Human cytomegalovirus contains a tegument protein that enhances transcription from promoters with upstream ATF and AP-1 cis-acting elements

Insights into the Transcriptome of Human Cytomegalovirus: A Comprehensive Review

Human cytomegalovirus (HCMV) is a widespread pathogen that poses significant risks to immunocompromised individuals. Its genome spans over 230 kbp and potentially encodes over 200 open-reading frames. The HCMV transcriptome consists of various types of RNAs, including messenger RNAs (mRNAs), long non-coding RNAs (lncRNAs), circular RNAs (circRNAs), and microRNAs (miRNAs), with emerging insights into their biological functions. HCMV mRNAs are involved in crucial viral processes, such as viral replication, transcription, and translation regulation, as well as immune modulation and other effects on host cells. Additionally, four lncRNAs (RNA1.2, RNA2.7, RNA4.9, and RNA5.0) have been identified in HCMV, which play important roles in lytic replication like bypassing acute antiviral responses, promoting cell movement and viral spread, and maintaining HCMV latency. CircRNAs have gained attention for their important and diverse biological functions, including association with different diseases, acting as microRNA sponges, regulating parental gene expression, and serving as translation templates. Remarkably, HCMV encodes miRNAs which play critical roles in silencing human genes and other functions. This review gives an overview of human cytomegalovirus and current research on the HCMV transcriptome during lytic and latent infection.

Subviral Dense Bodies of Human Cytomegalovirus Enhance Interferon-Beta Responses in Infected Cells and Impair Progeny Production

(1) Background: Infection with human cytomegalovirus (HCMV) leads to the production and release of subviral particles, termed Dense Bodies (DB). They are enclosed by a membrane resembling the viral envelope. This membrane mediates the entrance of DBs into cells in a way that is comparable to virus infection. HCMV attachment and entry trigger the induction of interferon synthesis and secretion, and the subsequent expression of interferon-regulated genes (IRGs) that might inhibit replication of the virus. Recently, we demonstrated that DBs induce a robust interferon response in the absence of infection. Little is known thus far, including how DBs influence HCMV infection and virus-host interaction. (2) Methods: Purified DBs were used to study the impact on virus replication and on the innate defense mechanisms of the cell. (3) Results: The incubation of cells with DBs at the time of infection had little effect on viral genome replication. Preincubation of DBs, however, led to a marked reduction in viral release from infected cells. These cells showed an enhancement of the cytopathic effect, associated with a moderate increase in early apoptosis. Despite virus-induced mechanisms to limit the interferon response, the induction of interferon-regulated genes (IRGs) was upregulated by DB treatment. (4) Conclusions: DBs sensitize cells against viral infection, comparable to the effects of interferons. The activities of these particles need to be considered when studying viral-host interaction.

Critical Role for the Human Cytomegalovirus Major Immediate Early Proteins in Recruitment of RNA Polymerase II and H3K27Ac To an Enhancer-Like Element in OriLyt

Human cytomegalovirus (HCMV) is an opportunistic pathogen that infects most of the population. The complex 236 kbp genome encodes more than 170 open reading frames, whose expression is temporally regulated by both viral transcriptional regulators and cellular factors that control chromatin and transcription. Here, we have used state of the art genomic technologies to investigate the viral transcriptome in conjunction with 2 key transcriptional regulators: Pol II and H3K27Ac. Although it is well known that the major immediate early (IE) proteins activate early gene expression through both direct and indirect interactions, and that histone modifications play an important role in regulating viral gene expression, the role of the IE proteins in modulating viral chromatin is not fully understood. To address this question, we have used a virus engineered for conditional expression of the IE proteins combined with RNA and Chromatin immunoprecipitation (ChIP) analyses to assess the role of these proteins in modulating both viral chromatin and gene expression. Our results show that (i) there is an enhancer-like element in OriLyt that is extraordinarily enriched in H3K27Ac; (ii) in addition to activation of viral gene expression, the IE proteins play a critical role in recruitment of Pol II and H3K27Ac to this element. IMPORTANCE HCMV is an important human pathogen associated with complications in transplant patients and birth defects. The complex program of viral gene expression is regulated by both viral proteins and host factors. Here, we have investigated the role of the immediate early proteins in regulating the viral epigenome. Our results show that the viral immediate early proteins bring about an enormous enrichment of H3K27Ac marks at the OriLyt RNA4.9 promoter, concomitant with an increase in RNA4.9 expression. This epigenetic characteristic adds importantly to the view that OriLyt has structural and functional characteristics of a strong enhancer that, we now discover, is regulated by IE proteins.

Subviral Dense Bodies of Human Cytomegalovirus Induce an Antiviral Type I Interferon Response

(1) Background: Cells infected with the human cytomegalovirus (HCMV) produce subviral particles, termed dense bodies (DBs), both in-vitro and in-vivo. They are released from cells, comparable to infectious virions, and are enclosed by a membrane that resembles the viral envelope and mediates the entry into cells. To date, little is known about how the DB uptake influences the gene expression in target cells. The purpose of this study was to investigate the impact of DBs on cells, in the absence of a viral infection. (2) Methods: Mass spectrometry, immunoblot analyses, siRNA knockdown, and a CRISPR-CAS9 knockout, were used to investigate the changes in cellular gene expression following a DB exposure; (3) Results: A number of interferon-regulated genes (IRGs) were upregulated after the fibroblasts and endothelial cells were exposed to DBs. This upregulation was dependent on the DB entry and mediated by the type I interferon signaling through the JAK-STAT pathway. The induction of IRGs was mediated by the sensing of the DB-introduced DNA by the pattern recognition receptor cGAS. (4) Conclusions: The induction of a strong type I IFN response by DBs is a unique feature of the HCMV infection. The release of DBs may serve as a danger signal and concomitantly contribute to the induction of a strong, antiviral immune response.

