Roscovitine, a specific inhibitor of cellular cyclin-dependent kinases, inhibits herpes simplex virus DNA synthesis in the presence of viral early proteins

Anti-herpes simplex virus activities and mechanisms of marine derived compounds

Herpes simplex virus (HSV) is the most widely prevalent herpes virus worldwide, and the herpetic encephalitis and genital herpes caused by HSV infection have caused serious harm to human health all over the world. Although many anti-HSV drugs such as nucleoside analogues have been ap-proved for clinical use during the past few decades, important issues, such as drug resistance, toxicity, and high cost of drugs, remain unresolved. Recently, the studies on the anti-HSV activities of marine natural products, such as marine polysaccharides, marine peptides and microbial secondary metabolites are attracting more and more attention all over the world. This review discusses the recent progress in research on the anti-HSV activities of these natural compounds obtained from marine organisms, relating to their structural features and the structure-activity relationships. In addition, the recent findings on the different anti-HSV mechanisms and molecular targets of marine compounds and their potential for therapeutic application will also be summarized in detail.

BMS-265246, a Cyclin-Dependent Kinase Inhibitor, Inhibits the Infection of Herpes Simplex Virus Type 1

Herpes simplex virus type 1 (HSV-1) infections are prevalent illnesses that can cause mucocutaneous ulcerative disease, keratitis, and genital herpes. In patients with compromised immune systems, the infection can lead to serious problems, such as encephalitis. Additionally, neonatal infections can cause brain problems and even death. Current first-line antiviral drugs are nucleoside analog inhibitors that target viral polymerase, and resistant strains have emerged. As a result, new drugs with distinct action modes are needed. Recent research indicates that cyclin-dependent kinases (CDKs) are prospective antiviral targets. Thus, CDK inhibitors may be effective antiviral agents against HSV-1 infection. In this study, we examined a panel of CDK inhibitors that target CDKs in the present study. BMS-265246 (BMS), a CDK 1/2 inhibitor, was found to effectively limit HSV-1 multiplication in Vero, HepG2, and Hela cells. A mechanism of action study suggested that BMS inhibits the early stages of viral replication when added early in the viral infection. The suppression of multiple steps in viral replication by BMS was revealed when HSV-1 infected cells were treated at different time periods in the viral life cycle. Our results suggest that BMS is a potent anti-HSV-1 agent and unique in that it may interfere with multiple steps in HSV-1 replication.

Simple and rapid high-throughput assay to identify HSV-1 ICP0 transactivation inhibitors

Herpes simplex virus 1 (HSV-1) is a ubiquitous virus that results in lifelong infections due to its ability to cycle between lytic replication and latency. As an obligate intracellular pathogen, HSV-1 exploits host cellular factors to replicate and aid in its life cycle. HSV-1 expresses infected cell protein 0 (ICP0), an immediate-early regulator, to stimulate the transcription of all classes of viral genes via its E3 ubiquitin ligase activity. Here we report an automated, inexpensive, and rapid high-throughput approach to examine the effects of small molecule compounds on ICP0 transactivator function in cells. Two HSV-1 reporter viruses, KOS6β (wt) and dlx3.1-6β (ICP0-null mutant), were used to monitor ICP0 transactivation activity through the HSV-1 ICP6 promoter:lacz expression cassette. A ≥10-fold difference in β-galactosidase activity was observed in cells infected with KOS6β compared to dlx3.1-6β, demonstrating that ICP0 potently transactivates the ICP6 promoter. We established the robustness and reproducibility with a Z'-factor score of ≥0.69, an important criterium for high-throughput analyses. Approximately 19,000 structurally diverse compounds were screened and 76 potential inhibitors of the HSV-1 transactivator ICP0 were identified. We expect this assay will aid in the discovery of novel inhibitors and tools against HSV-1 ICP0. Using well-annotated compounds could identify potential novel factors and pathways that interact with ICP0 to promote HSV-1 gene expression.

Nucleic acid binding mechanism of flavone derivative, riviciclib: Structural analysis to unveil anticancer potential

Despite burgeoned knowledge about the origin, growth, tissue interactions, and spread of cancer in recent years, the functional complexity and unique survival ability of cancer cells still make it difficult to target them. Riviciclib is a semi-synthetic derivative of rohitukine and possesses anticancer potential. Inhibition of nucleic acid activity in an uncontrolled dividing cell can form the basis for the development of new-age cancer therapeutics. The present study reports the molecular interaction between riviciclib and nucleic acid (DNA/tRNA) using spectroscopic and molecular docking studies in an attempt to comprehend its cellular toxicity as well as the nature and mode of binding between them. Vibrational spectroscopic results suggest that riviciclib intercalates DNA duplex and primarily binds with guanine, adenine, and thymine nucleobases. While in the case of riviciclib-tRNA complexation, riviciclib interacts mostly with uracil residues of the tRNA molecule. Besides nucleobases, riviciclib interacts with the sugar-phosphate backbone of both biomacromolecules. Conformationally, DNA alters from B-form to C-form, whereas tRNA shows no change in its native A-form. The order (10⁴ M^-1) of binding constant for riviciclib-nucleic acid complexation infer moderate to strong affinity of riviciclib with DNA and tRNA, respectively. Molecular docking explorations are further in corroboration with our spectroscopic outcomes.

Novel anti-flavivirus drugs targeting the nucleolar distribution of core protein

The risk of infectious diseases caused by Flavivirus is increasing globally. Here, we developed a novel high-throughput screening (HTS) system to evaluate the inhibitory effects of compounds targeting the nuclear localization of the flavivirus core protein. We screened 4000 compounds based on their ability to inhibit the nuclear localization of the core protein, and identified over 20 compounds including inhibitors for cyclin dependent kinase and glycogen synthase kinase. The efficacy of the identified compounds to suppress viral growth was validated in a cell-based infection system. Remarkably, the nucleolus morphology was affected by the treatment with the compounds, suggesting that the nucleolus function is critical for viral propagation. The present HTS system provides a useful strategy for the identification of antivirals against flavivirus by targeting the nucleolar localization of the core protein.

Early Steps in Herpes Simplex Virus Infection Blocked by a Proteasome Inhibitor

Viruses commandeer host cell 26S proteasome activity to promote viral entry, gene expression, replication, assembly, and egress. Proteasomal degradation activity is critical for herpes simplex virus (HSV) infection. The proteasome inhibitor bortezomib (also known as Velcade and PS-341) is a clinically effective antineoplastic drug that is FDA approved for treatment of hematologic malignancies such as multiple myeloma and mantle cell lymphoma. Low nanomolar concentrations of bortezomib inhibited infection by HSV-1, HSV-2, and acyclovir-resistant strains. Inhibition coincided with minimal cytotoxicity. Bortezomib did not affect attachment of HSV to cells or inactivate the virus directly. Bortezomib acted early in HSV infection by perturbing two distinct proteasome-dependent steps that occur within the initial hours of infection: the transport of incoming viral nucleocapsids to the nucleus and the virus-induced disruption of host nuclear domain 10 (ND10) structures. The combination of bortezomib with acyclovir demonstrated synergistic inhibitory effects on HSV infection. Thus, bortezomib is a novel potential therapeutic for HSV with a defined mechanism of action.IMPORTANCE Viruses usurp host cell functions to advance their replicative agenda. HSV relies on cellular proteasome activity for successful infection. Proteasome inhibitors, such as MG132, block HSV infection at multiple stages of the infectious cycle. Targeting host cell processes for antiviral intervention is an unconventional approach that might limit antiviral resistance. Here we demonstrated that the proteasome inhibitor bortezomib, which is a clinically effective cancer drug, has the in vitro features of a promising anti-HSV therapeutic. Bortezomib inhibited HSV infection during the first hours of infection at nanomolar concentrations that were minimally cytotoxic. The mechanism of bortezomib's inhibition of early HSV infection was to halt nucleocapsid transport to the nucleus and to stabilize the ND10 cellular defense complex. Bortezomib and acyclovir acted synergistically to inhibit HSV infection. Overall, we present evidence for the repurposing of bortezomib as a novel antiherpesviral agent and describe specific mechanisms of action.

