalovirus (CMV) reactivation from latency following immune dysregulation remains a serious risk for patients, often causing substantial morbidity and mortality. Here, we demonstrate the CMV-encoded G protein-coupled receptor, US28, in coordination with cellular Ephrin receptor A2, attenuates mitogen-activated protein kinase signaling, thereby limiting viral replication in latently infected primary monocytes. Furthermore, treatment of latently infected primary monocytes with dasatinib, a Food and Drug Association-approved kinase inhibitor used to treat a subset of leukemias, results in CMV reactivation. These ex vivo data correlate with our retrospective analyses of the Explorys electronic health record database, where we find dasatinib treatment is associated with a significant risk of CMV-associated disease (odds ratio 1.58, P = 0.0004). Collectively, our findings elucidate a signaling pathway that plays a central role in the balance between CMV latency and reactivation and identifies a common therapeutic cancer treatment that elevates the risk of CMV-associated disease.

The Human Cytomegalovirus β2.7 Long Non-Coding RNA Prevents Induction of Reactive Oxygen Species to Maintain Viral Gene Silencing during Latency

Human cytomegalovirus (HCMV) is a significant source of disease for the immunosuppressed and immunonaive. The treatment of HCMV is made more problematic by viral latency, a lifecycle stage in which the virus reduces its own gene expression and produces no infectious virus. The most highly expressed viral gene during HCMV latency is the viral β2.7 long non-coding RNA. Although we have recently shown that the β2.7 lncRNA lowers levels of reactive oxygen species (ROS) during infection in monocytes, how this impacts latency is unclear. We now show that β2.7 is important for establishing and maintaining HCMV latency by aiding the suppression of viral lytic gene expression and that this is directly related to its ability to quench reactive oxygen species (ROS). Consistent with this, we also find that exogenous inducers of ROS cause reactivation of latent HCMV. These effects can be compensated by treatment with an antioxidant to lower ROS levels. Finally, we show that ROS-mediated reactivation is independent of myeloid differentiation, but instead relies on NF-κB activation. Altogether, these results reveal a novel factor that is central to the complex process that underpins HCMV latency. These findings may be of particular relevance in the transplant setting, in which transplanted tissue/organs are subject to very high ROS levels, and HCMV reactivation poses a significant threat.

Design of a US28 ORF Deletion Virus in a Temperature-Sensitive Cytomegalovirus Strain Fails to Promote Lytic Replication in Hematopoietic Cells

Human cytomegalovirus (CMV) is a ubiquitous pathogen that latently resides in hematopoietic cells. Latently infected individuals with dysfunctional immune systems often experience CMV reactivation, which can cause devastating disease and mortality. While factors dictating the balance between latency and reactivation are not completely understood, CMV US28 is required for maintaining latent infection, and viral mutants that alter US28 function result in a lytic-like, rather than latent, infection in hematopoietic cells. In turn, viral lytic factors alter the host cell, making it challenging to characterize the US28-specific changes in the cellular milieu. To circumvent this, we generated a temperature-sensitive TB40/E recombinant virus, TB40/EgfpC510G (tsC510G), into which we engineered an amino acid change at position 510 (C510G) of IE2, as previously described in the CMV Towne strain. Using tsC510G, we then deleted the US28 ORF, termed tsC510G-US28Δ. Consistent with previous findings, tsC510G-US28Δ fails to undergo latency in Kasumi-3 cells at the permissive temperature. However, parallel cultures maintained at the non-permissive temperature showed a significant reduction in infectious center frequency, as measured by limiting dilution assay. Thus, we generated a new US28 mutant virus for use as a tool to study US28-specific changes in latently infected hematopoietic cells in the absence of induced lytic replication.

Polyploid giant cancer cells, EZH2 and Myc upregulation in mammary epithelial cells infected with high-risk human cytomegalovirus

Background

Human cytomegalovirus (HCMV) infection has been actively implicated in complex neoplastic processes. Beyond oncomodulation, the molecular mechanisms that might underlie HCMV-induced oncogenesis are being extensively studied. Polycomb repressive complex 2 (PRC2) proteins, in particular enhancer of zeste homolog 2 (EZH2) are associated with cancer progression. Nevertheless, little is known about EZH2 activation in the context of HCMV infection and breast oncogenesis.