Human Cytomegalovirus Utilizes Multiple Viral Proteins to Regulate the Basement Membrane Protein Nidogen 1

Nidogen 1 (NID1) is an important basement membrane protein secreted by many cell types. We previously found that human cytomegalovirus (HCMV) infection rapidly induced chromosome 1 breaks and that the basement membrane protein NID1, encoded near the 1q42 break site, was downregulated. We have now determined that the specific breaks in and of themselves did not regulate NID1, rather interactions between several viral proteins and the cellular machinery and DNA regulated NID1. We screened a battery of viral proteins present by 24 hours postinfection (hpi) when regulation was induced, including components of the incoming virion and immediate early (IE) proteins. Adenovirus (Ad) delivery of the tegument proteins pp71 and UL35 and the IE protein IE1 influenced steady-state (ss) NID1 levels. IE1's mechanism of regulation was unclear, while UL35 influenced proteasomal regulation of ss NID1. Real-time quantitative PCR (RT-qPCR) experiments determined that pp71 downregulated NID1 transcription. Surprisingly, WF28-71, a fibroblast clone that expresses minute quantities of pp71, suppressed NID1 transcription as efficiently as HCMV infection, resulting in the near absence of ss NID1. Sequence analysis of the region surrounding the 1q42 break sites and NID1 promoter revealed CCCTC-binding factor (CTCF) binding sites. Chromatin immunoprecipitation experiments determined that pp71 and CTCF were both bound at these two sites during HCMV infection. Expression of pp71 alone replicated this binding. Binding was observed as early as 1 hpi, and colocalization of pp71 and CTCF occurred as quickly as 15 min postinfection (pi) in infected cell nuclei. In fibroblasts where CTCF was knocked down, Adpp71 infection did not decrease NID1 transcription nor ss NID1 protein levels. Our results emphasize another aspect of pp71 activity during infection and identify this viral protein as a key contributor to HCMV's efforts to eliminate NID1. Further, we show, for the first time, direct interaction between pp71 and the cellular genome. IMPORTANCE We have found that human cytomegalovirus (HCMV) utilizes multiple viral proteins in multiple pathways to regulate a ubiquitous cellular basement membrane protein, nidogen-1 (NID1). The extent of the resources and the redundant methods that the virus has evolved to affect this control strongly suggest that its removal provides a life cycle advantage to HCMV. Our discoveries that one of the proteins that HCMV uses to control NID1, pp71, binds directly to the cellular DNA and can exert control when present in vanishingly small quantities may have broad implications in a wide range of infection scenarios. Dysregulation of NID1 in an immunocompetent host is not known to manifest complications during infection; however, in the naive immune system of a developing fetus, disruption of this developmentally critical protein could initiate catastrophic HCMV-induced birth defects.

Control of Immediate Early Gene Expression for Human Cytomegalovirus Reactivation

Human cytomegalovirus (HCMV) is a beta herpesvirus that persists for life in the majority of the world's population. The persistence of HCMV in the human population is due to the exquisite ability of herpesviruses to establish a latent infection that evades elimination by the host immune response. How the virus moves into and out of the latent state has been an intense area of research focus and debate. The prevailing paradigm is that the major immediate early promoter (MIEP), which drives robust expression of the major immediate early (MIE) transactivators, is epigenetically silenced during the establishment of latency, and must be reactivated for the virus to exit latency and re-enter productive replication. While it is clear that the MIEP is silenced by the association of repressive chromatin remodeling factors and histone marks, the mechanisms by which HCMV de-represses MIE gene expression for reactivation are less well understood. We have identified alternative promoter elements within the MIE locus that drive a second or delayed phase of MIE gene expression during productive infection. In the context of reactivation in THP-1 macrophages and primary CD34+ human progenitor cells, MIE transcripts are predominantly derived from initiation at these alternative promoters. Here we review the mechanisms by which alternative viral promoters might tailor the control of viral gene expression and the corresponding pattern of infection to specific cell types. Alternative promoter control of the HCMV MIE locus increases versatility in the system and allows the virus to tightly repress viral gene expression for latency but retain the ability to sense and respond to cell type-specific host cues for reactivation of replication.

Activator protein-1 transactivation of the major immediate early locus is a determinant of cytomegalovirus reactivation from latency

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that latently infects hematopoietic cells and has the ability to reactivate when triggered by immunological stress. This reactivation causes significant morbidity and mortality in immune-deficient patients, who are unable to control viral dissemination. While a competent immune system helps prevent clinically detectable viremia, a portrait of the factors that induce reactivation following the proper cues remains incomplete. Our understanding of the complex molecular mechanisms underlying latency and reactivation continues to evolve. We previously showed the HCMV-encoded G protein-coupled receptor US28 is expressed during latency and facilitates latent infection by attenuating the activator protein-1 (AP-1) transcription factor subunit, c-fos, expression and activity. We now show AP-1 is a critical component for HCMV reactivation. Pharmacological inhibition of c-fos significantly attenuates viral reactivation. In agreement, infection with a virus in which we disrupted the proximal AP-1 binding site in the major immediate early (MIE) enhancer results in inefficient reactivation compared to WT. Concomitantly, AP-1 recruitment to the MIE enhancer is significantly decreased following reactivation of the mutant virus. Furthermore, AP-1 is critical for derepression of MIE-driven transcripts and downstream early and late genes, while immediate early genes from other loci remain unaffected. Our data also reveal MIE transcripts driven from the MIE promoter, the distal promoter, and the internal promoter, iP2, are dependent upon AP-1 recruitment, while iP1-driven transcripts are AP-1-independent. Collectively, our data demonstrate AP-1 binding to and activation of the MIE enhancer is a key molecular process controlling reactivation from latency.

A molecular mechanism for probabilistic bet hedging and its role in viral latency

Probabilistic bet hedging is a generalized diversification strategy to maximize fitness in unpredictable environments and has been proposed as an evolutionary basis for herpesvirus latency. However, the molecular mechanisms enabling probabilistic bet hedging have remained elusive. Here, we find that the human herpesvirus cytomegalovirus—a major cause of birth defects and transplant failures—utilizes stochastic variability in the abundance of a protein packaged into individual viral particles to enable probabilistic bet hedging between alternate viral states.

Probabilistic bet hedging, a strategy to maximize fitness in unpredictable environments by matching phenotypic variability to environmental variability, is theorized to account for the evolution of various fate-specification decisions, including viral latency. However, the molecular mechanisms underlying bet hedging remain unclear. Here, we report that large variability in protein abundance within individual herpesvirus virion particles enables probabilistic bet hedging between viral replication and latency. Superresolution imaging of individual virions of the human herpesvirus cytomegalovirus (CMV) showed that virion-to-virion levels of pp71 tegument protein—the major viral transactivator protein—exhibit extreme variability. This super-Poissonian tegument variability promoted alternate replicative strategies: high virion pp71 levels enhance viral replicative fitness but, strikingly, impede silencing, whereas low virion pp71 levels reduce fitness but promote silencing. Overall, the results indicate that stochastic tegument packaging provides a mechanism enabling probabilistic bet hedging between viral replication and latency.

Expanding the Known Functional Repertoire of the Human Cytomegalovirus pp71 Protein

The human cytomegalovirus pp71 protein is packaged within the tegument of infectious virions and performs multiple functions in host cells to prime them for productive, lytic replication. Here we review the known and hypothesized functions of pp71 in regulating proteolysis, infection outcome (lytic or latent), histone deposition, transcription, translation, immune evasion, cell cycle progression, and pathogenesis. We also highlight recent advances in CMV-based vaccine candidates informed by an improved understanding of pp71 function.