Herpes Simplex Virus 1 ICP22 Suppresses CD80 Expression by Murine Dendritic Cells

Herpes simplex virus type 1 (HSV-1) has the ability to delay its clearance from the eye during ocular infection. Here, we show that ocular infection of mice with HSV-1 suppressed expression of the costimulatory molecule CD80 but not CD86 in the cornea. The presence of neutralizing anti-HSV-1 antibodies did not alleviate this suppression. At the cellular level, HSV-1 consistently downregulated the expression of CD80 by dendritic cells (DCs) but not by other antigen-presenting cells. Furthermore, flow cytometric analysis of HSV-1-infected corneal cells during a 7-day period reduced CD80 expression in DCs but not in B cells, macrophages, or monocytes. This suppression was associated with the presence of virus. Similar results were obtained using infected or transfected spleen cells or bone marrow-derived DCs. A combination of roscovitine treatment, transfection with immediate early genes (IE), and infection with a recombinant HSV-1 lacking the ICP22 gene shows the importance of ICP22 in downregulation of the CD80 promoter but not the CD86 promoter in vitro and in vivo At the mechanistic level, we show that the HSV-1 immediate early gene ICP22 binds the CD80 promoter and that this interaction is required for HSV-1-mediated suppression of CD80 expression. Conversely, forced expression of CD80 by ocular infection of mice with a recombinant HSV-1 exacerbated corneal scarring in infected mice. Taken together, these studies identify ICP22-mediated suppression of CD80 expression in dendritic cells as central to delayed clearance of the virus and limitation of the cytopathological response to primary infection in the eye.IMPORTANCE HSV-1-induced eye disease is a major public health problem. Eye disease is associated closely with immune responses to the virus and is exacerbated by delayed clearance of the primary infection. The immune system relies on antigen-presenting cells of the innate immune system to activate the T cell response. We found that HSV-1 utilizes a robust and finely targeted mechanism of local immune evasion. It downregulates the expression of the costimulatory molecule CD80 but not CD86 on resident dendritic cells irrespective of the presence of anti-HSV-1 antibodies. The effect is mediated by direct binding of HSV-1 ICP22, the product of an immediate early gene of HSV-1, to the promoter of CD80. This immune evasion mechanism dampens the host immune response and, thus, reduces eye disease in ocularly infected mice. Therefore, ICP22 may be a novel inhibitor of CD80 that could be used to modulate the immune response.

The HIV Integrase Inhibitor Raltegravir Inhibits Felid Alphaherpesvirus 1 Replication by Targeting both DNA Replication and Late Gene Expression

Alphaherpesvirus-associated ocular infections in humans caused by human alphaherpesvirus 1 (HHV-1) remain challenging to treat due to the frequency of drug application required and the potential for the selection of drug-resistant viruses. Repurposing on-the-market drugs is a viable strategy to accelerate the pace of drug development. It has been reported that the human immunodeficiency virus (HIV) integrase inhibitor raltegravir inhibits HHV-1 replication by targeting the DNA polymerase accessory factor and limits terminase-mediated genome cleavage of human betaherpesvirus 5 (HHV-5). We have previously shown, both in vitro and in vivo, that raltegravir can also inhibit the replication of felid alphaherpesvirus 1 (FeHV-1), a common ocular pathogen of cats with a pathogenesis similar to that of HHV-1 ocular disease. In contrast to what was reported for HHV-1, we were unable to select for a raltegravir-resistant FeHV-1 strain in order to define any basis for drug action. A candidate-based approach to explore the mode of action of raltegravir against FeHV-1 showed that raltegravir did not impact FeHV-1 terminase function, as described for HHV-5. Instead, raltegravir inhibited DNA replication, similarly to HHV-1, but by targeting the initiation of viral DNA replication rather than elongation. In addition, we found that raltegravir specifically repressed late gene expression independently of DNA replication, and both activities are consistent with inhibition of ICP8. Taken together, these results suggest that raltegravir could be a valuable therapeutic agent against herpesviruses.IMPORTANCE The rise of drug-resistant herpesviruses is a longstanding concern, particularly among immunocompromised patients. Therefore, therapies targeting viral proteins other than the DNA polymerase that may be less likely to lead to drug-resistant viruses are urgently needed. Using FeHV-1, an alphaherpesvirus closely related to HHV-1 that similarly causes ocular herpes in its natural host, we found that the HIV integrase inhibitor raltegravir targets different stages of the virus life cycle beyond DNA replication and that it does so without developing drug resistance under the conditions tested. This shows that the drug could provide a viable strategy for the treatment of herpesvirus infections.

MODERN ETHIOTROPIC CHEMOTHERAPY OF HERPESVIRUS INFECTIONS: ADVANCES, NEW TRENDS AND PERSPECTIVES. ALPHAHERPESVIRUSES (PART II)

A key role in the treatment of herpesviral infections is played by modified nucleosides and their predecessors - acyclovir, its L-valine ester (valaciclovir) and famciclovir (prodrug of penciclovir). The biological activity of compounds of this class is determined by their similarity to natural nucleosides. After phosphorylation by viral thymidine kinase and then cell enzymes to the triphosphate forms, acyclovir and penciclovir inhibit the activity of viral DNA polymerase and synthesis of viral DNA. The increasing role of herpesvirus infections in human infectious pathology, as well as the development of drug resistance in viruses, mainly in patients with immunodeficiencies of various origins, necessitate the search for new compounds possessing anti-herpesvirus activity, using as a biological target not DNA polymerase, but other viral proteins and enzymes, unique or different from cellular proteins, performing similar functions.

Timing Is Everything

N. Drayman et al. in their recent article (mBio 8:e01612-17, 2017, https://doi.org/10.1128/mBio.01612-17) have used dynamic proteomics and machine learning to show that the cell cycle state of any individual cell affects the outcome of a productive herpes simplex virus 1 (HSV-1) infection. Cells infected from early G₁ through S were most permissive for expression of genes from the HSV-1 genome, whereas cells infected in late G₂ to mitosis were much less so. Most of the infected cells that underwent mitosis became permanently nonpermissive for HSV-1 gene expression afterward. The cell cycle stage accounted for 60% of the success of infection, and cell density and motility accounted for most of the rest. To successfully reactivate, HSV-1 must express its genes in neurons and cells of the spinosum and granulosum epidermis strata. These cells are permanently in the cell cycle stages most permissive for HSV-1 gene expression, and none reenters mitosis, thus maximizing the efficiency of a successful HSV-1 reactivation before the adaptive immunity can control it.

The ERK-1 function is required for HSV-1-mediated G1/S progression in HEP-2 cells and contributes to virus growth

The herpes simplex virus 1 is able to readdress different cellular pathways including cell cycle to facilitate its replication and spread. During infection, the progression of the cell cycle from G1 to S phase makes the cellular replication machinery accessible to viral DNA replication. In this work we established that HSV-1, in asynchronized HEp-2 cells, strictly controls cell cycle progression increasing S-phase population from 9 hours post infection until the end of HSV-1 replication. The G1/S phases progression depends on two important proteins, cyclin E and CDK2. We demonstrate that their phosphorylated status and then their activity during the infection is strongly correlated to viral replication events. In addition, HSV-1 is able to recruit and distribute ERK1/2 proteins in a spatio-temporal fashion, highlighting its downstream regulatory effects on cellular processes. According with this data, using chemical inhibitor U0126 and ERK dominant negative cells we found that the lack of ERK1 activity affects cyclin E protein accumulation, viral gene transcription and percentage of the cells in S phase, during the viral replication. These data suggested a complex interaction between ERK, cell cycle progression and HSV-1 replication.

