Absence of IE1 p72 protein function during low-multiplicity infection by human cytomegalovirus results in a broad block to viral delayed-early gene expression

Exploring the Potential of Cytomegalovirus-Based Vectors: A Review

Viral vectors have emerged as powerful tools for delivering and expressing foreign genes, playing a pivotal role in gene therapy. Among these vectors, cytomegalovirus (CMV) stands out as a promising viral vector due to its distinctive attributes including large packaging capacity, ability to achieve superinfection, broad host range, capacity to induce CD8+ T cell responses, lack of integration into the host genome, and other qualities that make it an appealing vector candidate. Engineered attenuated CMV strains such as Towne and AD169 that have a ~15 kb genomic DNA deletion caused by virus passage guarantee human safety. CMV's large genome enables the efficient incorporation of substantial foreign genes as demonstrated by CMV vector-based therapies for SIV, tuberculosis, cancer, malaria, aging, COVID-19, and more. CMV is capable of reinfecting hosts regardless of prior infection or immunity, making it highly suitable for multiple vector administrations. In addition to its broad cellular tropism and sustained high-level gene expression, CMV triggers robust, virus-specific CD8+ T cell responses, offering a significant advantage as a vaccine vector. To date, successful development and testing of murine CMV (MCMV) and rhesus CMV (RhCMV) vectors in animal models have demonstrated the efficacy of CMV-based vectors. These investigations have explored the potential of CMV vectors for vaccines against HIV, cancer, tuberculosis, malaria, and other infectious pathogens, as well as for other gene therapy applications. Moreover, the generation of single-cycle replication CMV vectors, produced by deleting essential genes, ensures robust safety in an immunocompromised population. The results of these studies emphasize CMV's effectiveness as a gene delivery vehicle and shed light on the future applications of a CMV vector. While challenges such as production complexities and storage limitations need to be addressed, ongoing efforts to bridge the gap between animal models and human translation continue to fuel the optimism surrounding CMV-based vectors. This review will outline the properties of CMV vectors and discuss their future applications as well as possible limitations.

The human cytomegalovirus decathlon: Ten critical replication events provide opportunities for restriction

Human cytomegalovirus (HCMV) is a ubiquitous human pathogen that can cause severe disease in immunocompromised individuals, transplant recipients, and to the developing foetus during pregnancy. There is no protective vaccine currently available, and with only a limited number of antiviral drug options, resistant strains are constantly emerging. Successful completion of HCMV replication is an elegant feat from a molecular perspective, with both host and viral processes required at various stages. Remarkably, HCMV and other herpesviruses have protracted replication cycles, large genomes, complex virion structure and complicated nuclear and cytoplasmic replication events. In this review, we outline the 10 essential stages the virus must navigate to successfully complete replication. As each individual event along the replication continuum poses as a potential barrier for restriction, these essential checkpoints represent potential targets for antiviral development.

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

Revisiting promyelocytic leukemia protein targeting by human cytomegalovirus immediate-early protein 1

Promyelocytic leukemia (PML) bodies are nuclear organelles implicated in intrinsic and innate antiviral defense. The eponymous PML proteins, central to the self-organization of PML bodies, and other restriction factors found in these organelles are common targets of viral antagonism. The 72-kDa immediate-early protein 1 (IE1) is the principal antagonist of PML bodies encoded by the human cytomegalovirus (hCMV). IE1 is believed to disrupt PML bodies by inhibiting PML SUMOylation, while PML was proposed to act as an E3 ligase for IE1 SUMOylation. PML targeting by IE1 is considered to be crucial for hCMV replication at low multiplicities of infection, in part via counteracting antiviral gene induction linked to the cellular interferon (IFN) response. However, current concepts of IE1-PML interaction are largely derived from mutant IE1 proteins known or predicted to be metabolically unstable and globally misfolded. We performed systematic clustered charge-to-alanine scanning mutagenesis and identified a stable IE1 mutant protein (IE1cc172-176) with wild-type characteristics except for neither interacting with PML proteins nor inhibiting PML SUMOylation. Consequently, IE1cc172-176 does not associate with PML bodies and is selectively impaired for disrupting these organelles. Surprisingly, functional analysis of IE1cc172-176 revealed that the protein is hypermodified by mixed SUMO chains and that IE1 SUMOylation depends on nucleosome rather than PML binding. Furthermore, a mutant hCMV expressing IE1cc172-176 was only slightly attenuated compared to an IE1-null virus even at low multiplicities of infection. Finally, hCMV-induced expression of cytokine and IFN-stimulated genes turned out to be reduced rather than increased in the presence of IE1cc172-176 relative to wild-type IE1. Our findings challenge present views on the relationship of IE1 with PML and the role of PML in hCMV replication. This study also provides initial evidence for the idea that disruption of PML bodies upon viral infection is linked to activation rather than inhibition of innate immunity.

Bright and Early: Inhibiting Human Cytomegalovirus by Targeting Major Immediate-Early Gene Expression or Protein Function

The human cytomegalovirus (HCMV), one of eight human herpesviruses, establishes lifelong latent infections in most people worldwide. Primary or reactivated HCMV infections cause severe disease in immunosuppressed patients and congenital defects in children. There is no vaccine for HCMV, and the currently approved antivirals come with major limitations. Most approved HCMV antivirals target late molecular processes in the viral replication cycle including DNA replication and packaging. “Bright and early” events in HCMV infection have not been exploited for systemic prevention or treatment of disease. Initiation of HCMV replication depends on transcription from the viral major immediate-early (IE) gene. Alternative transcripts produced from this gene give rise to the IE1 and IE2 families of viral proteins, which localize to the host cell nucleus. The IE1 and IE2 proteins are believed to control all subsequent early and late events in HCMV replication, including reactivation from latency, in part by antagonizing intrinsic and innate immune responses. Here we provide an update on the regulation of major IE gene expression and the functions of IE1 and IE2 proteins. We will relate this insight to experimental approaches that target IE gene expression or protein function via molecular gene silencing and editing or small chemical inhibitors.

Murine cytomegalovirus infection in mice results in an acute inflammatory reaction in peripheral nerves

Human cytomegalovirus (CMV) infection is asymptomatic in immunocompetent individuals. However, it can lead to disease in immunodeficient population. Little is known of the mechanisms underlying the pathogenicity of the virus. We investigated the impact of CMV infection on mouse nervous system. Peripheral nerves but not spinal cord was permissive to MCMV during acute infection. Activated CD8⁺ T cells, monocytes/macrophages and cytokine expression were increased in the blood and sciatic nerves of infected mice, which exhibited transient sensory dysfunction. This study indicates that systemic MCMV infection leads to a dissemination of MCMV into peripheral nerves, which is associated with a local inflammation but not nerve tissue damage in the acute phase.

Roles of Non-coding RNAs During Herpesvirus Infection

Non-coding RNAs (ncRNAs) play essential roles in multiple aspects of the life cycles of herpesviruses and contribute to lifelong persistence of herpesviruses within their respective hosts. In this chapter, we discuss the types of ncRNAs produced by the different herpesvirus families during infection, some of the cellular ncRNAs manipulated by these viruses, and the overall contributions of ncRNAs to the viral life cycle, influence on the host environment, and pathogenesis.

Chromatin-Remodeling Factor SPOC1 Acts as a Cellular Restriction Factor against Human Cytomegalovirus by Repressing the Major Immediate Early Promoter