Human Cytomegalovirus Enters the Primary CD34+ Hematopoietic Progenitor Cells Where It Establishes Latency by Macropinocytosis

Viral entry is targeted by immunological and pharmacological measures to inhibit viral infection. Human cytomegalovirus (HCMV) entry into cells where it initiates productive infection has been well studied, but its entry into cell types where it establishes latency has not. Therefore, we examined the entry of HCMV into CD34⁺ hematopoietic progenitor cells where the virus establishes latency. We determined that HCMV enters into the primary CD34⁺ hematopoietic progenitor cells in which it establishes latency by macropinocytosis. The capsid-associated tegument protein pp150 is released from maturing endosomes and migrates to the nucleus, whereas other tegument proteins, including pp71, remain endosome associated in the cytoplasm. The inhibition of macropinocytosis impairs entry, thereby diminishing latency-associated transcription and reducing viral reactivation. We conclude that HCMV virions enter CD34⁺ cells by macropinocytosis but fail to fully uncoat or disassemble their tegument layers, leading to the establishment of latency.IMPORTANCE Virion entry is targeted by antivirals and natural immunity to prevent infection. Natural preexisting immunity is ineffective at clearing an HCMV infection, and an incomplete understanding of the viral glycoproteins and cellular receptors that mediate entry has hampered inhibitor development. Nevertheless, HCMV entry remains a viable drug target. Our characterization here of HCMV entry into primary CD34⁺ hematopoietic progenitor cells through macropinocytosis and our comparison to viral entry into fibroblast cells highlight virion uncoating and tegument disassembly as a divergence point between productive and latent infections. Further definition of tegument disassembly may permit the development of interventions to inhibit this process to block productive infection or to trigger it in incompletely differentiated cells to prevent the seeding of the latent reservoirs that make HCMV infections incurable.

Who's Driving? Human Cytomegalovirus, Interferon, and NFκB Signaling

As essential components of the host's innate immune response, NFκB and interferon signaling are critical determinants of the outcome of infection. Over the past 25 years, numerous Human Cytomegalovirus (HCMV) genes have been identified that antagonize or modulate the signaling of these pathways. Here we review the biology of the HCMV factors that alter NFκB and interferon signaling, including what is currently known about how these viral genes contribute to infection and persistence, as well as the major outstanding questions that remain.

Molecular Determinants and the Regulation of Human Cytomegalovirus Latency and Reactivation

Human cytomegalovirus (HCMV) is a beta herpesvirus that establishes a life-long persistence in the host, like all herpesviruses, by way of a latent infection. During latency, viral genomes are maintained in a quieted state. Virus replication can be reactivated from latency in response to changes in cellular signaling caused by stress or differentiation. The past decade has brought great insights into the molecular basis of HCMV latency. Here, we review the complex persistence of HCMV with consideration of latent reservoirs, viral determinants and their host interactions, and host signaling and the control of cellular and viral gene expression that contributes to the establishment of and reactivation from latency.

The Human CMV IE1 Protein: An Offender of PML Nuclear Bodies

PML nuclear bodies (PML-NBs) are SUMOylation-dependent, highly complex protein assemblies that accumulate in the interchromosomal territories of the cell nucleus. Research of the last two decades revealed that many viruses have evolved effector proteins that modify PML-NBs. This correlates with antagonization of individual PML-NB components which act as host cell restriction factors. The multifunctional immediate-early protein IE1 of human cytomegalovirus directly interacts with the PML protein resulting in a disruption of the dot-like structure of PML-NBs. This review summarizes recent advances on the functional consequences of PML-NB modification by IE1. In particular, we describe that PML exerts a novel co-regulatory role during the interferon response which is abrogated by IE1. Via binding to PML, IE1 is able to compromise both intrinsic antiviral defense mechanisms and classical innate immune responses. These interactions of IE1 with innate host defenses are crucial for the onset of lytic replication and, consequently, may represent promising targets for antiviral strategies.

Interaction of Human Cytomegalovirus Tegument Proteins ppUL35 and ppUL35A with Sorting Nexin 5 Regulates Glycoprotein B (gpUL55) Localization

Human cytomegalovirus (HCMV) is a widespread human pathogen that causes asymptomatic infection in healthy individuals but poses a serious threat to immunocompromised patients. During the late phase of HCMV infection, the viral capsid is transported to the cytoplasmic viral assembly center (cVAC), where it is enclosed by the tegument protein layer and the viral envelope. The cVAC consists of circularly arranged vesicles from the trans-Golgi and endosomal networks. The HCMV gene UL35 encodes ppUL35 and its shorter form, ppUL35A. We have previously shown that the UL35 gene is involved in HCMV assembly, but it is unknown how UL35 proteins regulate viral assembly. Here we show that sorting nexin 5 (SNX5), a component of the retromer and part of the retrograde transport pathway, interacts with UL35 proteins. Expression of wild-type proteins but not mutants defective in SNX5 binding resulted in the cellular redistribution of the cation-independent mannose-6-phosphate receptor (CI-M6PR), indicating that UL35 proteins bind and negatively regulate SNX5 to modulate cellular transport pathways. Furthermore, binding of UL35 proteins to SNX5 was required for efficient viral replication and for transport of the most abundant HCMV glycoprotein B (gB; gpUL55) to the cVAC. These results indicate that ppUL35 and ppUL35A control the localization of the essential gB through the regulation of a retrograde transport pathway. Thus, this work is the first to define a molecular interaction between a tegument protein and a vesicular transport factor to regulate glycoprotein localization.IMPORTANCE Human cytomegalovirus is ubiquitously present in the healthy population, but reactivation or reinfection can cause serious, life-threatening infections in immunocompromised patients. For completion of its lytic cycle, human cytomegalovirus induces formation of an assembly center where mature virus particles are formed from multiple viral proteins. Viral glycoproteins use separate vesicular pathways for transport to the assembly center, which are incompletely understood. Our research identified a viral structural protein which affects the localization of one of the major glycoproteins. We could link this change in glycoprotein localization to an interaction of the structural protein with a cellular protein involved in regulation of vesicle transport. This increases our understanding of how the virus intersects into cellular regulatory pathways to enhance its own 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.

Inhibition of human cytomegalovirus immediate-early gene expression and replication by the ethyl acetate (EtOAc) fraction of Elaeocarpus sylvestris in vitro

Background

In immunocompromised patients, human cytomegalovirus (HCMV) infection can lead to severe, life-threatening diseases, such as pneumonitis, hepatitis, gastrointestinal tract disease, and retinitis. We previously reported that a 70% ethanol extract of Elaeocarpus sylvestris leaves (ESE) inhibits human cytomegalovirus (HCMV) replication in vitro. In the present study, we determined the solvent fraction of ESE that inhibits HCMV replication using activity-guided fractionation.

Methods

Activity-guided fractionation of ESE was performed to determine the solvent fraction that inhibits HCMV replication. Effects of solvent fractions on HCMV lytic gene expression and major immediate-early (MIE) enhancer/promoter activity were further investigated.