Novel Role for Protein Inhibitor of Activated STAT 4 (PIAS4) in the Restriction of Herpes Simplex Virus 1 by the Cellular Intrinsic Antiviral Immune Response

Small ubiquitin-like modifier (SUMO) is used by the intrinsic antiviral immune response to restrict viral pathogens, such as herpes simplex virus 1 (HSV-1). Despite characterization of the host factors that rely on SUMOylation to exert their antiviral effects, the enzymes that mediate these SUMOylation events remain to be defined. We show that unconjugated SUMO levels are largely maintained throughout infection regardless of the presence of ICP0, the HSV-1 SUMO-targeted ubiquitin ligase. Moreover, in the absence of ICP0, high-molecular-weight SUMO-conjugated proteins do not accumulate if HSV-1 DNA does not replicate. These data highlight the continued importance for SUMO signaling throughout infection. We show that the SUMO ligase protein inhibitor of activated STAT 4 (PIAS4) is upregulated during HSV-1 infection and localizes to nuclear domains that contain viral DNA. PIAS4 is recruited to sites associated with HSV-1 genome entry through SUMO interaction motif (SIM)-dependent mechanisms that are destabilized by ICP0. In contrast, PIAS4 accumulates in replication compartments through SIM-independent mechanisms irrespective of ICP0 expression. Depletion of PIAS4 enhances the replication of ICP0-null mutant HSV-1, which is susceptible to restriction by the intrinsic antiviral immune response. The mechanisms of PIAS4-mediated restriction are synergistic with the restriction mechanisms of a characterized intrinsic antiviral factor, promyelocytic leukemia protein, and are antagonized by ICP0. We provide the first evidence that PIAS4 is an intrinsic antiviral factor. This novel role for PIAS4 in intrinsic antiviral immunity contrasts with the known roles of PIAS proteins as suppressors of innate immunity.

IMPORTANCE

Posttranslational modifications with small ubiquitin-like modifier (SUMO) proteins regulate multiple aspects of host immunity and viral replication. The protein inhibitor of activated STAT (PIAS) family of SUMO ligases is predominantly associated with the suppression of innate immune signaling. We now identify a unique and contrasting role for PIAS proteins as positive regulators of the intrinsic antiviral immune response to herpes simplex virus 1 (HSV-1) infection. We show that PIAS4 relocalizes to nuclear domains that contain viral DNA throughout infection. Depletion of PIAS4, either alone or in combination with the intrinsic antiviral factor promyelocytic leukemia protein, significantly impairs the intrinsic antiviral immune response to HSV-1 infection. Our data reveal a novel and dynamic role for PIAS4 in the cellular-mediated restriction of herpesviruses and establish a new functional role for the PIAS family of SUMO ligases in the intrinsic antiviral immune response to DNA virus infection.

Remote Activation of Host Cell DNA Synthesis in Uninfected Cells Signaled by Infected Cells in Advance of Virus Transmission

Viruses modulate cellular processes and metabolism in diverse ways, but these are almost universally studied in the infected cell itself. Here, we study spatial organization of DNA synthesis during multiround transmission of herpes simplex virus (HSV) using pulse-labeling with ethynyl nucleotides and cycloaddition of azide fluorophores. We report a hitherto unknown and unexpected outcome of virus-host interaction. Consistent with the current understanding of the single-step growth cycle, HSV suppresses host DNA synthesis and promotes viral DNA synthesis in spatially segregated compartments within the cell. In striking contrast, during progressive rounds of infection initiated at a single cell, we observe that infection induces a clear and pronounced stimulation of cellular DNA replication in remote uninfected cells. This induced DNA synthesis was observed in hundreds of uninfected cells at the extended border, outside the perimeter of the progressing infection. Moreover, using pulse-chase analysis, we show that this activation is maintained, resulting in a propagating wave of host DNA synthesis continually in advance of infection. As the virus reaches and infects these activated cells, host DNA synthesis is then shut off and replaced with virus DNA synthesis. Using nonpropagating viruses or conditioned medium, we demonstrate a paracrine effector of uninfected cell DNA synthesis in remote cells continually in advance of infection. These findings have significant implications, likely with broad applicability, for our understanding of the ways in which virus infection manipulates cell processes not only in the infected cell itself but also now in remote uninfected cells, as well as of mechanisms governing host DNA synthesis.

IMPORTANCE

We show that during infection initiated by a single particle with progressive cell-cell virus transmission (i.e., the normal situation), HSV induces host DNA synthesis in uninfected cells, mediated by a virus-induced paracrine effector. The field has had no conception that this process occurs, and the work changes our interpretation of virus-host interaction during advancing infection and has implications for understanding controls of host DNA synthesis. Our findings demonstrate the utility of chemical biology techniques in analysis of infection processes, reveal distinct processes when infection is examined in multiround transmission versus single-step growth curves, and reveal a hitherto-unknown process in virus infection, likely relevant for other viruses (and other infectious agents) and for remote signaling of other processes, including transcription and protein synthesis.

Discovery of the antiviral activities of pharmacologic cyclin-dependent kinase inhibitors: from basic to applied science

'Owing to the strong potential as antivirals exhibited to date by PCIs, cellular proteins are now starting to be considered more often as valid targets for antiviral drugs'

The differential mobilization of histones H3.1 and H3.3 by herpes simplex virus 1 relates histone dynamics to the assembly of viral chromatin

During lytic infections, HSV-1 genomes are assembled into unstable nucleosomes. The histones required for HSV-1 chromatin assembly, however, are in the cellular chromatin. We have shown that linker (H1) and core (H2B and H4) histones are mobilized during HSV-1 infection, and proposed that the mobilized histones are available for assembly into viral chromatin. However, the actual relevance of histone mobilization remained unknown. We now show that canonical H3.1 and variant H3.3 are also mobilized during HSV-1 infection. Mobilization required no HSV-1 protein expression, although immediate early or early proteins enhanced it. We used the previously known differential association of H3.3 and H3.1 with HSV-1 DNA to test the relevance of histone mobilization. H3.3 binds to HSV-1 genomes first, whereas H3.1 only binds after HSV-1 DNA replication initiates. Consistently, H3.3 and H3.1 were differentially mobilized. H3.1 mobilization decreased with HSV-1 DNA replication, whereas H3.3 mobilization was largely unaffected by it. These results support a model in which previously mobilized H3.1 is immobilized by assembly into viral chromatin during HSV-1 DNA replication, whereas H3.3 is mobilized and assembled into HSV-1 chromatin throughout infection. The differential mobilizations of H3.3 and H3.1 are consistent with their differential assembly into viral chromatin. These data therefore relate nuclear histone dynamics to the composition of viral chromatin and provide the first evidence that histone mobilization relates to viral chromatin assembly.

H3.1 is typically assembled into chromatin during DNA replication-dependent chromatin assembly. However, histones undergo exchange with those not bound in chromatin. During such exchanges, DNA replication-independent chromatin assembly incorporates histone variants, such as H3.3. The HSV-1 genomes are chromatinized, albeit in unstable nucleosomes. The viral genomes initially associate with H3.3, then associate with H3.1 only after HSV-1 DNA replication initiates. These differential interactions are consistent with the DNA replication-independent or -dependent assembly of H3.3 or H3.1, respectively, in cellular chromatin. We have shown that linker (H1) and core (H2B and H4) histones are mobilized during HSV-1 infection, but the significance of this mobilization remained unknown. We now find that H3.3 and H3.1 are also mobilized during infection. H3.3 is mobilized to a similar extent before or after HSV-1 DNA replication, which is consistent with its DNA replication-independent assembly into HSV-1 chromatin. In contrast, H3.1 mobilization decreases during HSV-1 DNA replication, which is consistent with the assembly of previously mobilized H3.1 into HSV-1 chromatin concomitant with HSV-1 DNA replication. The mobilizations of H3.1 and H3.3 are consistent with their kinetics of association with HSV-1 genomes, providing the first indication that histone mobilization relates to the assembly of viral chromatin.