The cellular protein SPOC1 (survival time-associated PHD [plant homeodomain] finger protein in ovarian cancer 1) acts as a regulator of chromatin structure and the DNA damage response. It binds H3K4me2/3-containing chromatin and promotes DNA condensation by recruiting corepressors such as KAP-1 and H3K9 methyltransferases. Previous studies identified SPOC1 as a restriction factor against human adenovirus (HAdV) infection that is antagonized by E1B-55K/E4-orf6-dependent proteasomal degradation. Here, we demonstrate that, in contrast to HAdV-infected cells, SPOC1 is transiently upregulated during the early phase of human cytomegalovirus (HCMV) replication. We show that the expression of immediate early protein 1 (IE1) is sufficient and necessary to induce SPOC1. Additionally, we discovered that during later stages of infection, SPOC1 is downregulated in a glycogen synthase kinase 3β (GSK-3β)-dependent manner. We provide evidence that SPOC1 overexpression severely impairs HCMV replication by repressing the initiation of viral immediate early (IE) gene expression. Consistently, we observed that SPOC1-depleted primary human fibroblasts displayed an augmented initiation of viral IE gene expression. This occurs in a multiplicity of infection (MOI)-dependent manner, a defining hallmark of intrinsic immunity. Interestingly, repression requires the presence of high SPOC1 levels at the start of infection, while later upregulation had no negative impact, suggesting distinct temporal roles of SPOC1 during the HCMV replicative cycle. Mechanistically, we observed a highly specific association of SPOC1 with the major immediate early promoter (MIEP), strongly suggesting that SPOC1 inhibits HCMV replication by MIEP binding and the subsequent recruitment of heterochromatin-building factors. Thus, our data add SPOC1 as a novel factor to the endowment of a host cell to restrict cytomegalovirus infections.IMPORTANCE Accumulating evidence indicates that during millennia of coevolution, host cells have developed a sophisticated compilation of cellular factors to restrict cytomegalovirus infections. Defining this equipment is important to understand cellular barriers against viral infection and to develop strategies to utilize these factors for antiviral approaches. So far, constituents of PML nuclear bodies and interferon gamma-inducible protein 16 (IFI16) were known to mediate intrinsic immunity against HCMV. In this study, we identify the chromatin modulator SPOC1 as a novel restriction factor against HCMV. We show that preexisting high SPOC1 protein levels mediate a silencing of HCMV gene expression via a specific association with an important viral cis-regulatory element, the major immediate early promoter. Since SPOC1 expression varies between cell types, this factor may play an important role in tissue-specific defense against HCMV.

SUMOylation of IE2p86 is required for efficient autorepression of the human cytomegalovirus major immediate-early promoter

The human cytomegalovirus (HCMV) IE2p86 protein is pivotal for coordinated regulation of viral gene expression. Besides functioning as a promiscuous transactivator, IE2p86 is also known to negatively regulate its own transcription. This occurs via direct binding of IE2p86 to a 14-bp palindromic DNA element located between the TATA box and the transcription start site of the major immediate-early promoter (MIEP), which is referred to as the cis repression signal (CRS). However, the exact mechanism of IE2p86-based autorepression is still unclear. By testing a series of IE2p86 mutants in transient expression assays, we found that not only did a DNA binding-deficient mutant of IE2p86 fail to repress the MIEP, but SUMOylation-negative mutants also failed to repress it. This finding was further supported by infection studies with primary fibroblasts harbouring a MIEP-driven transgene as a reporter. Here, we observed that a recombinant HCMV expressing SUMOylation-negative IE2p86 was defective in transgene downregulation, in contrast to wild-type HCMV. Interestingly, however, a double-mutant virus in which both the SUMO acceptor sites and the SUMO interaction motif (SIM) of IE2p86 were inactivated regained the ability to silence the MIEP. This correlated with increased expression levels of the IE2 isoforms IE2p40 and IE2p60, suggesting that these late proteins may contribute to MIEP suppression, thus compensating for the loss of IE2p86 SUMOylation. In summary, our results show that autorepression of the MIEP is not only regulated by late isoforms of IE2, but also depends on posttranslational SUMO modification, revealing a novel mechanism to fine-tune the expression of this important viral gene region.

Multiplex CRISPR/Cas9 system impairs HCMV replication by excising an essential viral gene

Anti-HCMV treatments used in immunosuppressed patients reduce viral replication, but resistant viral strains can emerge. Moreover, these drugs do not target latently infected cells. We designed two anti-viral CRISPR/Cas9 strategies to target the UL122/123 gene, a key regulator of lytic replication and reactivation from latency. The singleplex strategy contains one gRNA to target the start codon. The multiplex strategy contains three gRNAs to excise the complete UL122/123 gene. Primary fibroblasts and U-251 MG cells were transduced with lentiviral vectors encoding Cas9 and one or three gRNAs. Both strategies induced mutations in the target gene and a concomitant reduction of immediate early (IE) protein expression in primary fibroblasts. Further detailed analysis in U-251 MG cells showed that the singleplex strategy induced 50% of indels in the viral genome, leading to a reduction in IE protein expression. The multiplex strategy excised the IE gene in 90% of all viral genomes and thus led to the inhibition of IE protein expression. Consequently, viral genome replication and late protein expression were reduced by 90%. Finally, the production of new viral particles was nearly abrogated. In conclusion, the multiplex anti-UL122/123 CRISPR/Cas9 system can target the viral genome efficiently enough to significantly prevent viral replication.

miRNA-mediated targeting of human cytomegalovirus reveals biological host and viral targets of IE2

Human cytomegalovirus (HCMV) impacts more than one-half of the human population owing to its capacity to manipulate the cell and create latent reservoirs in the host. Despite an extensive understanding of HCMV biology during acute infection in fibroblasts, the molecular basis for latency in myeloid cells remains incomplete. This knowledge gap is due largely to the fact that the existing genetic systems require virus rescue in fibroblasts, precluding the study of genes that are essential during acute infection, yet likely play unique roles in myeloid cells or the establishment of latency. Here we present a solution to address this restriction. Through the exploitation of a hematopoietic-specific microRNA, we demonstrate a one-step recombineering approach that enables gene silencing only in cells associated with latency. As a proof of concept, here we describe a TB40/E variant that undergoes hematopoietic targeting of the Immediate Early-2 (IE2) gene to explore its function during infection of myeloid cells. While virus replication of the hematopoietic-targeted IE2 variant was unimpaired in fibroblasts, we observed a >100-fold increase in virus titers in myeloid cells. Virus replication in myeloid cells demonstrated that IE2 has a significant transcriptional footprint on both viral and host genes. These data implicate IE2 as an essential mediator of virus biology in myeloid cells and illustrate the utility of cell-specific microRNA-based targeting.

Human cytomegalovirus IE1 downregulates Hes1 in neural progenitor cells as a potential E3 ubiquitin ligase

Congenital human cytomegalovirus (HCMV) infection is the leading cause of neurological disabilities in children worldwide, but the mechanisms underlying these disorders are far from well-defined. HCMV infection has been shown to dysregulate the Notch signaling pathway in human neural progenitor cells (NPCs). As an important downstream effector of Notch signaling, the transcriptional regulator Hairy and Enhancer of Split 1 (Hes1) is essential for governing NPC fate and fetal brain development. In the present study, we report that HCMV infection downregulates Hes1 protein levels in infected NPCs. The HCMV 72-kDa immediate-early 1 protein (IE1) is involved in Hes1 degradation by assembling a ubiquitination complex and promoting Hes1 ubiquitination as a potential E3 ubiquitin ligase, followed by proteasomal degradation of Hes1. Sp100A, an important component of PML nuclear bodies, is identified to be another target of IE1-mediated ubiquitination. A C-terminal acidic region in IE1, spanning amino acids 451 to 475, is required for IE1/Hes1 physical interaction and IE1-mediated Hes1 ubiquitination, but is dispensable for IE1/Sp100A interaction and ubiquitination. Our study suggests a novel mechanism linking downregulation of Hes1 protein to neurodevelopmental disorders caused by HCMV infection. Our findings also complement the current knowledge of herpesviruses by identifying IE1 as the first potential HCMV-encoded E3 ubiquitin ligase.

Congenital human cytomegalovirus (HCMV) infection is the leading cause of neurological disabilities in children, but the underlying pathogenesis of this infection remains unclear. Hes1, an important effector of Notch signaling, governs the fate of neural progenitor cells (NPCs) and fetal brain development. Here we demonstrate that: (1) HCMV infection results in loss of Hes1 protein in NPCs; (2) the HCMV immediate-early 1 protein (IE1) mediates Hes1 protein downregulation through direct interaction, which requires amino acids 451–475; (3) IE1 assembles a Hes1 ubiquitination complex and mediates Hes1 ubiquitination; and (4) IE1 also assembles an Sp100A ubiquitination complex and mediates Sp100A ubiquitination, but does not require amino acids 451–475. These results suggest that HCMV IE1 is a potential E3 ubiquitin ligase. Downregulation of Hes1 by HCMV infection and IE1 implies a novel mechanism linking Hes1 depletion to virus-induced neuropathogenesis.