Results

Among the solvent fractions tested, the EtOAc fraction of ESE markedly reduced HCMV lytic gene expression and viral replication in vitro without exerting significant cytotoxic effects against human foreskin fibroblasts (HFF). Furthermore, the EtOAc fraction negatively affected HCMV MIE enhancer/promoter activity.

Conclusion

Our data collectively indicate that the EtOAc fraction of ESE contains active constituents that inhibit HCMV MIE enhancer/promoter activity and viral replication. The EtOAc fraction of ESE is a good source of novel drug candidates for treatment of HCMV-associated diseases.

Human Cytomegalovirus Enhances A. actinomycetemcomitans Adherence to Cells

Adherence of Actinobacillus actinomycetemcomitans to epithelial cells is an important step in periodontal disease pathogenesis. Recent publications describe the subgingival presence of a wide array of viruses [ e.g., human cytomegalo-virus (hCMV)]. Since viruses can increase cellular susceptibility for bacterial adherence, we investigated whether hCMV renders epithelial cells more prone to adherence by Actinobacillus actinomycetemcomitans. Cultivated HeLa and primary epithelial cells were shown to be semi-permissive for hCMV infection, which resulted in increased bacterial adherence. This increase correlated with viral concentrations, was evident in all Actinobacillus actinomycetemcomitans strains examined, and increased during the first 24 hrs, followed by a slight decrease. Immediate early antigen expression was not correlated with the increased adherence of Actinobacillus actinomycetemcomitans. The results confirmed our hypothesis that the adherence of Actinobacillus actinomycetemcomitans is influenced by hCMV in vitro.

Cytomegalovirus and immunotherapy: opportunistic pathogen, novel target for cancer and a promising vaccine vector

Cytomegalovirus (CMV) is a β-herpesvirus that infects most people in the world and is almost always asymptomatic in the healthy host. However, CMV persists for life, requiring continuous immune surveillance to prevent disease and thus, CMV is a frequent complication in immune compromised patients. Many groups have been exploring the potential for adoptive T-cell therapies to control CMV reactivation as well as the progression of solid tumors harboring CMV. In addition, CMV itself is being explored as a vaccine vector for eliciting potent T-cell responses. This review will discuss key features of the basic biology of CMV-specific T cells as well as highlighting unanswered questions and ongoing work in the development of T-cell-based immunotherapies to target CMV.

Stimulatory effects of human cytomegalovirus tegument protein pp71 lead to increased expression of CCL2 (monocyte chemotactic protein-1) during infection

Human cytomegalovirus (CMV) is the most common infectious cause of congenital birth defects in developed countries. Studies of infected amniotic fluid and placentae show CMV infection leads to a pro-inflammatory shift in cytokine profiles with implications for pathogenesis of foetal disease. ELISA, immunofluorescence and real-time-PCR assays were used to investigate CCL2 (monocyte chemotactic protein-1) and TNF-α changes following CMV infection of human fibroblasts, as well as following transient expression of CMV gene products in HeLa cells. Infection of human fibroblasts with CMV AD169 resulted in increased cytoplasmic and extracellular expression of CCL2 during early stages of infection, followed by marked downregulation of the chemokine at late times. Induction of CCL2 was not observed with CMV clinical strain Merlin, consistent with the postulated immune-evasion potential of this genetically intact WT strain. Comparison between live and UV-irradiated virus infections showed that changes in CCL2 levels were a direct response to active CMV replication. There were no significant changes in TNF-α expression during a parallel time-course of CMV infection. In transient transfection assays, overexpression of CMV tegument protein pp71 resulted in intracellular and extracellular upregulation of CCL2 protein. mRNA analysis showed that pp71-induced elevation in CCL2 was mediated through transcriptional upregulation. The data showed that CMV-induced upregulation of CCL2 during early stages of infection was mediated, at least in part, by stimulation of viral pp71, which may contribute to viral pathogenesis through enhanced virus dissemination.

Cytomegalovirus and glioma: putting the cart before the horse

In 1908, Oluf Bang and Vilhelm Ellerman laid the foundation for theory of oncoviruses by demonstrating that the avian erythroblastosis (a form of chicken leukaemia) could be transmitted by cell-free extracts. Since then, it has been shown very convincingly that viruses can directly cause several human cancers by various mechanisms. Epidemiological data imply that viruses are the second most important risk factor for cancer development in humans, exceeded only by tobacco consumption. Although the ability of certain viruses (hepatitis B and C, human papillomavirus, etc) to cause cancer has been time tested and proven scientifically, there are several other potential viral candidates whose role in oncogenesis is more controversial. One such controversial scenario involves the role of cytomegalovirus (CMV) in malignant gliomas, the most common form of primary brain tumour. CMV first attracted attention about a decade ago when CMV gene products were found in glioma tissue but not in normal brain. Since this initial observation, several different groups have shown an oncomodulatory effect of CMV; however, direct association between CMV infection and incidence of glioma is lacking. In this review, we will evaluate the evidence, both preclinical and clinical, regarding the possible role of CMV in gliomagenesis and maintenance. We will also critically evaluate the rationale for using antiviral drugs in the treatment of patients with glioma.

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.

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.

Inhibition of human cytomegalovirus replication by overexpression of CREB1

Expression of the human cytomegalovirus (HCMV) major immediate-early (MIE) genes is regulated by a strong enhancer-containing promoter with multiple binding sites for various transcription factors, including cyclic AMP response element binding protein 1 (CREB1). Here we show that overexpression of CREB1 potently blocked MIE transcription and HCMV replication. Surprisingly, CREB1 still exhibited strong inhibition of the MIE promoter when all five CREB binding sites within the enhancer were mutated, suggesting that CREB1 regulated the MIE gene expression indirectly. Promoter deletion analysis and site-directed mutagenesis identified the region between -130 and -50 upstream of the transcription start site of the MIE gene as the "CREB1 responsive region". Mutations of SP1/3 and NF-κB binding sites within this region interrupted the inhibitory effect induced by CREB1 overexpression. Our findings suggest that overexpression of CREB1 can cause repression of HCMV replication and may contribute to the development of new anti-HCMV strategies.

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.

The human cytomegalovirus US27 gene product enhances cell proliferation and alters cellular gene expression

Human cytomegalovirus (HCMV) is a prevalent pathogen worldwide. Although generally harmless in healthy individuals, HCMV can pose a serious threat to immune compromised individuals and developing fetuses in utero. HCMV encodes four genes predicted to give rise to G protein-coupled receptors (GPCRs): US27, US28, UL33, and UL78. The US28 gene product is a functional chemokine receptor that enhances cell growth in some cell types but induces apoptosis in others. In contrast, the US27 gene product has not been demonstrated to signal either constitutively or in a ligand-induced manner. In this study, US27 was expressed in transfected cells, and both cell proliferation and DNA synthesis were significantly increased compared to control cells. PCR array analysis revealed that expression of US27 led to changes in a limited number of cellular genes, but genes that were up-regulated included the pro-survival factor Bcl-x, AP-1 transcription factor components jun and fos, and the IL-6 family cytokine oncostatin M. These results demonstrate that US27 can impact host cell physiology and may shed light on the function of this orphan viral GPCR.