Phosphoproteomic analyses reveal signaling pathways that facilitate lytic gammaherpesvirus replication

Lytic gammaherpesvirus (GHV) replication facilitates the establishment of lifelong latent infection, which places the infected host at risk for numerous cancers. As obligate intracellular parasites, GHVs must control and usurp cellular signaling pathways in order to successfully replicate, disseminate to stable latency reservoirs in the host, and prevent immune-mediated clearance. To facilitate a systems-level understanding of phosphorylation-dependent signaling events directed by GHVs during lytic replication, we utilized label-free quantitative mass spectrometry to interrogate the lytic replication cycle of murine gammaherpesvirus-68 (MHV68). Compared to controls, MHV68 infection regulated by 2-fold or greater ca. 86% of identified phosphopeptides - a regulatory scale not previously observed in phosphoproteomic evaluations of discrete signal-inducing stimuli. Network analyses demonstrated that the infection-associated induction or repression of specific cellular proteins globally altered the flow of information through the host phosphoprotein network, yielding major changes to functional protein clusters and ontologically associated proteins. A series of orthogonal bioinformatics analyses revealed that MAPK and CDK-related signaling events were overrepresented in the infection-associated phosphoproteome and identified 155 host proteins, such as the transcription factor c-Jun, as putative downstream targets. Importantly, functional tests of bioinformatics-based predictions confirmed ERK1/2 and CDK1/2 as kinases that facilitate MHV68 replication and also demonstrated the importance of c-Jun. Finally, a transposon-mutant virus screen identified the MHV68 cyclin D ortholog as a viral protein that contributes to the prominent MAPK/CDK signature of the infection-associated phosphoproteome. Together, these analyses enhance an understanding of how GHVs reorganize and usurp intracellular signaling networks to facilitate infection and replication.

Potential of protein kinase inhibitors for treating herpesvirus-associated disease

Herpesviruses are ubiquitous human pathogens that establish lifelong persistent infections. Clinical manifestations range from mild self-limiting outbreaks such as childhood rashes and cold sores to the more severe and life-threatening outcomes of disseminated infection, encephalitis, and cancer. Nucleoside analog drugs that target viral DNA replication provide the primary means of treatment. However, extended use of these drugs can result in selection for drug-resistant strains, particularly in immunocompromised patients. In this review we will present recent observations about the participation of cellular protein kinases in herpesvirus biology and discuss the potential for targeting these protein kinases as well as the herpesvirus-encoded protein kinases as an anti-herpesvirus therapeutic strategy.

Centromere architecture breakdown induced by the viral E3 ubiquitin ligase ICP0 protein of herpes simplex virus type 1

The viral E3 ubiquitin ligase ICP0 protein has the unique property to temporarily localize at interphase and mitotic centromeres early after infection of cells by the herpes simplex virus type 1 (HSV-1). As a consequence ICP0 induces the proteasomal degradation of several centromeric proteins (CENPs), namely CENP-A, the centromeric histone H3 variant, CENP-B and CENP-C. Following ICP0-induced centromere modification cells trigger a specific response to centromeres called interphase Centromere Damage Response (iCDR). The biological significance of the iCDR is unknown; so is the degree of centromere structural damage induced by ICP0. Interphase centromeres are complex structures made of proximal and distal protein layers closely associated to CENP-A-containing centromeric chromatin. Using several cell lines constitutively expressing GFP-tagged CENPs, we investigated the extent of the centromere destabilization induced by ICP0. We show that ICP0 provokes the disappearance from centromeres, and the proteasomal degradation of several CENPs from the NAC (CENP-A nucleosome associated) and CAD (CENP-A Distal) complexes. We then investigated the nucleosomal occupancy of the centromeric chromatin in ICP0-expressing cells by micrococcal nuclease (MNase) digestion analysis. ICP0 expression either following infection or in cell lines constitutively expressing ICP0 provokes significant modifications of the centromeric chromatin structure resulting in higher MNase accessibility. Finally, using human artificial chromosomes (HACs), we established that ICP0-induced iCDR could also target exogenous centromeres. These results demonstrate that, in addition to the protein complexes, ICP0 also destabilizes the centromeric chromatin resulting in the complete breakdown of the centromere architecture, which consequently induces iCDR.

HSV-1 genome subnuclear positioning and associations with host-cell PML-NBs and centromeres regulate LAT locus transcription during latency in neurons

Major human pathologies are caused by nuclear replicative viruses establishing life-long latent infection in their host. During latency the genomes of these viruses are intimately interacting with the cell nucleus environment. A hallmark of herpes simplex virus type 1 (HSV-1) latency establishment is the shutdown of lytic genes expression and the concomitant induction of the latency associated (LAT) transcripts. Although the setting up and the maintenance of the latent genetic program is most likely dependent on a subtle interplay between viral and nuclear factors, this remains uninvestigated. Combining the use of in situ fluorescent-based approaches and high-resolution microscopic analysis, we show that HSV-1 genomes adopt specific nuclear patterns in sensory neurons of latently infected mice (28 days post-inoculation, d.p.i.). Latent HSV-1 genomes display two major patterns, called “Single” and “Multiple”, which associate with centromeres, and with promyelocytic leukemia nuclear bodies (PML-NBs) as viral DNA-containing PML-NBs (DCP-NBs). 3D-image reconstruction of DCP-NBs shows that PML forms a shell around viral genomes and associated Daxx and ATRX, two PML partners within PML-NBs. During latency establishment (6 d.p.i.), infected mouse TGs display, at the level of the whole TG and in individual cells, a substantial increase of PML amount consistent with the interferon-mediated antiviral role of PML. “Single” and “Multiple” patterns are reminiscent of low and high-viral genome copy-containing neurons. We show that LAT expression is significantly favored within the “Multiple” pattern, which underlines a heterogeneity of LAT expression dependent on the viral genome copy number, pattern acquisition, and association with nuclear domains. Infection of PML-knockout mice demonstrates that PML/PML-NBs are involved in virus nuclear pattern acquisition, and negatively regulate the expression of the LAT. This study demonstrates that nuclear domains including PML-NBs and centromeres are functionally involved in the control of HSV-1 latency, and represent a key level of host/virus interaction.

After an initial lytic infection, many viruses establish a lifelong latent infection that hides them from the host immune system activity until reactivation. To understand the resurgence of the associated diseases, it is indispensable to acquire a better knowledge of the different mechanisms involved in the antiviral defense. During latency, viral genomes of nuclear-replicative viruses, such as herpes simplex virus type 1 (HSV-1), are stored in the nucleus of host cells in a non-integrated form. Latency establishment is associated with a drastic change in HSV-1 gene expression program that is maintained until reactivation occurs. The last two decades of research has revealed that the functional organization of the cell nucleus, so-called nuclear architecture, is a major factor of regulation of cellular genes expression. Nonetheless, the role of nuclear architecture on HSV-1 gene expression has been widely overlooked. Here we describe that the genome of HSV-1 selectively interacts with two major nuclear structures, the promyelocytic nuclear bodies (PMLNBs or ND10) and the centromeres. We provide evidence supporting that these nuclear domains directly influence the behavior of latent viral genomes and their transcriptional activity. Overall, this study demonstrates that nuclear architecture is a major parameter driving the highly complex HSV-1 latency process.

Analysis of viral and cellular factors influencing herpesvirus-induced nuclear envelope breakdown

Herpesvirus nucleocapsids are translocated from their assembly site in the nucleus to the cytosol by acquisition of a primary envelope at the inner nuclear membrane which subsequently fuses with the outer nuclear membrane. This transport through the nuclear envelope requires homologs of the conserved herpesviral pUL31 and pUL34 proteins which form the nuclear egress complex (NEC). In its absence, 1,000-fold less virus progeny is produced. We isolated a UL34-negative mutant of the alphaherpesvirus pseudorabies virus (PrV), PrV-ΔUL34Pass, which regained replication competence after serial passages in cell culture by inducing nuclear envelope breakdown (NEBD) (B. G. Klupp, H. Granzow, and T. C. Mettenleiter, J. Virol. 85:8285-8292, 2011). To test whether this phenotype is unique, passaging experiments were repeated with a UL31 deletion mutant. After 60 passages, the resulting PrV-ΔUL31Pass replicated similarly to wild-type PrV. Ultrastructural analyses confirmed escape from the nucleus via NEBD, indicating an inherent genetic disposition in herpesviruses. To identify the mutated viral genes responsible for this phenotype, the genome of PrV-ΔUL34Pass was sequenced and compared to the genomes of parental PrV-Ka and PrV-ΔUL34. Targeted sequencing of PrV-ΔUL31Pass disclosed congruent mutations comprising genes encoding tegument proteins (pUL49, pUL46, pUL21, pUS2), envelope proteins (gI, pUS9), and protease pUL26. To investigate involvement of cellular pathways, different inhibitors of cellular kinases were tested. While induction of apoptosis or inhibition of caspases had no specific effect on the passaged mutants, roscovitine, a cyclin-dependent kinase inhibitor, and U0126, an inhibitor of MEK1/2, specifically impaired replication of the passaged mutants, indicating involvement of mitosis-related processes in herpesvirus-induced NEBD.