Human Cytomegalovirus Immediate-Early 1 Protein Rewires Upstream STAT3 to Downstream STAT1 Signaling Switching an IL6-Type to an IFNγ-Like Response

The human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) is best known for activating transcription to facilitate viral replication. Here we present transcriptome data indicating that IE1 is as significant a repressor as it is an activator of host gene expression. Human cells induced to express IE1 exhibit global repression of IL6- and oncostatin M-responsive STAT3 target genes. This repression is followed by STAT1 phosphorylation and activation of STAT1 target genes normally induced by IFNγ. The observed repression and subsequent activation are both mediated through the same region (amino acids 410 to 445) in the C-terminal domain of IE1, and this region serves as a binding site for STAT3. Depletion of STAT3 phenocopies the STAT1-dependent IFNγ-like response to IE1. In contrast, depletion of the IL6 receptor (IL6ST) or the STAT kinase JAK1 prevents this response. Accordingly, treatment with IL6 leads to prolonged STAT1 instead of STAT3 activation in wild-type IE1 expressing cells, but not in cells expressing a mutant protein (IE1dl410-420) deficient for STAT3 binding. A very similar STAT1-directed response to IL6 is also present in cells infected with a wild-type or revertant hCMV, but not an IE1dl410-420 mutant virus, and this response results in restricted viral replication. We conclude that IE1 is sufficient and necessary to rewire upstream IL6-type to downstream IFNγ-like signaling, two pathways linked to opposing actions, resulting in repressed STAT3- and activated STAT1-responsive genes. These findings relate transcriptional repressor and activator functions of IE1 and suggest unexpected outcomes relevant to viral pathogenesis in response to cytokines or growth factors that signal through the IL6ST-JAK1-STAT3 axis in hCMV-infected cells. Our results also reveal that IE1, a protein considered to be a key activator of the hCMV productive cycle, has an unanticipated role in tempering viral replication.

Our previous work has shown that the human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) modulates host cell signaling pathways involving proteins of the signal transducer and activator of transcription (STAT) family. IE1 has also long been known to facilitate viral replication by activating transcription. In this report we demonstrate that IE1 is as significant a repressor as it is an activator of host gene expression. Many genes repressed by IE1 are normally induced via STAT3 signaling triggered by interleukin 6 (IL6) or related cytokines, whereas many genes activated by IE1 are normally induced via STAT1 signaling triggered by interferon gamma (IFNγ). Our results suggest that the repression of STAT3- and the activation of STAT1-responsive genes by IE1 are coupled. By targeting STAT3, IE1 rewires upstream STAT3 to downstream STAT1 signaling. Consequently, genes normally induced by IL6 are repressed while genes normally induced by IFNγ become responsive to IL6 in the presence of IE1. We also demonstrate that, by switching an IL6 to an IFNγ-like response, IE1 tempers viral replication. These results suggest an unanticipated dual role for IE1 in either promoting or limiting hCMV propagation and demonstrate how a key viral regulatory protein merges two central cellular signaling pathways to divert cytokine responses relevant to hCMV pathogenesis.

Human cytomegalovirus IE1 protein alters the higher-order chromatin structure by targeting the acidic patch of the nucleosome

Human cytomegalovirus (hCMV) immediate early 1 (IE1) protein associates with condensed chromatin of the host cell during mitosis. We have determined the structure of the chromatin-tethering domain (CTD) of IE1 bound to the nucleosome core particle, and discovered that the specific interaction between IE1-CTD and the H2A-H2B acidic patch impairs the compaction of higher-order chromatin structure. Our results suggest that IE1 loosens up the folding of host chromatin during hCMV infections.

DOI:http://dx.doi.org/10.7554/eLife.11911.001

Most of the DNA in a cell is tightly wrapped around groups of proteins called histones, which gives the impression of beads on a string. These bead-like structures are called nucleosomes, and interactions between histones in different nucleosomes can link one nucleosome to another, to package the DNA into a very condensed form.

Viruses sometimes interact with this condensed DNA; for example, a virus called human cytomegalovirus is known to attach to condensed DNA when cells are preparing to divide. But the consequences of these interactions are not always clear.

Now, Fang, Chen et al. have worked out the three-dimensional structure of a protein from the cytomegalovirus while it is attached to a nucleosome. This structure revealed that the viral protein connects to same part of the histones that otherwise helps pull the nucleosomes together.

Further experiments then compared how the cytomegalovirus protein attaches to nucleosomes with the interaction between nucleosomes and a similar protein from a different virus. Both viral proteins were seen to interact with the same part of the histone protein, but in different ways.

Next, Fang, Chen et al. showed that the DNA is more loosely packed when the cytomegalovirus protein is attached to the nucleosomes. This was not the case for the similar protein from the other virus. The experiments show that small differences in the ways viral proteins interact with condensed DNA can change their effects on DNA packaging. Additionally, these findings may help scientists to better understand how the binding of the cytomegalovirus protein to the nucleosomes might affect this virus’s ability to infect or cause illness in humans.

DOI:http://dx.doi.org/10.7554/eLife.11911.002

Biology and pathogenesis of cytomegalovirus in periodontal disease

Human periodontitis is associated with a wide range of bacteria and viruses and with complex innate and adaptive immune responses. Porphyromonas gingivalis, Tannerella forsythia, Aggregatibacter actinomycetemcomitans, Treponema denticola, cytomegalovirus and other herpesviruses are major suspected pathogens of periodontitis, and a combined herpesvirus–bacterial periodontal infection can potentially explain major clinical features of the disease. Cytomegalovirus infects periodontal macrophages and T‐cells and elicits a release of interleukin‐1β and tumor necrosis factor‐α. These proinflammatory cytokines play an important role in the host defense against the virus, but they also have the potential to induce alveolar bone resorption and loss of periodontal ligament. Gingival fibroblasts infected with cytomegalovirus also exhibit diminished collagen production and release of an increased level of matrix metalloproteinases. This article reviews innate and adaptive immunity to cytomegalovirus and suggests that immune responses towards cytomegalovirus can play roles in controlling, as well as in exacerbating, destructive periodontal disease.

The human cytomegalovirus non-coding Beta2.7 RNA as a novel therapeutic for Parkinson's disease--Translational research with no translation

Human cytomegalovirus (HCMV) encodes abundant numbers of microRNAs (miRNAs) and other non-coding RNAs (ncRNAs) whose functions are presently under intense investigation. In this chapter, we discuss the function of one of the more well characterised virus-encoded ncRNAs, derived from the viral major early gene (Beta2.7). This RNA plays an anti-apoptotic role during infection by directly interacting with mitochondrial complex I to help maintain high levels of ATP production and by preventing the stress induced re-localisation of retinoid/interferon-induced mortality-19 protein, GRIM-19. We then go on to describe how an 800 nucleotide sub-domain of the Beta2.7 transcript, p137, has been exploited in the development of a novel therapeutic for the treatment of Parkinson's disease.

Analysis of the functional interchange between the IE1 and pp71 proteins of human cytomegalovirus and ICP0 of herpes simplex virus 1

Human cytomegalovirus (HCMV) immediate early protein IE1 and the tegument protein pp71 are required for efficient infection. These proteins have some functional similarities with herpes simplex virus 1 (HSV-1) immediate early protein ICP0, which stimulates lytic HSV-1 infection and derepresses quiescent HSV-1 genomes. All three proteins counteract antiviral restriction mediated by one or more components of promyelocytic leukemia (PML) nuclear bodies, and IE1 and pp71, acting together, almost completely complement ICP0 null mutant HSV-1. Here, we investigated whether ICP0 might substitute for IE1 or pp71 during HCMV infection. Using human fibroblasts that express ICP0, IE1, or pp71 in an inducible manner, we found that ICP0 stimulated replication of both wild-type (wt) and pp71 mutant HCMV while IE1 increased wt HCMV plaque formation and completely complemented the IE1 mutant. Although ICP0 stimulated IE2 expression from IE1 mutant HCMV and increased the number of IE2-positive cells, it could not compensate for IE1 in full lytic replication. These results are consistent with previous evidence that both IE1 and IE2 are required for efficient HCMV gene expression, but they also imply that IE2 functionality is influenced specifically by IE1, either directly or indirectly, and that IE1 may include sequences that have HCMV-specific functions. We discovered a mutant form of IE1 (YL2) that fails to stimulate HCMV infection while retaining 30 to 80% of the activity of the wt protein in complementing ICP0 null mutant HSV-1. It is intriguing that the YL2 mutation is situated in the region of IE1 that is shared with IE2 and which is highly conserved among primate cytomegaloviruses.

IMPORTANCE

Herpesvirus gene expression can be repressed by cellular restriction factors, one group of which is associated with structures known as ND10 or PML nuclear bodies (PML NBs). Regulatory proteins of several herpesviruses interfere with PML NB-mediated repression, and in some cases their activities are transferrable between different viruses. For example, the requirement for ICP0 during herpes simplex virus 1 (HSV-1) infection can be largely replaced by ICP0-related proteins expressed by other alphaherpesviruses and even by a combination of the unrelated IE1 and pp71 proteins of human cytomegalovirus (HCMV). Here, we report that ICP0 stimulates gene expression and replication of wt HCMV but cannot replace the need for IE1 during infection by IE1-defective HCMV mutants. Therefore, IE1 includes HCMV-specific functions that cannot be replaced by ICP0.