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.

Human cytomegalovirus pp71 stimulates major histocompatibility complex class i presentation of IE1-derived peptides at immediate early times of infection

Suppression of major histocompatibility complex (MHC) class I-mediated presentation of human cytomegalovirus (HCMV) peptides is an important mechanism to avoid CD8 T lymphocyte recognition and killing of infected cells. Of particular interest is how MHC class I presentation of essential regulatory immediate early (IE) proteins of HCMV can be effectively compromised at times when known viral immunoevasins are not abundantly expressed. The tegument protein pp71 had been suggested to be involved in MHC class I downregulation. Intriguingly, this polypeptide is also critically engaged in the initial derepression of the major IE gene locus, leading to enhanced expression of IE proteins IE1-pp72 and IE2-pp86. Using a set of viral mutants, we addressed the role of pp71 in MHC class I presentation of IE1-pp72-derived peptides. We show that the amount of "incoming" pp71 positively correlates with IE1-pp72 protein levels and with the presentation of IE1-derived peptides. This indicates that the amount of the IE1 protein, induced by pp71, rather than a putative immunoevasive function of the tegument protein, determines MHC class I antigen presentation of IE1-derived peptides. This process proved to be independent of the presence of pp65, which had been reported to interfere with IE1 presentation. It may thus be beneficial for the success of HCMV replication to limit the level of pp71 delivered from infecting particles in order to avoid critical levels of MHC class I presentation of IE protein-derived peptides.

The replication defect of ICP0-null mutant herpes simplex virus 1 can be largely complemented by the combined activities of human cytomegalovirus proteins IE1 and pp71

Herpes simplex virus 1 (HSV-1) immediate-early protein ICP0 is required for efficient lytic infection and productive reactivation from latency and induces derepression of quiescent viral genomes. Despite being unrelated at the sequence level, ICP0 and human cytomegalovirus proteins IE1 and pp71 share some functional similarities in their abilities to counteract antiviral restriction mediated by components of cellular nuclear structures known as ND10. To investigate the extent to which IE1 and pp71 might substitute for ICP0, cell lines were developed that express either IE1 or pp71, or both together, in an inducible manner. We found that pp71 dissociated the hDaxx-ATRX complex and inhibited accumulation of these proteins at sites juxtaposed to HSV-1 genomes but had no effect on the promyelocytic leukemia protein (PML) or Sp100. IE1 caused loss of the small ubiquitin-like modifier (SUMO)-conjugated forms of PML and Sp100 and inhibited the recruitment of these proteins to HSV-1 genome foci but had little effect on hDaxx or ATRX in these assays. Both IE1 and pp71 stimulated ICP0-null mutant plaque formation, but neither to the extent achieved by ICP0. The combination of IE1 and pp71, however, inhibited recruitment of all ND10 proteins to viral genome foci, stimulated ICP0-null mutant HSV-1 plaque formation to near wild-type levels, and efficiently induced derepression of quiescent HSV-1 genomes. These results suggest that ND10-related intrinsic resistance results from the additive effects of several ND10 components and that the effects of IE1 and pp71 on subsets of these components combine to mirror the overall activities of ICP0.

Coordination of late gene transcription of human cytomegalovirus with viral DNA synthesis: recombinant viruses as potential therapeutic vaccine candidates

INTRODUCTION

During productive infection, human cytomegalovirus (HCMV) genes are expressed in a temporal cascade, with temporal phases designated as immediate-early (IE), early, and late. The major IE (MIE) genes, UL123 and UL122 (IE1/IE2), play a critical role in subsequent viral gene expression and the efficiency of viral replication. The early viral genes encode proteins necessary for viral DNA replication. Following viral DNA replication, delayed-early and late viral genes are expressed which encode structural proteins for the virion. The late genes can be divided into two broad classes. At early times the gamma-1 or leaky-late class are expressed at low levels after infection and are dramatically upregulated at late times. In contrast, the gamma-2 or 'true' late genes are expressed exclusively after viral DNA replication. Expression of true late (gamma-2 class) viral genes is completely prevented by inhibition of viral DNA synthesis.

AREAS COVERED

This review addresses the viral genes required for HCMV late gene transcription. Recombinant viruses that are defective for late gene transcription allow for early viral gene expression and viral DNA synthesis, but not infectious virus production. Since current HCMV prophylaxis is limited by several shortcomings, the use of defective recombinant viruses to induce HCMV cell-mediated and humoral immunity is discussed.

EXPERT OPINION

HCMV DNA replication and late gene transcription are not completely linked. Viral-encoded trans-acting factors are required. Recombinant viruses proficient in MIE and early viral gene expression and defective in late gene expression may be an alternative therapeutic vaccine candidates for the induction of cell-mediated and humoral immunity.

Tale of a tegument transactivator: the past, present and future of human CMV pp71

Herpesviruses assemble large virions capable of delivering to a newly infected cell not only the viral genome, but also viral proteins packaged within the tegument layer between the DNA-containing capsid and the lipid envelope. In this review, we describe the tegument transactivator of the β-herpesvirus human CMV, the pp71 protein. We present the known mechanistic features through which it activates viral gene expression during a lytic infection but fails to do so when the virus establishes latency, and describe how pp71 stimulates the cell cycle and may help infected cells avoid detection by the adaptive immune system. A historical overview of pp71 is extended with current perceptions of its roles during human CMV infections and suggestions for future avenues of experimentation.

BclAF1 restriction factor is neutralized by proteasomal degradation and microRNA repression during human cytomegalovirus infection

Cell proteins can restrict the replication of viruses. Here, we identify the cellular BclAF1 protein as a human cytomegalovirus restriction factor and describe two independent mechanisms the virus uses to decrease its steady-state levels. Immediately following infection, the viral pp71 and UL35 proteins, which are delivered to cells within virions, direct the proteasomal degradation of BclAF1. Although BclAF1 reaccumulates through the middle stages of infection, it is subsequently down-regulated at late times by miR-UL112-1, a virus-encoded microRNA. In the absence of BclAF1 neutralization, viral gene expression and replication are inhibited. These data identify two temporally and mechanistically distinct functions used by human cytomegalovirus to down-regulate a cellular antiviral protein.

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.