Methods

Herein, we identified EZH2 as a downstream target for HCMV-induced Myc upregulation upon acute and chronic infection with high-risk strains using a human mammary epithelial model.

Findings

We detected polyploidy and CMV-transformed HMECs (CTH) cells harboring HCMV and dynamically undergoing the giant cells cycle. Acquisition of embryonic stemness markers positively correlated with EZH2 and Myc expression. EZH2 inhibitors curtail sustained CTH cells’ malignant phenotype. Besides harboring polyploid giant cancer cells (PGCCs), tumorigenic breast biopsies were characterized by an enhanced EZH2 and Myc expression, with a strong positive correlation between EZH2 and Myc expression, and between PGCC count and EZH2/Myc expression in the presence of HCMV. Further, we isolated two HCMV strains from EZH2^HighMyc^High basal-like tumors which replicate in MRC5 cells and transform HMECs toward CTH cells after acute infection.

Interpretation

Our data establish a potential link between HCMV-induced Myc activation, the subsequent EZH2 upregulation, and polyploidy induction. These data support the proposed tumorigenesis properties of EZH2/Myc, and allow the isolation of two oncogenic HCMV strains from EZH2^HighMyc^High basal breast tumors while identifying EZH2 as a potential therapeutic target in the management of breast cancer, particularly upon HCMV infection.

Funding

This work was supported by grants from the University of Franche-Comté (UFC) (CR3300), the Région Franche-Comté (2021-Y-08292 and 2021-Y-08290) and the Ligue contre le Cancer (CR3304) to Georges Herbein. Zeina Nehme is a recipient of a doctoral scholarship from the municipality of Habbouch. Sandy Haidar Ahmad is recipient of a doctoral scholarship from Lebanese municipality. Ranim El Baba is a recipient of a doctoral scholarship from Hariri foundation for sustainable human development.

Human Cytomegalovirus IE2 Both Activates and Represses Initiation and Modulates Elongation in a Context-Dependent Manner

Human cytomegalovirus (HCMV) immediate-early 2 (IE2) protein is a multifunctional transcription factor that is essential for lytic HCMV infection. IE2 functions as an activator of viral early genes, negatively regulates its own promoter, and is required for viral replication. The mechanisms by which IE2 executes these distinct functions are incompletely understood. Using PRO-Seq, which profiles nascent transcripts, and a recently developed DFF-chromatin immunoprecipitation (DFF-ChIP; employs chromatin digestion by the endonuclease DNA fragmentation factor prior to IP) approach that resolves occupancy and local chromatin environment, we show that IE2 controls viral gene transcription in three distinct capacities during late HCMV infection and reveal mechanisms that involve direct binding of IE2 to viral DNA. IE2 represses a subset of viral promoters by binding within their core promoter regions and blocking the assembly of preinitiation complexes (PICs). Remarkably, IE2 forms a repressive complex at the major immediate-early promoter region involving direct association of IE2 with nucleosomes and TBP. IE2 stimulates transcription by binding nearby, but not within, core promoter regions. In addition, IE2 functions as a direct roadblock to transcription elongation. At one locus, this function of IE2 appears to be important for the synthesis of a spliced viral RNA. Consistent with the minimal observed effects of IE2 depletion on host gene transcription, IE2 does not functionally engage the host genome. Our results reveal mechanisms of transcriptional control by IE2, uncover a previously unknown function of IE2 as a Pol II elongation modulator, and demonstrate that DFF-ChIP is a useful tool for probing transcription factor occupancy and interactions between transcription factors and nucleosomes at high resolution.

Herpesvirus Replication Compartments: Dynamic Biomolecular Condensates?

Recent progress has provided clear evidence that many RNA-viruses form cytoplasmic biomolecular condensates mediated by liquid-liquid phase separation to facilitate their replication. In contrast, seemingly contradictory data exist for herpesviruses, which replicate their DNA genomes in nuclear membrane-less replication compartments (RCs). Here, we review the current literature and comment on nuclear condensate formation by herpesviruses, specifically with regard to RC formation. Based on data obtained with human cytomegalovirus (human herpesvirus 5), we propose that liquid and homogenous early RCs convert into more heterogeneous RCs with complex properties over the course of infection. We highlight how the advent of DNA replication leads to the maturation of these biomolecular condensates, likely by adding an additional DNA scaffold.