Immediate-Early (IE) gene regulation of cytomegalovirus: IE1- and pp71-mediated viral strategies against cellular defenses

Three crucial hurdles hinder studies on human cytomegalovirus (HCMV): strict species specificity, differences between in vivo and in vitro infection, and the complexity of gene regulation. Ever since the sequencing of the whole genome was first accomplished, functional studies on individual genes have been the mainstream in the CMV field. Gene regulation has therefore been elucidated in a more detailed fashion. However, viral gene regulation is largely controlled by both cellular and viral components. In other words, viral gene expression is determined by the virus-host interaction. Generally, cells respond to viral infection in a defensive pattern; at the same time, viruses try to counteract the cellular defense or else hide in the host (latency). Viruses evolve effective strategies against cellular defense in order to achieve replicative success. Whether or not they are successful, cellular defenses remain in the whole viral replication cycle: entry, immediate-early (IE) gene expression, early gene expression, DNA replication, late gene expression, and viral egress. Many viral strategies against cellular defense, and which occur in the immediate-early time of viral infection, have been documented. In this review, we will summarize the documented biological functions of IE1 and pp71 proteins, especially with regard to how they counteract cellular intrinsic defenses.

Human cytomegalovirus pUL79 is an elongation factor of RNA polymerase II for viral gene transcription

In this study, we have identified a unique mechanism in which human cytomegalovirus (HCMV) protein pUL79 acts as an elongation factor to direct cellular RNA polymerase II for viral transcription during late times of infection. We and others previously reported that pUL79 and its homologues are required for viral transcript accumulation after viral DNA synthesis. We hypothesized that pUL79 represented a unique mechanism to regulate viral transcription at late times during HCMV infection. To test this hypothesis, we analyzed the proteome associated with pUL79 during virus infection by mass spectrometry. We identified both cellular transcriptional factors, including multiple RNA polymerase II (RNAP II) subunits, and novel viral transactivators, including pUL87 and pUL95, as protein binding partners of pUL79. Co-immunoprecipitation (co-IP) followed by immunoblot analysis confirmed the pUL79-RNAP II interaction, and this interaction was independent of any other viral proteins. Using a recombinant HCMV virus where pUL79 protein is conditionally regulated by a protein destabilization domain ddFKBP, we showed that this interaction did not alter the total levels of RNAP II or its recruitment to viral late promoters. Furthermore, pUL79 did not alter the phosphorylation profiles of the RNAP II C-terminal domain, which was critical for transcriptional regulation. Rather, a nuclear run-on assay indicated that, in the absence of pUL79, RNAP II failed to elongate and stalled on the viral DNA. pUL79-dependent RNAP II elongation was required for transcription from all three kinetic classes of viral genes (i.e. immediate-early, early, and late) at late times during virus infection. In contrast, host gene transcription during HCMV infection was independent of pUL79. In summary, we have identified a novel viral mechanism by which pUL79, and potentially other viral factors, regulates the rate of RNAP II transcription machinery on viral transcription during late stages of HCMV infection.

In this study, we report a novel mechanism used by human cytomegalovirus (HCMV) to regulate the elongation rate of RNA polymerase II (RNAP II) to facilitate viral transcription during late stages of infection. Recently, we and others have identified several viral factors that regulate gene expression during late infection. These factors are functionally conserved among beta- and gamma- herpesviruses, suggesting a unique transcriptional regulation shared by viruses of these two subfamilies. However, the mechanism remains elusive. Here we show that HCMV pUL79, one of these factors, interacts with RNAP II as well as other viral factors involved in late gene expression. We have started to elucidate the nature of the pUL79-RNAP II interaction, finding that pUL79 does not alter the protein levels of RNAP II or its recruitment to viral promoters. However, during late times of infection, pUL79 helps RNAP II efficiently elongate along the viral DNA template to transcribe HCMV genes. Host genes are not regulated by this pUL79-mediated mechanism. Therefore, our study discovers a previously uncharacterized mechanism where RNAP II activity is modulated by viral factor pUL79, and potentially other viral factors as well, for coordinated viral transcription.

Long ncRNAs expressed during human cytomegalovirus infections

ABSTRACT: 

Human cytomegalovirus (HCMV) has a tremendous coding capacity within its dsDNA genome that has allowed it to coevolve with its host. Transcription of the virus genome is not limited to protein-coding genes; in fact, most of the transcription from the HCMV genome during lytic replication generates viral ncRNAs that are not translated into protein. Four long ncRNAs (RNA5.0, RNA4.9, RNA1.2 and RNA2.7) account for the majority of HCMV transcription during lytic replication. Here, we review the expression and function of these long ncRNAs in the context of virus replication and pathogenesis. Long ncRNAs may contribute to HCMV evasion of the host response and manipulate cellular and viral programs to successfully persist throughout the lifetime of its host.

Restriction factors against human CMV

Cellular proteins called 'restriction factors' (RFs) form an important component of the innate immune response to viral replication. However, viruses have learned how to antagonize RFs through mechanisms that are specific for each virus. Here, we summarize the general hallmarks of RFs before going on to discuss the specific strategies recruited by some key RFs that strive to hold human CMV (HCMV) infection back, as well as the counter-restriction mechanisms employed by the virus to overcome this innate defense. Such RFs include the cellular constituents of nuclear domain 10 (ND10), and IFI16, a nuclear member of the PYHIN protein family. Viral regulatory proteins, such as IE1 or pp71, try to oppose the ND10-induced blockade of virus replication by either modifying or disrupting this RF. IFI16, on the other hand, inhibits virus DNA synthesis by downregulating the transcription of viral gene UL54; the intruding virus attempts to antagonize IFI16 by mislocalizing it from the nucleus to the cytoplasm via the action of viral protein UL97. Finally, we consider how Viperin, a RF initially thought to inhibit HCMV maturation late during infection, has actually been demonstrated to enhance virus maturation by increasing lipid metabolism and enhancing virus envelopment.

The human cytomegalovirus IE1 protein: past and present developments

ABSTRACT: 

Human cytomegalovirus (HCMV), a member of the β-herpesvirus subfamily, is an important pathogen that infects the majority of the human population. The evolutionary success of HCMV largely depends on its ability to evade host defense systems and establish a lifelong persistence after primary infection. In fact, HCMV has dedicated a considerable part of its gene products to manipulate or disable immune effector processes. This review focuses on the major immediate–early protein IE1 – a multifunctional key regulator that has the capacity to counteract the first host defense activities. We summarize the known structural and mechanistic features by which IE1 modulates innate immune mechanisms as well as other cellular processes, and discuss how the individual functions of IE1 contribute to the success of a lytic HCMV infection.

Human cytomegalovirus induces upregulation of arginase II: possible implications for vasculopathies

Both human cytomegalovirus (HCMV) and arginase II (ARG II) have been implicated in the pathogenesis of cardiovascular diseases. The effects of HCMV on ARG II are unknown. The aim of this study was to investigate the effects of HCMV on ARG II expression in endothelial and vascular smooth muscle cells (SMC) both in vitro and ex vivo. Endothelial and SMC were infected with either HCMV or UV-irradiated HCMV. Expression of ARG II, endothelial or inducible nitric oxide synthase (eNOS and iNOS, respectively) and viral immediate early (IE) was quantified using quantitative PCR. Ganciclovir and short interfering RNA were used to determine the viral gene mediating the effects on ARG II. Detection of viral antigens and ARG II expression was performed by immunofluorescence or immunohistochemistry. HCMV infection increased both ARG II mRNA and protein levels in the examined cells; this effect was mediated by the HCMV IE2-p86 protein. The upregulation of ARG II was accompanied by a downregulation of eNOS but an induction of iNOS in HCMV-infected endothelial cells. Both eNOS and iNOS expressions were induced in HCMV-infected SMC. ARG II was abundantly expressed in endothelial cells, foam cells and SMC and was importantly significantly upregulated in HCMV-immunoreactive human carotid atherosclerotic plaques. HCMV IE2-p86 mediates ARG II upregulation in vitro and ARG II is co-expressed with HCMV antigens in human carotid atherosclerotic plaques. We speculate that HCMV may contribute to endothelial dysfunction via ARG II induction and reduced eNOS production.