Human cytomegalovirus infection increases mitochondrial biogenesis

Fibroblasts infected by Human Cytomegalovirus (CMV) undergo a robust increase in mitochondrial biogenesis with a corresponding increase in mitochondrial activity that is partly dependent on the viral anti-apoptotic pUL37x1 protein (vMIA). The increased respiration activity is blocked by the mitochondrial translation inhibitor chloramphenicol, which additionally suppresses viral production. Intriguingly, chloramphenicol and pUL37x1 depletion have different effects on respiration capacity but similar effects on CMV production, suggesting that pUL37x1 promotes viral replication by efficient utilization of new mitochondria. These results argue for a role of pUL37x1 beyond controlling apoptosis.

Cell death-independent functions of granzymes: hit viruses where it hurts

Granule exocytosis by cytotoxic lymphocytes is the key mechanism of our immune response to eliminate virus-infected cells. These lytic granules contain the pore-forming protein perforin and a set of five serine proteases called granzymes (GrA, GrB, GrH, GrK, GrM) that display distinct substrate specificities. Granzymes have mostly been studied for their ability to induce cell death. However, viruses have evolved many inhibitors to effectively block apoptosis. Evidence is emerging that granzymes also use noncytotoxic strategies to inhibit viral replication and potential viral reactivation from latency. Granzymes directly cleave viral or host cell proteins that are required in the viral life cycle. Furthermore, granzymes induce a pro-inflammatory cytokine response to create an antiviral environment. In this review, we summarize and discuss these novel strategies by which the immune system counteracts viral infections, and we will address the potential therapeutic applications that could emerge from this intriguing mechanism.

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 immediate early gene expression in the osteosarcoma line U2OS is repressed by the cell protein ATRX

The control of human cytomegalovirus (HCMV) immediate early (IE) gene expression in infected human fibroblasts was compared with that in the U2OS human osteosarcoma cells. Viral IE expression was stimulated by the virion protein pp71 and repressed by the cell protein hDaxx in fibroblasts, as expected from published data. Neither of these events occurred in infected U2OS cells, suggesting that this cell line lacks one or more factors that repress HCMV IE expression. The chromatin remodeling factor ATRX is absent from U2OS cells, therefore the effect of introducing this protein by electroporation of plasmid DNA was investigated. Provision of ATRX inhibited HCMV IE expression, and the presence of the HCMV-specified virion phosphoprotein pp71 overcame the repression. The experiments demonstrate that ATRX can act as a cellular intrinsic antiviral defense in U2OS cells by blocking gene expression from incoming HCMV genomes. In contrast, ATRX did not affect the replication of herpes simplex virus type 1, showing that there are differences in the way U2OS cells respond to the presence of the herpesviral genomes.

Tegument protein control of latent herpesvirus establishment and animation

Herpesviruses are successful pathogens that infect most vertebrates as well as at least one invertebrate species. Six of the eight human herpesviruses are widely distributed in the population. Herpesviral infections persist for the life of the infected host due in large part to the ability of these viruses to enter a non-productive, latent state in which viral gene expression is limited and immune detection and clearance is avoided. Periodically, the virus will reactivate and enter the lytic cycle, producing progeny virus that can spread within or to new hosts. Latency has been classically divided into establishment, maintenance, and reactivation phases. Here we focus on demonstrated and postulated molecular mechanisms leading to the establishment of latency for representative members of each human herpesvirus family. Maintenance and reactivation are also briefly discussed. In particular, the roles that tegument proteins may play during latency are highlighted. Finally, we introduce the term animation to describe the initiation of lytic phase gene expression from a latent herpesvirus genome, and discuss why this step should be separated, both molecularly and theoretically, from reactivation.

The activator protein 1 binding motifs within the human cytomegalovirus major immediate-early enhancer are functionally redundant and act in a cooperative manner with the NF-{kappa}B sites during acute infection

Human cytomegalovirus (HCMV) infection causes a rapid induction of c-Fos and c-Jun, the major subunits of activator protein 1 (AP-1), which in turn have been postulated to activate the viral immediate-early (IE) genes. Accordingly, the major IE promoter (MIEP) enhancer, a critical control region for initiating lytic HCMV infection and reactivation from the latent state, contains one well-characterized AP-1 site and a second candidate interaction site. In this study we explored the role of these AP-1 elements in the context of the infection. We first show that the distal candidate AP-1 motif binds c-Fos/c-Jun heterodimers (AP-1 complex) and confers c-Fos/c-Jun-mediated activity to a core promoter. Site-directed mutagenesis studies indicate that both AP-1 response elements are critical for 12-O-tetradecanoylphorbol-13-acetate (TPA)-enhanced MIEP activity in transient-transfection assays. In marked contrast to the results obtained with the isolated promoter, disruption of the AP-1 recognition sites of the MIEP in the context of the infectious HCMV genome has no significant influence on the expression of the MIE protein IE1 or viral replication in different cell types. Moreover, a chimeric murine CMV driven by the HCMV MIEP (hMCMV-ES) with the two AP-1 binding sites mutated is not compromised in virulence, is able to grow and disseminate to different organs of the newborn mice as efficiently as the parental virus, and is competent in reactivation. We show, however, that combined inactivation of the enhancer AP-1 and NF-κB recognition sites leads to an attenuation of the hMCMV-ES in the neonatal murine infection model, not observed when each single element is abolished. Altogether, these results underline the functional redundancy of the MIEP elements, highlighting the plasticity of this region, which probably evolved to ensure maximal transcriptional performance across many diverse environments.

Noncytotoxic inhibition of cytomegalovirus replication through NK cell protease granzyme M-mediated cleavage of viral phosphoprotein 71

Granzyme M (GrM) is highly expressed in cytotoxic granules of NK cells, which provide the first line of defense against viral pathogens. GrM knockout mice show increased susceptibility toward murine CMV infection. Although GrM is a potent inducer of cell death, the mechanism by which GrM eliminates viruses remains elusive. In this paper, we show that purified human GrM in combination with the perforin-analog streptolysin O (SLO) strongly inhibited human CMV (HCMV) replication in fibroblasts in the absence of host cell death. In a proteomic approach, GrM was highly specific toward the HCMV proteome and most efficiently cleaved phosphoprotein 71 (pp71), an HCMV tegument protein that is critical for viral replication. Cleavage of pp71 occurred when viral lysates were incubated with purified GrM, when intact cells expressing recombinant pp71 were challenged with living cytotoxic effector cells, and when HCMV-infected fibroblasts were incubated with SLO and purified GrM. GrM directly cleaved pp71 after Leu(439), which coincided with aberrant cellular localization of both pp71 cleavage fragments as determined by confocal immunofluorescence. In a luciferase reporter assay, cleavage of pp71 after Leu(439) by GrM completely abolished the ability of pp71 to transactivate the HCMV major immediate-early promoter, which is indispensable for effective HCMV replication. Finally, GrM decreased immediate-early 1 protein expression in HCMV-infected fibroblasts. These results indicate that the NK cell protease GrM mediates cell death-independent antiviral activity by direct cleavage of a viral substrate.