Human MicroRNAs Attenuate the Expression of Immediate Early Proteins and HCMV Replication during Lytic and Latent Infection in Connection with Enhancement of Phosphorylated RelA/p65 (Serine 536) That Binds to MIEP

Attenuating the expression of immediate early (IE) proteins is essential for controlling the lytic replication of human cytomegalovirus (HCMV). The human microRNAs (hsa-miRs), miR-200b-3p and miR-200c-3p, have been identified to bind the 3′-untranslated region (3′-UTR) of the mRNA encoding IE proteins. However, whether hsa-miRs can reduce IE72 expression and HCMV viral load or exhibit a crosstalk with the host cellular signaling machinery, most importantly the NF-κB cascade, has not been evaluated. In this study, argonaute-crosslinking and immunoprecipitation-seq revealed that miR-200b-3p and miR-200c-3p bind the 3′-UTR of UL123, which is a gene that encodes IE72. The binding of these miRNAs to the 3′-UTR of UL123 was verified in transfected cells stably expressing GFP. We used miR-200b-3p/miR-200c-3p mimics to counteract the downregulation of these miRNA after acute HCMV infection. This resulted in reduced IE72/IE86 expression and HCMV VL during lytic infection. We determined that IE72/IE86 alone can inhibit the phosphorylation of RelA/p65 at the Ser⁵³⁶ residue and that p-Ser⁵³⁶ RelA/p65 binds to the major IE promoter/enhancer (MIEP). The upregulation of miR-200b-3p and miR-200c-3p resulted in the phosphorylation of RelA/p65 at Ser⁵³⁶ through the downregulation of IE, and the binding of the resultant p-Ser⁵³⁶ RelA/p65 to MIEP resulted in a decreased production of pro-inflammatory cytokines. Overall, miR-200b-3p and miR-200c-3p—together with p-Ser⁵³⁶ RelA/p65—can prevent lytic HCMV replication during acute and latent infection

Modulation of host cell signaling during cytomegalovirus latency and reactivation

Background

Human cytomegalovirus (HCMV) resides latently in cells of the myeloid compartment, including CD34+hematopoietic progenitor cells and circulating monocytes. Healthy hosts maintain the virus latently, and this infection is, for the most part, asymptomatic. However, given the proper external cues, HCMV reactivates from latency, at which point the virus disseminates, causing disease. The viral and cellular factors dictating the balance between these phases of infection are incompletely understood, though a large body of literature support a role for viral-mediated manipulation of host cell signaling.

Main body

To establish and maintain latency, HCMV has evolved various means by which it usurps host cell factors to alter the cellular environment to its own advantage, including altering host cell signaling cascades. As early as virus entry into myeloid cells, HCMV usurps cellular signaling to change the cellular milieu, and this regulation includes upregulation, as well as downregulation, of different signaling cascades. Indeed, given proper reactivation cues, this signaling is again altered to allow for transactivation of viral lytic genes.

Conclusions

HCMV modulation of host cell signaling is not binary, and many of the cellular pathways altered are finely regulated, wherein the slightest modification imparts profound changes to the cellular milieu. It is also evident that viral-mediated cell signaling differs not only between these phases of infection, but also is myeloid cell type specific. Nonetheless, understanding the exact pathways and the means by which HCMV mediates them will undoubtedly provide novel targets for therapeutic intervention.

Modulation of host cell signaling during cytomegalovirus latency and reactivation

Background

Human cytomegalovirus (HCMV) resides latently in cells of the myeloid compartment, including CD34⁺ hematopoietic progenitor cells and circulating monocytes. Healthy hosts maintain the virus latently, and this infection is, for the most part, asymptomatic. However, given the proper external cues, HCMV reactivates from latency, at which point the virus disseminates, causing disease. The viral and cellular factors dictating the balance between these phases of infection are incompletely understood, though a large body of literature support a role for viral-mediated manipulation of host cell signaling.

Main body

To establish and maintain latency, HCMV has evolved various means by which it usurps host cell factors to alter the cellular environment to its own advantage, including altering host cell signaling cascades. As early as virus entry into myeloid cells, HCMV usurps cellular signaling to change the cellular milieu, and this regulation includes upregulation, as well as downregulation, of different signaling cascades. Indeed, given proper reactivation cues, this signaling is again altered to allow for transactivation of viral lytic genes.