CDK INHIBITORY NUCLEOSIDE ANALOGS PREVENT TRANSCRIPTION FROM VIRAL GENOMES

Targeting viral proteins has lead to many successful antivirals. Yet, such antivirals rapidly select for resistance, tend to be active against only a few related viruses, and require previous characterization of the target proteins. Alternatively, antivirals may be targeted to cellular proteins. Replication of many viruses requires cellular CDKs and pharmacological CDK inhibitors (PCIs), such as the purine-based roscovitine (Rosco), are proving safe in clinical trials against cancer. Rosco inhibits replication of wild-type or (multi-)drug resistant HIV, HCMV, EBV, VZV, and HSV-1 and 2. However, the antiviral mechanisms of purine PCIs remain unknown. Our objective is to characterize these mechanisms using HSV as a model We have shown that Rosco prevents initiation of transcription from viral, but not cellular, genomes. This inhibition is promoter independent, but genome dependent, and requires no viral proteins. This is a novel antiviral mechanism and a previously unknown activity for purine PCIs.

Core histones H2B and H4 are mobilized during infection with herpes simplex virus 1

The infecting genomes of herpes simplex virus 1 (HSV-1) are assembled into unstable nucleosomes soon after nuclear entry. The source of the histones that bind to these genomes has yet to be addressed. However, infection inhibits histone synthesis. The histones that bind to HSV-1 genomes are therefore most likely those previously bound in cellular chromatin. In order for preexisting cellular histones to associate with HSV-1 genomes, however, they must first disassociate from cellular chromatin. Consistently, we have shown that linker histones are mobilized during HSV-1 infection. Chromatinization of HSV-1 genomes would also require the association of core histones. We therefore evaluated the mobility of the core histones H2B and H4 as measures of the mobilization of H2A-H2B dimers and the more stable H3-H4 core tetramer. H2B and H4 were mobilized during infection. Their mobilization increased the levels of H2B and H4 in the free pools and decreased the rate of H2B fast chromatin exchange. The histones in the free pools would then be available to bind to HSV-1 genomes. The mobilization of H2B occurred independently from HSV-1 protein expression or DNA replication although expression of HSV-1 immediate-early (IE) or early (E) proteins enhanced it. The mobilization of core histones H2B and H4 supports a model in which the histones that associate with HSV-1 genomes are those that were previously bound in cellular chromatin. Moreover, this mobilization is consistent with the assembly of H2A-H2B and H3-H4 dimers into unstable nucleosomes with HSV-1 genomes.

Role of a cdk5-associated protein, p35, in herpes simplex virus type 1 replication in vivo

Previous studies have shown that herpes simplex virus type 1 (HSV-1) replication is inhibited by the cyclin-dependent kinase (cdk) inhibitor roscovitine. One roscovitine-sensitive cdk that functions in neurons is cdk5, which is activated in part by its binding partner, p35. Because HSV establishes latent infections in sensory neurons, we sought to determine the role p35 plays in HSV-1 replication in vivo. For these studies, wild-type (wt) and p35−/− mice were infected with HSV-1 using the mouse ocular model of HSV latency and reactivation. The current results indicate that p35 is an important determinant of viral replication in vivo.

MHV-68 Open Reading Frame 20 is a nonessential gene delaying lung viral clearance

Recently, it has been demonstrated that the MHV-68 ORF20-encoded gene product induces cell-cycle arrest at the G2/M phase, followed by apoptosis. To study the role of this conserved gene in vivo, two independent ORF20-deficient MHV-68 viruses and their revertants were constructed. As the replication in vitro of both mutants followed similar kinetics to that of the wild-type and revertant viruses, ORF20 is therefore a nonessential virus gene. No cell cycle arrest could be observed upon infection of cells with wild type MHV-68 or mutant viruses. In addition, no major differences were detected between mock- and virus-infected cells when protein and inactivation levels of the mitotic promoter factor cdc2/cyclinB were analyzed. Following intranasal infection, the recovery of mutant, revertant and wild-type viruses in the lungs was similar. With the ORF20-deficient viruses, however, there was a significant delay of four days in clearance of virus from the lungs. Surprisingly, the magnitude and cell population distribution in the exudates of the lung was essentially similar to mice infected with wild-type, revertant or ORF20-deleted viruses. Subsequent establishment of latency was normal for both mutants, demonstrating that ORF20 does not play a critical role in establishment of a persistent infection. These results indicate that while expression of ORF20 may impact on the pathogenicity of the infection, the observed induction of G2/M arrest in ORF20-expressing cells may not be the primary function of ORF20 in the context of viral infection.

Compounds that target host cell proteins prevent varicella-zoster virus replication in culture, ex vivo, and in SCID-Hu mice

Varicella-zoster virus (VZV) replicates in quiescent T cells, neurons, and skin cells. In cultured fibroblasts (HFFs), VZV induces host cyclin expression and cyclin-dependent kinase (CDK) activity without causing cell cycle progression. CDK1/cyclin B1 phosphorylates the major viral transactivator, and the CDK inhibitor roscovitine prevents VZV mRNA transcription. We investigated the antiviral effects of additional compounds that target CDKs or other cell cycle enzymes in culture, ex vivo, and in vivo. Cytotoxicity and cell growth arrest doses were determined by Neutral Red assay. Antiviral effects were evaluated in HFFs by plaque assay, genome copy number, and bioluminescence. Positive controls were acyclovir (400 microM) and phosphonoacetic acid (PAA, 1 mM). Test compounds were roscovitine, aloisine A, and purvalanol A (CDK inhibitors), aphidicolin (inhibits human and herpesvirus DNA polymerase), l-mimosine (indirectly inhibits human DNA polymerase), and DRB (inhibits casein kinase 2). All had antiviral effects below the concentrations required for cell growth arrest. Compounds were tested in skin organ culture at EC(99) doses; all prevented VZV replication in skin, except for aloisine A and purvalanol A. In SCID mice with skin xenografts, roscovitine (0.7 mg/kg/day) was as effective as PAA (36 mg/kg/day). The screening systems described here are useful models for evaluating novel antiviral drugs for VZV.

Classic swine fever virus NS2 protein leads to the induction of cell cycle arrest at S-phase and endoplasmic reticulum stress

Background

Classical swine fever (CSF) caused by virulent strains of Classical swine fever virus (CSFV) is a haemorrhagic disease of pigs, characterized by disseminated intravascular coagulation, thrombocytopoenia and immunosuppression, and the swine endothelial vascular cell is one of the CSFV target cells. In this report, we investigated the previously unknown subcellular localization and function of CSFV NS2 protein by examining its effects on cell growth and cell cycle progression.

Results

Stable swine umbilical vein endothelial cell line (SUVEC) expressing CSFV NS2 were established and showed that the protein localized to the endoplasmic reticulum (ER). Cellular analysis revealed that replication of NS2-expressing cell lines was inhibited by 20-30% due to cell cycle arrest at S-phase. The NS2 protein also induced ER stress and activated the nuclear transcription factor kappa B (NF-κB). A significant increase in cyclin A transcriptional levels was observed in NS2-expressing cells but was accompanied by a concomitant increase in the proteasomal degradation of cyclin A protein. Therefore, the induction of cell cycle arrest at S-phase by CSFV NS2 protein is associated with increased turnover of cyclin A protein rather than the down-regulation of cyclin A transcription.