Nuclear localization of tegument-delivered pp71 in human cytomegalovirus-infected cells is facilitated by one or more factors present in terminally differentiated fibroblasts

Herpesviral virions contain a tegument layer that consists primarily of viral proteins. The delivery of fully functional proteins to infected cells upon virion envelope fusion to the plasma membrane allows herpesviruses to modulate cellular activities prior to viral gene expression. Certain tegument proteins can also regulate viral processes. For example, the pp71 tegument protein encoded by the UL82 gene of human cytomegalovirus (HCMV) stimulates viral immediate early (IE) gene expression and thus acts to initiate the productive lytic infectious cycle. In terminally differentiated fibroblasts infected with HCMV, tegument-delivered pp71 traffics to the nucleus and degrades the cellular transcriptional corepressor Daxx to initiate viral IE gene expression and lytic replication. However, when HCMV infects incompletely differentiated cells, tegument-delivered pp71 remains in the cytoplasm, allowing the nucleus-localized Daxx protein to silence viral IE gene expression and promote the establishment of a latent infection in certain cell types. We sought to determine whether undifferentiated cells block the trafficking of tegument-delivered pp71 to the nucleus or whether differentiated cells facilitate the nuclear transport of tegument-delivered pp71. Heterogenous cell fusion experiments demonstrated that tegument-delivered pp71 found in the cytoplasm of undifferentiated NT2 cells could be driven into the nucleus by one or more factors provided by fully differentiated fibroblasts. Our data raise the intriguing possibility that latency is the default program launched by HCMV upon viral entry into cells and that lytic infection is initiated only in certain (differentiated) cells that can facilitate the delivery of incoming pp71 to the nucleus.

The human cytomegalovirus UL76 gene regulates the level of expression of the UL77 gene

BACKGROUND

Human cytomegalovirus (HCMV) can be reactivated under immunosuppressive conditions causing several fatal pneumonitis, hepatitis, retinitis, and gastrointestinal diseases. HCMV also causes deafness and mental retardation in neonates when primary infection has occurred during pregnancy. In the genome of HCMV at least 194 known open reading frames (ORFs) have been predicted, and approximately one-quarter, or 41 ORFs, are required for viral replication in cell culture. In contrast, the majority of the predicted ORFs are nonessential for viral replication in cell culture. However, it is also possible that these ORFs are required for the efficient viral replication in the host. The UL77 gene of HCMV is essential for viral replication and has a role in viral DNA packaging. The function of the upstream UL76 gene in the HCMV-infected cells is not understood. UL76 and UL77 are cistons on the same viral mRNA and a conventional 5' mRNA for UL77 has not been detected. The vast majority of eukaryotic mRNAs are monocistronic, i.e., they encode only a single protein.

METHODOLOGY/PRINCIPAL FINDINGS

To determine whether the UL76 ORF affects UL77 gene expression, we mutated UL76 by ORF frame-shifts, stop codons or deletion of the viral gene. The effect on UL77 protein expression was determined by either transfection of expression plasmids or infection with recombinant viruses. Mutation of UL76 ORF significantly increased the level of UL77 protein expression. However, deletion of UL76 upstream of the UL77 ORF had only marginal effects on viral growth.

CONCLUSIONS/SIGNIFICANCE

While UL76 is not essential for viral replication, the UL76 ORF is involved in regulation of the level of UL77 protein expression in a manner dependent on the translation re-initiation. UL76 may fine-tune the UL77 expression for the efficient viral replication in the HCMV- infected cells.

Functional properties of the human cytomegalovirus IE86 protein required for transcriptional regulation and virus replication

The human cytomegalovirus (HCMV) IE86 protein is essential for HCMV replication due to its ability to transactivate critical viral early promoters. In the current study, we performed a comprehensive mutational analysis between amino acids (aa) 535 and 545 of IE86 and assessed the impact of these mutations on IE86-mediated transcriptional activation. Using transient assays and complementing analysis with recombinant HCMV clones, we show that single amino acid mutations differentially impair the ability of IE86 to mediate transactivation of essential early gene promoters. The conserved tyrosine at amino acid 544 is critical for activation of the UL54 promoter in vitro and in the context of the viral genome. In contrast, mutation of the proline at position 535 disrupted activation of the UL54 promoter in transient assays but displayed activity similar to that of wild-type (WT) IE86 when assessed in the genomic context. To examine the underlying mechanism of this differential effect, glutathione S-transferase (GST) pulldown assays were performed, revealing that Y544 is critical for binding to the TATA binding protein (TBP), suggesting that this interaction is likely necessary for the ability of IE86 to activate the UL54 promoter. In contrast, mutation of either P535 or Y544 disrupted activation of the UL112-113 promoter both in vitro and in vivo, suggesting that interaction with TBP is not sufficient for IE86-mediated activation of this early promoter. Together, these studies demonstrate that IE86 activates early promoters by distinct mechanisms.

Histone deacetylases in viral infections

Chromatin remodeling and gene expression are regulated by histone deacetylases (HDACs) that condense the chromatin structure by deacetylating histones. HDACs comprise a group of enzymes that are responsible for the regulation of both cellular and viral genes at the transcriptional level. In mammals, a total of 18 HDACs have been identified and grouped into four classes, i.e., class I (HDACs 1, 2, 3, 8), class II (HDACs 4, 5, 6, 7, 9, 10), class III (Sirt1–Sirt7), and class IV (HDAC11). We review here the role of HDACs on viral replication and how HDAC inhibitors could potentially be used as new therapeutic tools in several viral infections.

Cellular and viral control over the initial events of human cytomegalovirus experimental latency in CD34+ cells

Human cytomegalovirus (HCMV) persists for the life of its host by establishing a latent infection. The identification of viral and cellular determinants of latency is the first step toward developing antiviral treatments that target and might clear or control the reservoir of latent virus. HCMV latency is established in CD34(+) cells when expression of viral immediate early (IE) proteins that initiate lytic infection is silenced. Viral IE gene expression during lytic infection is controlled by a cellular intrinsic immune defense mediated by promyelocytic leukemia nuclear body (PML-NB) proteins such as Daxx and histone deacetylases (HDACs). This defense is inactivated at the start of lytic infection by the HCMV virion tegument protein pp71, which upon viral entry traffics to the nucleus and induces Daxx degradation. Here we show that a similar defense is present, active, and not neutralized during experimental latency in CD34(+) cells infected in vitro because tegument-delivered pp71 remains in the cytoplasm. Artificial inactivation of this defense by HDAC inhibition or Daxx knockdown rescues viral IE gene expression upon infection of CD34(+) cells with a laboratory-adapted viral strain but not with clinical strains. Interestingly, coinfection of CD34(+) cells with clinical viral strains blocked the ability of an HDAC inhibitor to activate IE1 and early protein expression during infection with a laboratory-adapted strain. This suggests that in addition to the intrinsic defense, HCMV clinical strains contribute an HDAC-independent, trans-acting dominant means of control over viral gene expression during the early stages of experimental HCMV latency modeled in vitro in CD34(+) cells.