Human cytomegalovirus major immediate early 1 protein targets host chromosomes by docking to the acidic pocket on the nucleosome surface

The 72-kDa immediate early 1 (IE1) protein encoded by human cytomegalovirus (hCMV) is a nuclearly localized promiscuous regulator of viral and cellular transcription. IE1 has long been known to associate with host mitotic chromatin, yet the mechanisms underlying this interaction have not been specified. In this study, we identify the cellular chromosome receptor for IE1. We demonstrate that the viral protein targets human nucleosomes by directly binding to core histones in a nucleic acid-independent manner. IE1 exhibits two separable histone-interacting regions with differential binding specificities for H2A-H2B and H3-H4. The H2A-H2B binding region was mapped to an evolutionarily conserved 10-amino-acid motif within the chromatin-tethering domain (CTD) of IE1. Results from experimental approaches combined with molecular modeling indicate that the IE1 CTD adopts a β-hairpin structure, docking with the acidic pocket formed by H2A-H2B on the nucleosome surface. IE1 binds to the acidic pocket in a way similar to that of the latency-associated nuclear antigen (LANA) of the Kaposi's sarcoma-associated herpesvirus. Consequently, the IE1 and LANA CTDs compete for binding to nucleosome cores and chromatin. Our work elucidates in detail how a key viral regulator is anchored to human chromosomes and identifies the nucleosomal acidic pocket as a joint target of proteins from distantly related viruses. Based on the striking similarities between the IE1 and LANA CTDs and the fact that nucleosome targeting by IE1 is dispensable for productive replication even in "clinical" strains of hCMV, we speculate that the two viral proteins may serve analogous functions during latency of their respective viruses.

Transcription of true late (γ2) cytomegalovirus genes requires UL92 function that is conserved among beta- and gammaherpesviruses

Human cytomegalovirus-encoded UL92 plays an essential role in viral replication that has not been resolved. We show here that this gene controls the accumulation of true late (γ2) viral transcripts, a property shared with several other recently evaluated genes (UL79, UL87, UL91, and UL95) conserved among beta- and gammaherpesviruses. When the UL92 mutant virus was evaluated, function was fully complemented by either the natural protein or the homologous Rh127 protein from rhesus cytomegalovirus. N-terminal epitope-tagged UL92 protein is functional, follows complex early-late expression kinetics, and localizes in the nucleus within viral replication compartments. UL92 severely impacts the late (72-h postinfection) expression of nine genes encoding virion proteins (UL32, UL55, UL73, UL75, UL80, UL86, UL99, and UL115), as well as UL91 and itself, but does not influence the levels of UL44, UL82, or UL83 accumulation. Although viral DNA is made at normal levels, viral capsid accumulation in the nucleus is severely compromised in UL92 mutant virus-infected cells, and mature virions are not observed in the cytoplasm. Taken together, UL92 is a key regulator of late viral gene expression, apparently functioning with four other beta- or gammaherpesvirus gene products in a pattern that appears reminiscent of gene regulation in T4 DNA bacteriophage.

Cytomegalovirus UL91 is essential for transcription of viral true late (γ2) genes

Human cytomegalovirus-encoded UL91 is a betagamma gene that is essential for viral replication. Here we show that the 111-amino-acid (aa) UL91 protein controls accumulation of true-late (γ2) viral transcripts. The primate betaherpesvirus conserved N-terminal region from aa 1 to 71 is sufficient to fully reconstitute function. Evaluation of viral DNA, RNA, and antigen revealed that UL91 protein is expressed with leaky-late (γ1) kinetics, localizes in the nucleus without influencing viral DNA synthesis, and must be present from 48 h postinfection to support full expression of late viral transcripts and proteins. In the absence of UL91, viral capsid assembly in the nucleus of infected cells is significantly reduced, and mature, cytoplasmic virions fail to form. Taken together, the evidence shows that UL91 regulates late viral gene expression by a mechanism that is apparently conserved in betaherpesviruses and gammaherpesviruses.

Interplay between viruses and host sumoylation pathways

Post-translational modification by members of the small ubiquitin-like modifier (SUMO) family of proteins is important for the regulation of many cellular proteins and pathways. As obligate parasites, viruses must engage with the host cell throughout their replication cycles, and it is therefore unsurprising that there are many examples of interplay between viral proteins and the host sumoylation system. This article reviews recent advances in this field, summarizing information on sumoylated viral proteins, the varied ways in which viruses engage with SUMO-related pathways, and the consequences of these interactions for viral replication and engagement with innate and intrinsic immunity.

Viral product trafficking to mitochondria, mechanisms and roles in pathogenesis

A wide variety of viruses cause significant morbidity and mortality in humans. However, targeted antiviral therapies have been developed for only a subset of these viruses, with the majority of currently licensed antiviral drugs targeting viral entry, replication or exit steps during the viral life cycle. Due to increasing emergence of antiviral drug resistant viruses, the isolation of multiple viral subtypes, and toxicities of existing therapies, there remains an urgent need for the timely development of novel antiviral agents, including those targeting host factors essential for viral replication. This review summarizes viral products that target mitochondria and their effects on common mitochondria regulated pathways. These viral products and the mitochondrial pathways affected by them provide potential novel targets for the rational design of antiviral drugs. Viral products alter oxidative balance, mitochondrial permeability transition pore, mitochondrial membrane potential, electron transport and energy production. Moreover, viruses may cause the Warburg Effect, in which metabolism is reprogrammed to aerobic glycolysis as the main source of energy. Finally, viral products affect proapoptotic and antiapoptotic signaling, as well as mitochondrial innate immune signaling. Because of their importance for the generation of metabolic intermediates and energy as well as cell survival, mitochondrial pathways are targeted by multiple independent viral products. Structural modifications of existing drugs targeted to mitochondrial pathways may lead to the development of novel antiviral drugs with improved efficacy and reduced toxicity.

Histone modifications in herpesvirus infections

In eukaryotic cells, gene expression is not only regulated by transcription factors but also by several epigenetic mechanisms including post-translational modifications of histone proteins. There are numerous histone modifications described to date and methylation, acetylation, ubiquitination and phosphorylation are amongst the best studied. In parallel, certain viruses interact with the very same regulatory mechanisms, hereby manipulating the normal epigenetic landscape of the host cell, to fit their own replication needs. This review concentrates on herpesviruses specifically and how they interfere with the histone-modifying enzymes to regulate their replication cycles. Herpesviruses vary greatly with respect to the cell types they infect and the clinical diseases they cause, yet they share various common features including their capacity to encode viral proteins which affect and interfere with the normal functions of histone-modifying enzymes. Studying the epigenetic manipulation/dysregulation of herpesvirus-host interactions not only generates novel insights into the pathogenesis of these viruses but may also have important therapeutic implications.

The chromatin-tethering domain of human cytomegalovirus immediate-early (IE) 1 mediates associations of IE1, PML and STAT2 with mitotic chromosomes, but is not essential for viral replication

Human cytomegalovirus (HCMV) immediate-early (IE) 1 protein associates with chromosomes in mitotic cells using its carboxyl-terminal 16 aa region. However, the role of this IE1 activity in viral growth has not been evaluated in the context of mutant virus infection. We produced a recombinant HCMV encoding mutant IE1 with the carboxyl-terminal chromosome-tethering domain (CTD) deleted. This IE1(ΔCTD) virus grew like the wild-type virus in fibroblasts, indicating that the CTD is not essential for viral replication in permissive cells. Unlike wild-type virus infections, PML and STAT2, which interact with IE1, did not accumulate at mitotic chromosomes in IE1(ΔCTD) virus-infected fibroblasts, demonstrating that their associations with chromosomes are IE1 CTD-dependent. IE1 SUMOylation did not affect IE1 association with chromosomes. Our results provide genetic evidence that the CTD is required for the associations of IE1, PML and STAT2 with mitotic chromosomes, but that these IE1-related activities are not essential for viral replication in fibroblasts.