Conclusions

All the data suggest that CSFV NS2 protein modulate the cellular growth and cell cycle progression through inducing the S-phase arrest and provide a cellular environment that is advantageous for viral replication. These findings provide novel information on the function of the poorly characterized CSFV NS2 protein.

Human cytomegalovirus IE1 protein enhances herpes simplex virus type 1-induced syncytial formation in U373MG cells

Co-infection of herpes simplex virus type 1 (HSV-1) and human cytomegalovirus (HCMV) is not uncommon in immunocompromised hosts. Importantly, organ transplant recipients concurrently infected with HSV-1 and HCMV have a worse clinical outcome than recipients infected with a single virus. However, factors regulating the pathologic response in HSV-1, HCMV co-infected tissues are unclear. We investigated the potential biologic role of HCMV gene product immediate early 1 (IE1) protein in HSV-1-induced syncytial formation in U373MG cells. We utilized a co-infection model by infecting HSV-1 to U373MG cells constitutively expressing HCMV IE1 protein, UMG1-2. Syncytial formation was assessed by enumerating nuclei number per syncytium and number of syncytia. HSV-1-induced syncytial formation was enhanced after 24 hr in UMG1-2 cells compared with U373MG controls. The amplified phenotype in UMG1-2 cells was effectively suppressed by roscovitine in addition to inhibitors of viral replication. This is the first study to provide histological evidence of the contribution of HCMV IE1 protein to enhanced cytopathogenic responses in active HSV-1 infection.

Cyclin-dependent kinase 1/cyclin B1 phosphorylates varicella-zoster virus IE62 and is incorporated into virions

Varicella-zoster virus (VZV), an alphaherpesvirus restricted to humans, infects differentiated cells in vivo, including T lymphocytes, keratinocytes, and neurons, and spreads rapidly in confluent cultured dermal fibroblasts (HFFs). In VZV-infected HFFs, atypical expression of cyclins D3 and B1 occurs along with the induction of cyclin-dependent kinase (CDK) activity. A specific CDK1 inhibitor blocked VZV spread, indicating an important function for this cellular kinase in VZV replication. CDK activity assays of infected cells revealed a large viral phosphoprotein that was identified as being the major immediate-early transactivator, IE62. Since IE62 colocalized with CDK1/cyclin B1 by confocal microscopy, we investigated whether this cellular kinase complex interacts with IE62. Using recombinant fragments of IE62 spanning the entire amino acid sequence, we found that purified CDK1/cyclin B1 phosphorylated IE62 at residues T10, S245, and T680 in vitro. Immunoprecipitation of cyclin B1 from VZV-infected HFFs indicated that IE62 was included in the complex within infected cells. The full-length IE62 protein, obtained by immunoprecipitation from infected cells, was also phosphorylated by purified CDK1/cyclin B1. Based on IE62/CDK1/cyclin B1 colocalization near viral assembly regions, we hypothesized that these cellular proteins could be incorporated into VZV virions with IE62. Purified virions were analyzed by immunoblotting for the presence of CDK1 and cyclin B1, and active CDK1 and cyclin B1 were present in the VZV tegument with IE62 and were sensitive to detergent treatment. Thus, IE62 is a substrate for CDK1/cyclin B1, and virions could deliver the active cellular kinase to nondividing cells that normally do not express it.

The antiproliferative activity of melatonin in B65 rat dopaminergic neuroblastoma cells is related to the downregulation of cell cycle-related genes

A potential application of melatonin is its ability to rescue many cell types from cell death, because of its antioxidant properties. Likewise, recent studies suggest that melatonin may also be used as an anti-tumor drug, due to its anti-proliferative properties in tumor cells when administered at physiologic or pharmacologic doses. In the present study, we investigated the mechanisms involved in the apoptosis induced by acute exposure to melatonin and roscovitine in the rat dopaminergic neuroblastoma B65 cell line. Cell growth studies revealed that, at 24 hr of treatment, roscovitine blocked cell growth and induced apoptosis whereas melatonin delayed cell growth and induced a slight increase in the number of apoptotic nuclei. Melatonin also increased the percentage of cells in the G1-phase of the cell cycle, whereas roscovitine blocked cells in the G2/M-phase. Both compounds significantly downregulated the transcriptional activity of cdk4, while melatonin also downregulated cdk2 and cyclin D1. Taken together, our data show that melatonin at millimolar concentrations inhibits dopaminergic B65 proliferation, induces cell apoptosis, and modulates cell cycle progression by inhibiting the transcriptional activity of cyclins and cdks related to the progression of the G1-phase.

Linker histones are mobilized during infection with herpes simplex virus type 1

Histones interact with herpes simplex virus type 1 (HSV-1) genomes and localize to replication compartments early during infections. However, HSV-1 genomes do not interact with histones in virions and are deposited in nuclear domains devoid of histones. Moreover, late viral replication compartments are also devoid of histones. The processes whereby histones come to interact with HSV-1 genomes, to be later displaced, remain unknown. However, they would involve the early movement of histones to the domains containing HSV-1 genomes and the later movement away from them. Histones unbind from chromatin, diffuse through the nucleoplasm, and rebind at different sites. Such mobility is upregulated by, for example, phosphorylation or acetylation. We evaluated whether HSV-1 infection modulates histone mobility, using fluorescence recovery after photobleaching. All somatic H1 variants were mobilized to different degrees. H1.2, the most mobilized, was mobilized at 4 h and further so at 7 h after infection, resulting in increases in its "free" pools. H1.2 was mobilized to a "basal" degree under conditions of little to no HSV-1 protein expression. This basal mobilization required nuclear native HSV-1 genomes but was independent of HSV-1 proteins and most likely due to cellular responses. Mobilization above this basal degree, and increases in H1.2 free pools, however, depended on immediate-early or early HSV-1 proteins, but not on HSV-1 genome replication or late proteins. Linker histone mobilization is a novel consequence of cell-virus interactions, which is consistent with the dynamic interactions between histones and HSV-1 genomes during lytic infection; it may also participate in the regulation of viral gene expression.

Inhibition of the cyclin-dependent kinases at the beginning of human cytomegalovirus infection specifically alters the levels and localization of the RNA polymerase II carboxyl-terminal domain kinases cdk9 and cdk7 at the viral transcriptosome

We previously reported that defined components of the host transcription machinery are recruited to human cytomegalovirus immediate-early (IE) transcription sites, including cdk9 and cdk7 (S. Tamrakar, A. J. Kapasi, and D. H. Spector, J. Virol. 79:15477-15493, 2005). In this report, we further document the complexity of this site, referred to as the transcriptosome, through identification of additional resident proteins, including viral UL69 and cellular cyclin T1, Brd4, histone deacetylase 1 (HDAC1), and HDAC2. To examine the role of cyclin-dependent kinases (cdks) in the establishment of this site, we used roscovitine, a specific inhibitor of cdk1, cdk2, cdk7, and cdk9, that alters processing of viral IE transcripts and inhibits expression of viral early genes. In the presence of roscovitine, IE2, cyclin T1, Brd4, HDAC1, and HDAC2 accumulate at the transcriptosome. However, accumulation of cdk9 and cdk7 was specifically inhibited. Roscovitine treatment also resulted in decreased levels of cdk9 and cdk7 RNA. There was a corresponding reduction in cdk9 protein but only a modest decrease in cdk7 protein. However, overexpression of cdk9 does not compensate for the effects of roscovitine on cdk9 localization or viral gene expression. Delaying the addition of roscovitine until 8 h postinfection prevented all of the observed effects of the cdk inhibitor. These data suggest that IE2 and multiple cellular factors needed for viral RNA synthesis accumulate within the first 8 h at the viral transcriptosome and that functional cdk activity is required for the specific recruitment of cdk7 and cdk9 during this time interval.