Analysis of latent viral gene expression in natural and experimental latency models of human cytomegalovirus and its correlation with histone modifications at a latent promoter

Human cytomegalovirus (HCMV) is an opportunistic human pathogen that establishes a lifelong latent infection, which can reactivate periodically. If unchecked by a robust immune response, this reactivation can result in severe disease in immunocompromised patients. Reactivation of latent virus in myeloid progenitor cells is concomitant with cellular differentiation through regulation of the major immediate-early promoter (MIEP) by chromatin remodelling. In this study, we analysed the expression of the latent gene transcript UL81-82as (LUNA). LUNA is expressed in latently infected CD34(+) cells and its expression decreases as CD34(+) cells differentiate to immature dendritic cells. Upon maturation (and HCMV reactivation), a second wave of transcription occurs, consistent with expression during lytic infection. Furthermore, we show that the LUNA promoter is associated with acetylated histones during HCMV latency in experimentally and naturally infected CD34(+) cells, thus suggesting that latent gene promoters are, like the MIEP, regulated by post-translational modifications of their associated histone proteins.

The CREB site in the proximal enhancer is critical for cooperative interaction with the other transcription factor binding sites to enhance transcription of the major intermediate-early genes in human cytomegalovirus-infected cells

One of the two SP1 sites in the proximal enhancer of the human cytomegalovirus (HCMV) major immediate-early (MIE) promoter is essential for transcription in human fibroblast cells (H. Isomura, M. F. Stinski, A. Kudoh, T. Daikoku, N. Shirata, and T. Tsurumi, J. Virol. 79:9597-9607, 2005). Upstream of the two SP1 sites to -223 relative to the +1 transcription start site, there are an additional five DNA binding sites for eukaryotic transcription factors. We determined the effects of the various transcription factor DNA binding sites on viral MIE RNA transcription, viral gene expression, viral DNA synthesis, or infectious virus production. We prepared recombinant HCMV bacterial artificial chromosome (BAC) DNAs with either one site missing or one site present upstream of the two SP1 sites. Infectious recombinant HCMV BAC DNAs were transfected into various cell types to avoid the effect of the virion-associated transactivators. Regardless of the cell type, which included human fibroblast, endothelial, and epithelial cells, the CREB site had the most significant and independent effect on the MIE promoter. The other sites had a minor independent effect. However, the combination of the different transcription factor DNA binding sites was significantly stronger than multiple duplications of the CREB site. These findings indicate that the CREB site in the presence of the other sites has a major role for the replication of HCMV.

Properties of virion transactivator proteins encoded by primate cytomegaloviruses

Background

Human cytomegalovirus (HCMV) is a betaherpesvirus that causes severe disease in situations where the immune system is immature or compromised. HCMV immediate early (IE) gene expression is stimulated by the virion phosphoprotein pp71, encoded by open reading frame (ORF) UL82, and this transactivation activity is important for the efficient initiation of viral replication. It is currently recognized that pp71 acts to overcome cellular intrinsic defences that otherwise block viral IE gene expression, and that interactions of pp71 with the cell proteins Daxx and ATRX are important for this function. A further property of pp71 is the ability to enable prolonged gene expression from quiescent herpes simplex virus type 1 (HSV-1) genomes. Non-human primate cytomegaloviruses encode homologs of pp71, but there is currently no published information that addresses their effects on gene expression and modes of action.

Results

The UL82 homolog encoded by simian cytomegalovirus (SCMV), strain Colburn, was identified and cloned. This ORF, named S82, was cloned into an HSV-1 vector, as were those from baboon, rhesus monkey and chimpanzee cytomegaloviruses. The use of an HSV-1 vector enabled expression of the UL82 homologs in a range of cell types, and permitted investigation of their abilities to direct prolonged gene expression from quiescent genomes. The results show that all UL82 homologs activate gene expression, and that neither host cell type nor promoter target sequence has major effects on these activities. Surprisingly, the UL82 proteins specified by non-human primate cytomegaloviruses, unlike pp71, did not direct long term expression from quiescent HSV-1 genomes. In addition, significant differences were observed in the intranuclear localization of the UL82 homologs, and in their effects on Daxx. Strikingly, S82 mediated the release of Daxx from nuclear domain 10 substructures much more rapidly than pp71 or the other proteins tested. All UL82 homologs stimulated the early release of ATRX from nuclear domain 10.

Conclusion

All of the UL82 homolog proteins analysed activated gene expression, but surprising differences in other aspects of their properties were revealed. The results provide new information on early events in infection with cytomegaloviruses.

Proteasome inhibitor differentially regulates expression of the major immediate early genes of human cytomegalovirus in human central nervous system-derived cell lines

Proteasome inhibitor, which inhibits NF-kappaB activation, has been reported to activate c-Jun N-terminal kinase (JNK)-c-Jun pathway. In this study, we investigated the effects of proteasome inhibitor on the human cytomegalovirus (HCMV) major immediate early (MIE) gene expression in human central nervous system (CNS)-derived cell lines. Treatment of HCMV-infected 118MGC glioma and U373-MG astrocytoma cells with three proteasome inhibitors, MG132, clasto-lactacystin beta-lactone, and epoxomicin, suppressed MIE protein expression. In contrast, in HCMV-infected IMR-32 neuroblastoma cells, the proteasome inhibitors increased MIE protein expression, even in the presence of NF-kappaB inhibitor SN-50. A luciferase reporter assay demonstrated that MG132 markedly elevated the MIE promoter/enhancer (MIEP) activity in IMR-32 cells, but down-regulated it in 118MGC and U373-MG cells. Mutation in five cAMP response elements (CREs) within the MIEP resulted in a loss of the ability to respond to MG132 in IMR-32 cells. Moreover, Western blotting analysis revealed that MG132 induced c-Jun phosphorylation in all three CNS-derived cell lines, whereas a high level of activating transcription factor-2 (ATF-2) phosphorylation was observed only in IMR-32 cells. Finally, MG132-induced MIE protein expression was suppressed by JNK inhibitor that reduced the phosphorylation levels of both c-Jun and ATF-2. Taken together, these results suggest that the proteasome inhibitors activate CRE binding proteins consisting of c-Jun and ATF-2 through activating the JNK-c-Jun pathway, thereby inducing MIE protein synthesis in IMR-32 cells under the condition where NF-kappaB activity is inhibited.