The enhanced host-cell permissiveness of human cytomegalovirus is mediated by the Ras signaling pathway

Human cytomegalovirus utilizes cellular signal transduction pathways to activate viral or cellular transcription factors involved in the control of viral gene expression and DNA replication. In the present study, we demonstrate that Harvey-ras-transformed cells show increased permissiveness to human cytomegalovirus when compared to their parental non-transformed cells. Both the progeny viral yield and the protein levels were elevated in the human cytomegalovirus-infected Harvey-ras-transformed cells requiring active viral gene replication, as shown by the infection with UV-inactivated human cytomegalovirus. Inhibition of Ras or of key molecules of the Ras pathway, effectively suppressed viral infection in the Harvey-ras-transformed cells. On a cellular level, the human cytomegalovirus-infected Harvey-ras-transformed cells formed larger cellular foci, which were significantly higher in number, compared to the uninfected cells and preferentially recruited human cytomegalovirus virions, thereby incriminating human cytomegalovirus infection for the increased transformation of these cells. Furthermore, proliferation assays revealed a higher rate for the human cytomegalovirus-infected Harvey-ras-transformed cells compared to mock-infected cells, whereas human cytomegalovirus infection had no considerable effect on the proliferation of the non-transformed cells. Higher susceptibility to apoptosis was also detected in the human cytomegalovirus-infected ras-transformed cells, which in combination with the higher progeny virus reveals a mode by which human cytomegalovirus achieves efficient spread of infection in the cells expressing the oncogenic Harvey-ras (12V) gene. Collectively, our data suggest that human cytomegalovirus employs the host-cell Ras signaling pathway to ensue viral expression and ultimately successful propagation. Transformed cells with an activated Ras signaling pathway are therefore particularly susceptible to human cytomegalovirus infection.

Early inhibitors of human cytomegalovirus: state-of-art and therapeutic perspectives

Human cytomegalovirus (HCMV) infection is associated with severe morbidity and mortality in immunocompromised individuals, mainly transplant recipients and AIDS patients, and is the most frequent cause of congenital malformations in newborn children. To date, few drugs are licensed for the treatment of HCMV infections, most of which target the viral DNA polymerase and suffer from many drawbacks, including long-term toxicity, low potency, and poor bioavailability. In addition, the emergence of drug-resistant viral strains is becoming an increasing problem for disease management. Finally, none of the current anti-HCMV drugs have been approved for the treatment of congenital infections. For all these reasons, there is still a strong need for new anti-HCMV drugs with novel mechanisms of action. The first events of the virus replication cycle, including attachment, entry, immediate-early gene expression, and immediate-early functions—in particular that of Immediate-Early 2 protein—represent attractive targets for the development of novel antiviral compounds. Such inhibitors would block not only the expression of viral immediate-early proteins, which play a key role in the pathogenesis of HCMV infection, but also the host immunomodulation and the changes to cell physiology induced by the first events of virus infection. This review describes the current knowledge on the initial phases of HCMV replication, their validation as potential novel antiviral targets, and the development of compounds that block such processes.

Human cytomegalovirus IE1 protein elicits a type II interferon-like host cell response that depends on activated STAT1 but not interferon-γ

Human cytomegalovirus (hCMV) is a highly prevalent pathogen that, upon primary infection, establishes life-long persistence in all infected individuals. Acute hCMV infections cause a variety of diseases in humans with developmental or acquired immune deficits. In addition, persistent hCMV infection may contribute to various chronic disease conditions even in immunologically normal people. The pathogenesis of hCMV disease has been frequently linked to inflammatory host immune responses triggered by virus-infected cells. Moreover, hCMV infection activates numerous host genes many of which encode pro-inflammatory proteins. However, little is known about the relative contributions of individual viral gene products to these changes in cellular transcription. We systematically analyzed the effects of the hCMV 72-kDa immediate-early 1 (IE1) protein, a major transcriptional activator and antagonist of type I interferon (IFN) signaling, on the human transcriptome. Following expression under conditions closely mimicking the situation during productive infection, IE1 elicits a global type II IFN-like host cell response. This response is dominated by the selective up-regulation of immune stimulatory genes normally controlled by IFN-γ and includes the synthesis and secretion of pro-inflammatory chemokines. IE1-mediated induction of IFN-stimulated genes strictly depends on tyrosine-phosphorylated signal transducer and activator of transcription 1 (STAT1) and correlates with the nuclear accumulation and sequence-specific binding of STAT1 to IFN-γ-responsive promoters. However, neither synthesis nor secretion of IFN-γ or other IFNs seems to be required for the IE1-dependent effects on cellular gene expression. Our results demonstrate that a single hCMV protein can trigger a pro-inflammatory host transcriptional response via an unexpected STAT1-dependent but IFN-independent mechanism and identify IE1 as a candidate determinant of hCMV pathogenicity.

Alternative splicing of the human cytomegalovirus major immediate-early genes affects infectious-virus replication and control of cellular cyclin-dependent kinase

The major immediate-early (MIE) gene locus of human cytomegalovirus (HCMV) is the master switch that determines the outcomes of both lytic and latent infections. Here, we provide evidence that alteration in the splicing of HCMV (Towne strain) MIE genes affects infectious-virus replication, movement through the cell cycle, and cyclin-dependent kinase activity. Mutation of a conserved 24-nucleotide region in MIE exon 4 increased the abundance of IE1-p38 mRNA and decreased the abundance of IE1-p72 and IE2-p86 mRNAs. An increase in IE1-p38 protein was accompanied by a slight decrease in IE1-p72 protein and a significant decrease in IE2-p86 protein. The mutant virus had growth defects, which could not be complemented by wild-type IE1-p72 protein in trans. The phenotype of the mutant virus could not be explained by an increase in IE1-p38 protein, but prevention of the alternate splice returned the recombinant virus to the wild-type phenotype. The lower levels of IE1-p72 and IE2-p86 proteins correlated with a delay in early and late viral gene expression and movement into the S phase of the cell cycle. Mutant virus-infected cells had significantly higher levels of cdk-1 expression and enzymatic activity than cells infected with wild-type virus. The mutant virus induced a round-cell phenotype that accumulated in the G(2)/M compartment of the cell cycle with condensation and fragmentation of the chromatin. An inhibitor of viral DNA synthesis increased the round-cell phenotype. The round cells were characteristic of an abortive viral infection.

Human cytomegalovirus early protein pUL21a promotes efficient viral DNA synthesis and the late accumulation of immediate-early transcripts

We have previously reported that a newly annotated gene of human cytomegalovirus (HCMV), UL21a, encodes an early viral protein termed pUL21a. Most notably, the virions of a UL21a deletion virus had markedly reduced infectivity, indicating that UL21a is required to establish an efficient productive infection. In this study, we infected fibroblasts with equal numbers of DNA-containing viral particles and identified where in the viral life cycle pUL21a acted. The UL21a deletion virus entered cells and initiated viral gene expression efficiently; however, it synthesized viral DNA poorly and accumulated several immediate-early (IE) transcripts at reduced levels at late times of infection. The defect in viral DNA synthesis preceded that in gene expression, and inhibition of viral DNA synthesis reduced the late accumulation of IE transcripts in both wild-type and mutant virus-infected cells to equivalent levels. This suggests that reduced viral DNA synthesis is the cause of reduced IE gene expression in the absence of UL21a. The growth of UL21a deletion virus was similar to that of recombinant HCMV in which pUL21a expression was abrogated by stop codon mutations, and the defect was rescued in pUL21a-expressing fibroblasts. pUL21a expression in trans was sufficient to restore viral DNA synthesis and gene expression of mutant virus produced from normal fibroblasts, whereas mutant virus produced from complementing cells still exhibited the defect in normal fibroblasts. Thus, pUL21a does not promote the functionality of HCMV virions; rather, its de novo synthesis facilitates viral DNA synthesis, which is necessary for the late accumulation of IE transcripts and establishment of a productive infection.

Differential relocation and stability of PML-body components during productive human cytomegalovirus infection: detailed characterization by live-cell imaging

In controlling the switch from latency to lytic infection, the immediate early (IE) genes lie at the core of herpesvirus pathogenesis. To image the 72kDa human cytomegalovirus (HCMV) major IE protein (IE1-72K), a recombinant virus encoding IE1 fused with EGFP was constructed. Using this construct, the IE1-EGFP fusion was detected at ND10 (PML-bodies) within 2h post infection (p.i.) and the complete disruption of ND10 imaged through to 6h p.i. HCMV genomes and IE2-86K protein could be detected adjacent to the slowly degrading IE1-72K/ND10 foci. IE1-72K associates with metaphase chromatin, recruiting both PML and STAT2. hDaxx, STAT1 and IE2-86K did not re-locate to metaphase chromatin; the fate of hDaxx is particularly important as this protein contributes to an intrinsic barrier to HCMV infection. While IE1-72K participates in a complex with chromatin, PML, STAT2 and Sp100, IE1-72K releases hDaxx from ND10 yet does not appear to remain associated with it.