Emerin is hyperphosphorylated and redistributed in herpes simplex virus type 1-infected cells in a manner dependent on both UL34 and US3

Cells infected with wild-type herpes simplex virus type 1 (HSV-1) show disruption of the organization of the nuclear lamina that underlies the nuclear envelope. This disruption is reflected in changes in the localization and phosphorylation of lamin proteins. Here, we show that HSV-1 infection causes relocalization of the LEM domain protein emerin. In cells infected with wild-type virus, emerin becomes more mobile in the nuclear membrane, and in cells infected with viruses that fail to express UL34 protein (pUL34) and US3 protein (pUS3), emerin no longer colocalizes with lamins, suggesting that infection causes a loss of connection between emerin and the lamina. Infection causes hyperphosphorylation of emerin in a manner dependent upon both pUL34 and pUS3. Some emerin hyperphosphorylation can be inhibited by the protein kinase Cdelta (PKCdelta) inhibitor rottlerin. Emerin and pUL34 interact physically, as shown by pull-down and coimmunoprecipitation assays. Emerin expression is not, however, necessary for infection, since virus growth is not impaired in cells derived from emerin-null transgenic mice. The results suggest a model in which pUS3 and PKCdelta that has been recruited by pUL34 hyperphosphorylate emerin, leading to disruption of its connections with lamin proteins and contributing to the disruption of the nuclear lamina. Changes in emerin localization, nuclear shape, and lamin organization characteristic of cells infected with wild-type HSV-1 also occur in cells infected with recombinant virus that does not make viral capsids, suggesting that these changes occur independently of capsid envelopment.

Varicella-zoster virus infection of human foreskin fibroblast cells results in atypical cyclin expression and cyclin-dependent kinase activity

In its course of human infection, varicella-zoster virus (VZV) infects rarely dividing cells such as dermal fibroblasts, differentiated keratinocytes, mature T cells, and neurons, none of which are actively synthesizing DNA; however, VZV is able to productively infect them and use their machinery to replicate the viral genome. We hypothesized that VZV alters the intracellular environment to favor viral replication by dysregulating cell cycle proteins and kinases. Cyclin-dependent kinases (CDKs) and cyclins displayed a highly unusual profile in VZV-infected confluent fibroblasts: total amounts of CDK1, CDK2, cyclin B1, cyclin D3, and cyclin A protein increased, and kinase activities of CDK2, CDK4, and cyclin B1 were strongly and simultaneously induced. Cyclins B1 and D3 increased as early as 24 h after infection, concurrent with VZV protein synthesis. Confocal microscopy indicated that cyclin D3 overexpression was limited to areas of IE62 production, whereas cyclin B1 expression was irregular across the VZV plaque. Downstream substrates of CDKs, including pRb, p107, and GM130, did not show phosphorylation by immunoblotting, and p21 and p27 protein levels were increased following infection. Finally, although the complement of cyclin expression and high CDK activity indicated a progression through the S and G(2) phases of the cell cycle, DNA staining and flow cytometry indicated a possible G(1)/S blockade in infected cells. These data support earlier studies showing that pharmacological CDK inhibitors can inhibit VZV replication in cultured cells.

Five years of progress on cyclin-dependent kinases and other cellular proteins as potential targets for antiviral drugs

In 1997-1998, the pharmacological cyclin-dependent kinase (CDK) inhibitors (PCIs) were independently discovered to inhibit replication of human cytomegalovirus, herpes simplex virus type 1 and HIV-1. The results from small clinical trials against cancer were then suggesting that PCIs could be safe enough to be used clinically. It was thus hypothesized that PCIs could have the potential to be developed as novel antivirals targeting cellular proteins. Consequently, Antiviral Chemistry & Chemotherapy published in 2001 the first review on the potential of CDKs, and cellular proteins in general, as potential targets for antivirals. The viral functions inhibited by PCIs, or their cellular targets, were then just starting to be characterized. The antiviral spectrum of PCIs and their effects on viral disease were still mostly untested. Even their actual specificity was not yet completely characterized. In addition, cellular proteins were not accepted as valid targets for antivirals. Significant progress has been made in the last 5 years in understanding the antiviral activities of PCIs and the potential roles of cellular proteins in general as targets for antivirals. The first clinical trials of the antiviral activities of PCIs and other inhibitors of cellular protein kinases have now been scheduled. Herein, we review the progress made since the publication of the first review on PCIs as potential antiviral drugs and on CDKs, and cellular proteins in general, as potential targets for antiviral drugs. We also highlight the major issues that still need to be addressed before PCIs or other drugs targeting cellular proteins can be developed as clinical antivirals.

Emerging host cell targets for hepatitis C therapy

Chronic hepatitis C virus (HCV) infection is a major burden on humanity. The current HCV therapy has limited efficacy, and there is pressing need for new and more effective therapies. Host cell factors that are required for HCV infection, replication and/or pathogenesis represent potential therapeutic targets. Of particular interest are cellular receptors that mediate HCV entry, factors that facilitate HCV replication and assembly, and intracellular pathways involving lipid biosynthesis, oxidative stress and innate immune response. A crucial challenge now is to manipulate such cellular targets pharmacologically for chronic HCV treatment, without being limited by side effects.

Cyclin-dependent kinase activity is required for efficient expression and posttranslational modification of human cytomegalovirus proteins and for production of extracellular particles

We have previously shown that the addition of the cyclin-dependent kinase (cdk) inhibitor Roscovitine at the beginning of infection of cells with human cytomegalovirus (HCMV) significantly disrupts immediate-early gene expression and the progression of the infection. In the present study, we have examined the effects of cdk inhibition on late viral events by delaying addition of Roscovitine until 24 h postinfection. Although viral DNA replication was inhibited two- to threefold by treatment of infected cells with Roscovitine, the drop did not correspond to the 1- to 2-log-unit decrease in virus titer. Quantification of viral DNA in the supernatant from cells revealed that there was a significant reduction in the production or release of extracellular particles. We observed a lag in the expression of several viral proteins but there was a significant decrease in the steady-state levels of IE2-86. Likewise, the steady-state level of the essential tegument protein UL32 (pp150) was reduced. The levels of pp150 and IE2-86 mRNA were not greatly affected by treatment with Roscovitine and thus did not correlate with the reduced levels of protein. In contrast, the expression of the tegument protein ppUL69 was higher in drug-treated samples, and the protein accumulated in a hyperphosphorylated form. ppUL69 localized to intranuclear aggregates that did not overlap with viral replication centers in cells treated with Roscovitine. Taken together, these data indicate that cdk activity is required at multiple steps during HCMV infection, including the expression, modification, and localization of virus-encoded proteins.

Imaging immediate-early and strict-late promoter activity during oncolytic herpes simplex virus type 1 infection and replication in tumors

An increasing number of oncolytic viruses have been developed and studied for cancer therapy. In response to needs for non-invasive monitoring and imaging of oncolytic virotherapy, several different approaches, including a positron emission tomography-based method, a method using secreted marker peptides, and optical imaging-based methods, have been reported. Among these modalities, we utilized the luciferase-based bioluminescent assay/imaging systems to determine the kinetics and dynamics of a productive viral infection. The replication cycle of herpes simplex virus type 1 (HSV-1) is punctuated by a temporal cascade of three classes of viral genes: immediate-early (IE), early (E) and late (L) genes. U(L)39- and gamma(1)34.5-deleted, replication-conditional HSV-1 mutants that express firefly luciferase under the control of the IE4/5 or strict-late gC promoters were generated. These oncolytic viruses were examined in cultured cells and a mouse tumor model. IE promoter- and strict-late promoter-mediated luciferase expression was confirmed to indicate viral infection and replication, respectively. Incorporation of a strict-late promoter-driven luciferase cassette into oncolytic HSV-1 vectors would be useful for assessing tumor oncolysis in preclinical tumor treatment studies.