Conclusions

HCMV modulation of host cell signaling is not binary, and many of the cellular pathways altered are finely regulated, wherein the slightest modification imparts profound changes to the cellular milieu. It is also evident that viral-mediated cell signaling differs not only between these phases of infection, but also is myeloid cell type specific. Nonetheless, understanding the exact pathways and the means by which HCMV mediates them will undoubtedly provide novel targets for therapeutic intervention.

HCMV Antivirals and Strategies to Target the Latent Reservoir

Human cytomegalovirus (HCMV) is a ubiquitous human herpesvirus. In healthy people, primary infection is generally asymptomatic, and the virus can go on to establish lifelong latency in cells of the myeloid lineage. However, HCMV often causes severe disease in the immunosuppressed: transplant recipients and people living with AIDS, and also in the immunonaive foetus. At present, there are several antiviral drugs licensed to control HCMV disease. However, these are all faced with problems of poor bioavailability, toxicity and rapidly emerging viral resistance. Furthermore, none of them are capable of fully clearing the virus from the host, as they do not target latent infection. Consequently, reactivation from latency is a significant source of disease, and there remains an unmet need for treatments that also target latent infection. This review briefly summarises the most common HCMV antivirals used in clinic at present and discusses current research into targeting the latent HCMV reservoir.

Cytomegalovirus (CMV) Infection and Latency

Cytomegalovirus (CMV) is a herpesvirus that infects a majority of the human population worldwide [...].

Nutraceutical Curcumin with Promising Protection against Herpesvirus Infections and Their Associated Inflammation: Mechanisms and Pathways

Herpesviruses are DNA viruses that infect humans and animals with the ability to induce latent and lytic infections in their hosts, causing critical health complications. The enrolment of nutraceutical anti-herpesvirus drugs in clinical investigations with promising levels of reduced resistance, free or minimal cellular toxicity, and diverse mechanisms of action might be an effective way to defeat challenges that hurdle the progress of anti-herpesvirus drug development, including the problems with drug resistance and recurrent infections. Therefore, in this review, we aim to hunt down all investigations that feature the curative properties of curcumin, a principal bioactive phenolic compound of the spice turmeric, in regard to various human and animal herpesvirus infections and inflammation connected with these diseases. Curcumin was explored with potent antiherpetic actions against herpes simplex virus type 1 and type 2, human cytomegalovirus, Kaposi’s sarcoma-associated herpesvirus, Epstein–Barr virus, bovine herpesvirus 1, and pseudorabies virus. The mechanisms and pathways by which curcumin inhibits anti-herpesvirus activities by targeting multiple steps in herpesvirus life/infectious cycle are emphasized. Improved strategies to overcome bioavailability challenges that limit its use in clinical practice, along with approaches and new directions to enhance the anti-herpesvirus efficacy of this compound, are also reviewed. According to the reviewed studies, this paper presents curcumin as a promising natural drug for the prevention and treatment of herpesvirus infections and their associated inflammatory diseases.

CMV-encoded GPCR pUL33 activates CREB and facilitates its recruitment to the MIE locus for efficient viral reactivation

Human cytomegalovirus (HCMV) establishes life-long latent infection in hematopoietic progenitor cells and circulating monocytes in infected individuals. Myeloid differentiation coupled with immune dysregulation leads to viral reactivation, which can cause severe disease and mortality. Reactivation of latent virus requires chromatin reorganization and the removal of transcriptional repressors in exchange for transcriptional activators. While some factors involved in these processes are identified, a complete characterization of the viral and cellular factors involved in their upstream regulation remains elusive. Herein, we show the HCMV-encoded G protein-coupled receptor (GPCR), UL33, is expressed during latency. Although this viral GPCR is not required to maintain latent infection, our data reveal UL33-mediated signaling is important for efficient viral reactivation. Additionally, UL33 signaling induces cellular cyclic AMP response element binding protein (CREB1, referred to here as CREB) phosphorylation, a transcription factor that promotes reactivation when recruited to the major immediate early (MIE) enhancer/promoter. Finally, targeted pharmacological inhibition of CREB activity reverses the reactivation phenotype of the UL33 signaling-deficient mutant. In sum, our data reveal UL33-mediated signaling functions to activate CREB, resulting in successful viral reactivation.