Human cytomegalovirus IE72 protein interacts with the transcriptional repressor hDaxx to regulate LUNA gene expression during lytic infection

A putative latency-associated transcript (LUNA) complementary to the human cytomegalovirus (HCMV) UL81-82 region previously identified in seropositive donors' monocytes is also expressed during lytic infection. Thus, the LUNA promoter is active during both lytic and latent infection. Consequently, the mechanisms regulating this promoter may provide further insight into factors that determine whether the outcome of HCMV infection is latent or lytic. By transfection, the LUNA promoter exhibited low but reproducible activity. Substantial activation by virus infection suggested that a viral factor was important for LUNA expression during lytic infection. IE72, a known transactivator of viral promoters, activated the LUNA promoter in cotransfection assays. Furthermore, coinfection with wild-type HCMV but not an IE72 deletion virus (CR208) also activated the LUNA promoter. Finally, diminished LUNA gene expression in CR208 virus-infected cells supported a role for IE72 in LUNA gene expression. The initial regulation of herpesvirus immediate-early gene expression is associated with proteins found at cellular nuclear domain 10 (ND10) bodies, such as PML, hDaxx, and ATRX. hDaxx transfection repressed LUNA promoter activity. Furthermore, we observed binding of hDaxx to the LUNA promoter, which was abrogated by IE72 gene expression via direct interaction. Finally, we show that small interfering RNA (siRNA) knockdown of the hDaxx interaction partner ATRX rescued LUNA gene expression in CR208-infected cells. Overall, these data show that hDaxx/ATRX-mediated repression of LUNA during lytic infection absolutely requires IE72 gene expression. It also suggests that the targeting of cellular factors by IE72 is important throughout the different phases of HCMV gene expression during productive infection.

The human cytomegalovirus major immediate-early proteins as antagonists of intrinsic and innate antiviral host responses

The major immediate-early (IE) gene of human cytomegalovirus (CMV) is believed to have a decisive role in acute infection and its activity is an important indicator of viral reactivation from latency. Although a variety of gene products are expressed from this region, the 72-kDa IE1 and the 86-kDa IE2 nuclear phosphoproteins are the most abundant and important. Both proteins have long been recognized as promiscuous transcriptional regulators. More recently, a critical role of the IE1 and IE2 proteins in counteracting non-adaptive host cell defense mechanisms has been revealed. In this review we will briefly summarize the available literature on IE1- and IE2-dependent mechanisms contributing to CMV evasion from intrinsic and innate immune responses.

Physical requirements and functional consequences of complex formation between the cytomegalovirus IE1 protein and human STAT2

Our previous work has shown that efficient evasion from type I interferon responses by human cytomegalovirus (hCMV) requires expression of the 72-kDa immediate-early 1 (IE1) protein. It has been suggested that IE1 inhibits interferon signaling through intranuclear sequestration of the signal transducer and activator of transcription 2 (STAT2) protein. Here we show that physical association and subnuclear colocalization of IE1 and STAT2 depend on short acidic and serine/proline-rich low-complexity motifs in the carboxy-terminal region of the 491-amino-acid viral polypeptide. These motifs compose an essential core (amino acids 373 to 420) and an adjacent ancillary site (amino acids 421 to 445) for STAT2 interaction that are predicted to form part of a natively unstructured domain. The presence of presumably "disordered" carboxy-terminal domains enriched in low-complexity motifs is evolutionarily highly conserved across all examined mammalian IE1 orthologs, and the murine cytomegalovirus IE1 protein appears to interact with STAT2 just like the human counterpart. A recombinant hCMV specifically mutated in the IE1 core STAT2 binding site displays hypersensitivity to alpha interferon, delayed early viral protein accumulation, and attenuated growth in fibroblasts. However, replication of this mutant virus is specifically restored by knockdown of STAT2 expression. Interestingly, complex formation with STAT2 proved to be entirely separable from disruption of nuclear domain 10 (ND10), another key activity of IE1. Finally, our results demonstrate that IE1 counteracts the antiviral interferon response and promotes viral replication by at least two distinct mechanisms, one depending on sequestration of STAT2 and the other one likely involving ND10 interaction.

Hexamethylene bisacetamide can convert nonpermissive human cells to a permissive state for expressing the major immediate-early genes of human cytomegalovirus by up-regulating NF-kappaB activity

Expression of the major immediate-early (MIE) genes of human cytomegalovirus (HCMV) in the human thyroid papillary carcinoma cell line TPC-1 is repressed at the transcriptional level. However, treatment of these cells with hexamethylene bisacetamide (HMBA), a chemical inducer of differentiation, for 12 to 24 h before infection enabled the cells to support IE1 and IE2 gene expression and consequently HCMV replication. In HMBA-treated cells the transcription factor NF-kappaB was induced and the MIE promoter (MIEP) was activated. The presence of a NF-kappaB inhibitory peptide SN-50 or expression of a dominant negative IkappaBalpha protein during the HMBA pretreatment period efficiently prevented the HMBA-induced MIEP activation and MIE protein synthesis. Moreover, introduction of mutations into the NF-kappaB binding sites in the MIEP in a plasmid expressing the IE1 protein diminished its ability to express the protein in HMBA-treated cells. Therefore, the NF-kappaB activity previously induced in HMBA-treated cells and the NF-kappaB sites in the MIEP were shown to be essential for HCMV to respond to HMBA action and to express the MIE genes. Investigation of the mechanisms by which HMBA activates NF-kappaB revealed that degradation of IkappaBalpha and translocation of the phosphorylated NF-kappaB p65 subunit to the nucleus, both of which are known to be critical steps in NF-kappaB activation, are stimulated in the HMBA-treated cells. These results indicate that treatment of nonpermissive TPC-1 cells with HMBA induces MIE gene permissiveness by up-regulating NF-kappaB activity.

Temporal dynamics of cytomegalovirus chromatin assembly in productively infected human cells

The genomes of herpesviruses, including human cytomegalovirus (CMV), are double-stranded DNA molecules maintained as episomes during infection. The viral DNA lacks histones when encapsidated in the virion. However, it has been found histone associated inside infected cells, implying unidentified chromatin assembly mechanisms. Our results indicate that components of the host cell nucleosome deposition machinery target intranuclear CMV DNA, resulting in stepwise viral-chromatin assembly. CMV genomes undergo limited histone association and nucleosome assembly as early as 30 min after infection via DNA replication-independent mechanisms. Low average viral-genome chromatinization is maintained throughout the early stages of infection. The late phase of infection is characterized by a striking increase in average histone occupancy coupled with the process of viral-DNA replication. While the initial chromatinization affected all analyzed parts of the CMV chromosome, a subset of viral genomic regions, including the major immediate-early promoter, proved to be largely resistant to replication-dependent histone deposition. Finally, our results predict the likely requirement for an unanticipated chromatin disassembly process that enables packaging of histone-free DNA into progeny capsids.

Internal deletions of IE2 86 and loss of the late IE2 60 and IE2 40 proteins encoded by human cytomegalovirus affect the levels of UL84 protein but not the amount of UL84 mRNA or the loading and distribution of the mRNA on polysomes

The major immediate-early (IE) region of human cytomegalovirus encodes two IE proteins, IE1 72 and IE2 86, that are translated from alternatively spliced transcripts that differ in their 3' ends. Two other proteins that correspond to the C-terminal region of IE2 86, IE2 60 and IE2 40, are expressed at late times. In this study, we used IE2 mutant viruses to examine the mechanism by which IE2 86, IE2 60, and IE2 40 affect the expression of a viral DNA replication factor, UL84. Deletion of amino acids (aa) 136 to 290 of IE2 86 results in a significant decrease in UL84 protein during the infection. This loss of UL84 is both proteasome and calpain independent, and the stability of the protein in the context of infection with the mutant remains unaffected. The RNA for UL84 is expressed to normal levels in the mutant virus-infected cells, as are the RNAs for two other proteins encoded by this region, UL85 and UL86. Moreover, nuclear-to-cytoplasmic transport and the distribution of the UL84 mRNA on polysomes are unaffected. A region between aa 290 and 369 of IE2 86 contributes to the UL84-IE2 86 interaction in vivo and in vitro. IE2 86, IE2 60, and IE2 40 are each able to interact with UL84 in the mutant-infected cells, suggesting that these interactions may be important for the roles of UL84 and the IE2 proteins. Thus, these data have defined the contribution of IE2 86, IE2 60, and IE2 40 to the efficient expression of UL84 throughout the infection.