Tissue distribution, pharmacokinetics and identification of roscovitine metabolites in rat

The pharmacokinetics, biodistribution and the metabolic pathway of roscovitine were investigated in Sprague-Dawley rats after a single intravenous dose of 25 mg/kg. Blood, lungs, kidney, liver, testis, adipose tissue, spleen and brain were removed at different time-points. Plasma and tissue samples were analyzed using high performance liquid chromatography. The metabolites were identified using liquid chromatography/tandem mass spectrometry and nuclear magnetic resonance spectroscopy. Roscovitine (MW=354) was cleared rapidly from circulation and highly distributed to the tissues. The elimination half-life of roscovitine in plasma and tissues was short (<30 min). A major metabolite (M1) was observed mainly in plasma and in lower levels in all other tissues. M1 was identified as conversion of the hydroxyl-group at C2 to carboxylic acid (MW=368). A second metabolite (M2) was observed mainly in liver and kidney and identified as a hydroxylation product of the C8 of the purine-ring (MW=370). A third metabolite (M3) was found in several organs and corresponded to N-dealkylation of the N9-isopropyl side-chain (MW=312). Roscovitine concentrations in the brain were 30% of that observed in plasma, however no metabolites were detected in brain. In this investigation, three major metabolites of roscovitine were isolated and identified. Also, it was shown that roscovitine eliminates rapidly from both blood and tissues.

Antiviral Activity of CYC202 in HIV-1-infected Cells

There are currently 40 million individuals in the world infected with human immunodeficiency virus (HIV). The introduction of highly active antiretroviral therapy (HAART) has led to a significant reduction in AIDS-related morbidity and mortality. Unfortunately, up to 25% of patients discontinue their initial HAART regimen. Current HIV-1 inhibitors target the fusion of the virus to the cell and two viral proteins, reverse transcriptase and protease. Here, we examined whether other targets, such as an activated transcription factor, could be targeted to block HIV-1 replication. We specifically asked whether we could target a cellular kinase needed for HIV-1 transcription using CYC202 (R-roscovitine), a pharmacological cyclin-dependent kinase inhibitor. We targeted the cdk2-cyclin E complex in HIV-1-infected cells because both cdk2 and cyclin E are nonessential during mammalian development and are likely replaced by other kinases. We found that CYC202 effectively inhibits wild type and resistant HIV-1 mutants in T-cells, monocytes, and peripheral blood mononuclear cells at a low IC(50) and sensitizes these cells to enhanced apoptosis resulting in a dramatic drop in viral titers. Interestingly, the effect of CYC202 is independent of cell cycle stage and more specific for the cdk2-cyclin E complex. Finally, we show that cdk2-cyclin E is loaded onto the HIV-1 genome in vivo and that CYC202 is able to inhibit the uploading of this cdk-cyclin complex onto HIV-1 DNA. Therefore, targeting cellular enzymes necessary for HIV-1 transcription, which are not needed for cell survival, is a compelling strategy to inhibit wild type and mutant HIV-1 strains.

Coordination of transcription factor phosphorylation and histone methylation by the P-TEFb kinase during human immunodeficiency virus type 1 transcription

The human immunodeficiency virus type 1 (HIV-1) Tat protein recruits positive transcription elongation factor b (P-TEFb) to the transactivation response (TAR) RNA structure to facilitate formation of processive transcription elongation complexes (TECs). Here we examine the role of the Tat/TAR-specified cyclin-dependent kinase 9 (CDK9) kinase activity in regulation of HIV-1 transcription elongation and histone methylation. In HIV-1 TECs, P-TEFb phosphorylates the RNA polymerase II (RNAP II) carboxyl-terminal domain (CTD) and the transcription elongation factors SPT5 and Tat-SF1 in a Tat/TAR-dependent manner. Using in vivo chromatin immunoprecipitation analysis, we demonstrate the following distinct properties of the HIV-1 transcription complexes. First, the RNAP II CTD is phosphorylated at Ser 2 and Ser 5 near the promoter and at downstream coding regions. Second, the stable association of SPT5 with the TECs is dependent upon P-TEFb kinase activity. Third, P-TEFb kinase activity is critical for the induction of methylation of histone H3 at lysine 4 and lysine 36 on HIV-1 genes. Flavopiridol, a potent P-TEFb kinase inhibitor, inhibits CTD phosphorylation, stable SPT5 binding, and histone methylation, suggesting that its potent antiviral activity is due to its ability to inhibit several critical and unique steps in HIV-1 transcription elongation.

Roscovitine inhibits activation of promoters in herpes simplex virus type 1 genomes independently of promoter-specific factors

Flavopiridol, roscovitine, and other inhibitors of Cyclin-Dependent Kinases (CDK) inhibit the replication of a variety of viruses in vitro while proving nontoxic in human clinical trials of their effects against cancer. Consequently, these and other Pharmacological CDK inhibitors (PCIs) have been proposed as potential antivirals. Flavopiridol potently inhibits all tested CDKs and inhibits the transcription of most cellular and viral genes. In contrast, roscovitine and other purine PCIs inhibit with high potency only CDK1, CDK2, CDK5, and CDK7, and they specifically inhibit the expression of viral but not cellular genes. The levels at which purine PCIs inhibit gene expression are unknown, as are the factors which determine their specificity for expression of viral but not cellular genes. We show herein that roscovitine prevents the initiation of transcription of herpes simplex virus type 1 (HSV-1) genes but has no effect on transcription elongation. We further show that roscovitine does not inhibit the initiation or elongation of cellular transcription and that its inhibitory effects are specific for promoters in HSV-1 genomes. Therefore, we have identified a novel biological activity for PCIs, i.e., their ability to prevent the initiation of transcription. We have also identified genome location as one of the factors that determine whether the transcription of a given gene is inhibited by roscovitine. The activities of roscovitine on viral transcription resemble one of the antiherpesvirus activities of alpha interferon and could be used as a model for the development of novel antivirals. The genome-specific effects of roscovitine may also be important for its development against virus-induced cancers.

PEP8-TAT2, a membrane-permeable peptide, inhibits cyclin-Cdk2 activity in HeLa cells

Here we show that the PEP8-TAT2 peptide is effectively transduced into HeLa cells and that it inhibits cellular cyclin-Cdk2 activity. Like the PEP8 peptide, the PEP8-TAT2 peptide inhibits Cdk2 activity in vitro with an IC50 value of 5 nM, as determined by an immuno-complex kinase assay. It also inhibits DNA synthesis in and proliferation of cultured HeLa cells by arresting cell cycle at the G1/S transition. Further, the PEP8-TAT2 peptide inhibits cell death-associated Cdk2 activity and thereby prevents apoptotic progression in paclitaxel-treated cells. We propose that this inhibitor peptide is an effective agent to suppress the proliferation of human cancer cells, as well as apoptotic progression, by blocking cellular cyclin-dependent Cdk kinase activity.

Use of a multi-virus array for the study of human viral and retroviral pathogens: gene expression studies and ChIP-chip analysis

Background

Since the discovery of human immunodeficiency virus (HIV-1) twenty years ago, AIDS has become one of the most studied diseases. A number of viruses have subsequently been identified to contribute to the pathogenesis of HIV and its opportunistic infections and cancers. Therefore, a multi-virus array containing eight human viruses implicated in AIDS pathogenesis was developed and its efficacy in various applications was characterized.

Results

The amplified open reading frames (ORFs) of human immunodeficiency virus type 1, human T cell leukemia virus types 1 and 2, hepatitis C virus, Epstein-Barr virus, human herpesvirus 6A and 6B, and Kaposi's sarcoma-associated herpesvirus were spotted on glass slides and hybridized to DNA and RNA samples. Using a random priming method for labeling genomic DNA or cDNA probes, we show specific detection of genomic viral DNA from cells infected with the human herpesviruses, and effectively demonstrate the inhibitory effects of a cellular cyclin dependent kinase inhibitor on viral gene expression in HIV-1 and KSHV latently infected cells. In addition, we coupled chromatin immunoprecipitation with the virus chip (ChIP-chip) to study cellular protein and DNA binding.

Conclusions

An amplicon based virus chip representing eight human viruses was successfully used to identify each virus with little cross hybridization. Furthermore, the identity of both viruses was correctly determined in co-infected cells. The utility of the virus chip was demonstrated by a variety of expression studies. Additionally, this is the first demonstrated use of ChIP-chip analysis to show specific binding of proteins to viral DNA, which, importantly, did not require further amplification for detection.