Human cytomegalovirus DNA replication: antiviral targets and drugs

Human cytomegalovirus (HCMV) infection is associated with severe morbidity and mortality in immunocompromised individuals, in particular transplant recipients and AIDS patients, and is the most frequent congenital viral infection in humans. There are currently five drugs approved for HCMV treatment: ganciclovir and its prodrug valganciclovir, foscarnet, cidofovir and fomivirsen. These drugs have provided a major advance in HCMV disease management, but they suffer from poor bioavailability, significant toxicity and limited effectiveness, mainly due to the development of drug resistance. Fortunately, there are several novel and potentially very effective new compounds which are under pre-clinical and clinical evaluation and may address these limitations. This review focuses on HCMV proteins that are directly or indirectly involved in viral DNA replication and represent already established or potential novel antiviral targets, and describes both currently available drugs and new compounds against such protein targets.

Binding STAT2 by the acidic domain of human cytomegalovirus IE1 promotes viral growth and is negatively regulated by SUMO

The human cytomegalovirus (HCMV) 72-kDa immediate-early 1 (IE1) protein is thought to modulate cellular antiviral functions impacting on promyelocytic leukemia (PML) nuclear bodies and signal transducer and activator of transcription (STAT) signaling. IE1 consists of four distinct regions: an amino-terminal region required for nuclear localization, a large central hydrophobic region responsible for PML targeting and transactivation activity, an acidic domain, and a carboxyl-terminal chromatin tethering domain. We found that the acidic domain of IE1 is required for binding to STAT2. A mutant HCMV encoding IE1(Delta421-475) with the acidic domain deleted was generated. In mutant virus-infected cells, IE1(Delta421-475) failed to bind to STAT2. The growth of mutant virus was only slightly delayed at a high multiplicity of infection (MOI) but was severely impaired at a low MOI with low-level accumulation of viral proteins. When cells were pretreated with beta interferon, the mutant virus showed an additional 1,000-fold reduction in viral growth, even at a high MOI, compared to the wild type. The inhibition of STAT2 loading on the target promoter upon infection was markedly reduced with mutant virus. Furthermore, sumoylation of IE1 at this acidic domain was found to abolish the activity of IE1 to bind to STAT2 and repress the interferon-stimulated genes. Our results provide genetic evidence that IE1 binding to STAT2 requires the 55-amino-acid acidic domain and promotes viral growth by interfering with interferon signaling and demonstrate that this viral activity is negatively regulated by a cellular sumoylation pathway.

Transactivation of cellular genes involved in nucleotide metabolism by the regulatory IE1 protein of murine cytomegalovirus is not critical for viral replicative fitness in quiescent cells and host tissues

Despite its high coding capacity, murine CMV (mCMV) does not encode functional enzymes for nucleotide biosynthesis. It thus depends on cellular enzymes, such as ribonucleotide reductase (RNR) and thymidylate synthase (TS), to be supplied with deoxynucleoside triphosphates (dNTPs) for its DNA replication. Viral transactivation of these cellular genes in quiescent cells of host tissues is therefore a parameter of viral fitness relevant to pathogenicity. Previous work has shown that the IE1, but not the IE3, protein of mCMV transactivates RNR and TS gene promoters and has revealed an in vivo attenuation of the mutant virus mCMV-DeltaIE1. It was attractive to propose the hypothesis that lack of transactivation by IE1 and a resulting deficiency in the supply of dNTPs are the reasons for growth attenuation. Here, we have tested this hypothesis with the mutant virus mCMV-IE1-Y165C expressing an IE1 protein that selectively fails to transactivate RNR and TS in quiescent cells upon transfection while maintaining the capacity to disperse repressive nuclear domains (ND10). Our results confirm in vivo attenuation of mCMV-DeltaIE1, as indicated by a longer doubling time in host organs, whereas mCMV-IE1-Y165C replicated like mCMV-WT and the revertant virus mCMV-IE1-C165Y. Notably, the mutant virus transactivated RNR and TS upon infection of quiescent cells, thus indicating that IE1 is not the only viral transactivator involved. We conclude that transactivation of cellular genes of dNTP biosynthesis is ensured by redundancy and that attenuation of mCMV-DeltaIE1 results from the loss of other critical functions of IE1, with its function in the dispersal of ND10 being a promising candidate.

Repression of human cytomegalovirus major immediate early gene expression by the cellular transcription factor CCAAT displacement protein

Initiation of human cytomegalovirus (HCMV) productive infection is dependent on the major immediate early (MIE) genes ie1 and ie2. Several putative binding sites for CCAAT displacement protein (CDP or CUX1) were identified within the MIE promoter/regulatory region. Binding assays demonstrated binding of CUX1 to MIE-region oligonucleotides containing the CUX1 core binding sequence ATCGAT and mutagenesis of this sequence abrogated CUX1 binding. Furthermore, CUX1 repressed expression of a luciferase reporter construct controlled by the MIE promoter, and mutation of CUX1 binding sites within the promoter diminished this repressive function of CUX1. In the context of virus infection of HEK293 cells transfected with the CUX1 expression vector, CUX1 showed evidence of association with the HCMV MIE regulatory region and inhibited the capacity of the virus to express ie1 and ie2 transcripts, suggesting that this cellular factor regulates MIE gene expression following virus entry. These data identify a role for CUX1 in repressing HCMV gene expression essential for initiation of the replicative cycle.

Anti-Human Cytomegalovirus Activity of Constituents from Sasa Albo-Marginata (Kumazasa in Japan)

Background:

Effective new anti-human cytomegalovirus (HCMV) agents and regimens need to be developed. We examined the anti-HCMV properties of crude extract (True World Extract of Bambuseae sasa [TWEBS]) and five compounds ( p-coumaric acid, 3-hydroxy-4-methoxyben-zaldehyde [vanillin], p-hydroxybenzaldehyde, 3-hydroxypyridine and 4',5,7-trihydroxy-3',5'-dimethoxyflavone [tricin]), isolated from Sasa albo-marginata, a bamboo known in Japan as Sasa.

Methods:

Among TWEBS and five compounds screened in a plaque reduction assay, four showed anti-HCMV activity in the MRC-5 human embryonic fibroblast cell line. The anti-HCMV mechanisms of the TWEBS was examined by western blot analysis using primary antibody specific for an immediate early (IE) antigen of HCMV, for a structural late antigen of HCMV and for β-actin.

Results:

Treatment of cells with ⩾0.001% of TWEBS inhibited the observable cytopathic effects of HCMV on infected cells. Western blot analysis demonstrated that TWEBS decreased the expression of IE antigen and late antigen of HCMV in the infected cells. Next, we examined the anti-HCMV properties of five compounds isolated from TWEBS. In a viral plaque reduction assay, tricin showed dose-dependent inhibitory properties with a 50% effective concentration of 0.17 µug/ml (selective index =1,205.8).

Conclusions:

The hot water extract (TWEBS) of Sasa albo-marginata, with tricin isolated from it, has anti-HCMV activity in MRC-5 cells. TWEBS and/or tricin are a novel compound with potential anti-HCMV activity. Future studies should evaluate these findings in vivo.

Development of cell lines that provide tightly controlled temporal translation of the human cytomegalovirus IE2 proteins for complementation and functional analyses of growth-impaired and nonviable IE2 mutant viruses

The human cytomegalovirus (HCMV) IE2 86 protein is essential for viral replication. Two other proteins, IE2 60 and IE2 40, which arise from the C-terminal half of IE2 86, are important for later stages of the infection. Functional analyses of IE2 86 in the context of the infection have utilized bacterial artificial chromosomes as vectors to generate mutant viruses. One limitation is that many mutations result in debilitated or nonviable viruses. Here, we describe a novel system that allows tightly controlled temporal expression of the IE2 proteins and provides complementation of both growth-impaired and nonviable IE2 mutant viruses. The strategy involves creation of cell lines with separate lentiviruses expressing a bicistronic RNA with a selectable marker as the first open reading frame (ORF) and IE2 86, IE2 60, or IE2 40 as the second ORF. Induction of expression of the IE2 proteins occurs only following DNA recombination events mediated by Cre and FLP recombinases that delete the first ORF. HCMV encodes Cre and FLP, which are expressed at immediate-early (for IE2 86) and early-late (for IE2 40 and IE2 60) times, respectively. We show that the presence of full-length IE2 86 alone provides some complementation for virus production, but the correct temporal expression of IE2 86 and IE2 40 together has the most beneficial effect for early-late gene expression and synthesis of infectious virus. This approach for inducible protein translation can be used for complementation of other mutations as well as controlled expression of toxic cellular and microbial proteins.