Replication-initiator protein (UL9) of the herpes simplex virus 1 binds NFB42 and is degraded via the ubiquitin-proteasome pathway

Host Cell Neddylation Facilitates Alphaherpesvirus Entry in a Virus-Specific and Cell-Dependent Manner

Herpes simplex virus 1 (HSV-1) commandeers the host cell proteasome at several steps of its replication cycle, including entry. Here we demonstrate that HSV-2, pseudorabies virus (PRV), and bovine herpesvirus 1 (BoHV-1) entry are blocked by bortezomib, a proteasome inhibitor that is an FDA-approved cancer drug. Proteasome-dependent entry of HSV-1 is thought to be ubiquitin-independent. To interrogate further the proteasomal mechanism of entry, we determined the involvement of the ubiquitin-like molecule NEDD8 and the neddylation cascade in alphaherpesvirus entry and infection. MLN4924 is a small-molecule inhibitor of neddylation that binds directly to the NEDD8-activating enzyme. Cell treatment with MLN4924 inhibited plaque formation and infectivity by HSV-1, PRV, and BoHV-1 at noncytotoxic concentrations. Thus, the neddylation pathway is broadly important for alphaherpesvirus infection. However, the neddylation inhibitor had little effect on entry of the veterinary viruses but had a significant inhibitory effect on entry of HSV-1 and HSV-2 into seven different cell types. Washout experiments indicated that MLN4924's effect on viral entry was reversible. A time-of-addition assay suggested that the drug was acting on an early step in the entry process. Small interfering RNA (siRNA) knockdown of NEDD8 significantly inhibited HSV entry. In probing the neddylation-dependent step in entry, we found that MLN4924 dramatically blocked endocytic uptake of HSV from the plasma membrane by >90%. In contrast, the rate of HSV entry into cells that support direct fusion of HSV with the cell surface was unaffected by MLN4924. Interestingly, proteasome activity was less important for the endocytic internalization of HSV from the cell surface. The results suggest that the NEDD8 cascade is critical for the internalization step of HSV entry. IMPORTANCE Alphaherpesviruses are ubiquitous pathogens of humans and veterinary species that cause lifelong latent infections and significant morbidity and mortality. Host cell neddylation is important for cell homeostasis and for the infection of many viruses, including HSV-1, HSV-2, PRV, and BoHV-1. Inhibition of neddylation by a pharmacologic inhibitor or siRNA blocked HSV infection at the entry step. Specifically, the NEDD8 pathway was critically important for HSV-1 internalization from the cell surface by an endocytosis mechanism. The results expand our limited understanding of cellular processes that mediate HSV internalization. To our knowledge, this is the first demonstration of a function for the neddylation cascade in virus entry.

Inhibition of herpes simplex virus-1 infection by MBZM-N-IBT: in silico and in vitro studies

Introduction

The emergence of drug resistance and cross-resistance to existing drugs has warranted the development of new antivirals for Herpes simplex viruses (HSV). Hence, we have designed this study to evaluate the anti-viral activity of 1-[(2-methyl benzimidazole-1-yl) methyl]-2-oxo-indolin-3-ylidene] amino] thiourea (MBZM-N-IBT), against HSV-1.

Method

Molecular docking was performed to assess the affinity of MBZM-N-IBT for HSV-1 targets. This was validated by plaque assay, estimation of RNA and protein levels as well as time of addition experiments in vitro.

Result

Molecular docking analysis suggested the inhibitory capacity of MBZM-N-IBT against HSV-1. This was supported by the abrogation of the HSV-1 infectious viral particle formation with the IC₅₀ value of 3.619 µM. Viral mRNA levels were also reduced by 72% and 84% for UL9 and gC respectively. MBZM-N-IBT also reduced the protein synthesis for gC and ICP8 significantly. While mRNA of ICP8 was not significantly affected, its protein synthesis was reduced by 47%. The time of addition experiment revealed the capacity of MBZM-N-IBT to inhibit HSV-1 at early as well as late stages of infection in the Vero cells. Similar effect of MBZM-N-IBT was also noticed in the Raw 264.7 and BHK 21 cells after HSV-1 infection. Supported by the in silico data, this can be attributed to possible interference with multiple HSV targets including the ICP8, ICP27, UL42, UL25, UL15 and gB proteins.

Conclusion

These results along with the lack of acute oral toxicity and significant anti-inflammatory effects suggest its suitability for further evaluation as a non-nucleoside inhibitor of HSV.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-021-01581-5.

MG-132 interferes with iron cellular homeostasis and alters virulence of bovine herpesvirus 1

Bovine herpesvirus 1 (BoHV-1) requires an iron-replete cell host to replicate efficiently. BoHV-1 infection provokes an increase in ferritin levels and a decrease of transferrin receptor 1 (TfR-1) expression, ultimately lowering iron pool extent. Thus, cells try to limit iron availability for virus spread. It has been demonstrated that MG-132, a proteasome inhibitor, reduces BoHV-1 release. Since ferritin, the major iron storage protein in mammalian cells, undergoes proteasome-mediated degradation, herein, the influence of MG-132 on iron metabolism during BoHV-1 infection was examined. Following infection in bovine cells (MDBK), MG-132 reduced cell death and viral yield. Western blot analysis showed a significant ferritin accumulation, likely due to the inhibition of its proteasome-mediated degradation pathway. In addition, the concomitant down-regulation of TfR-1 expression, observed during infection, was counteracted by proteasome inhibitor. This trend may be explained by enhanced acidic vesicular organelles, detected by acridine orange staining, determining a reduction of intracellular pH, that promotes new synthesis of TfR-1 degraded in a recycling pathway. In addition, MG-132 influences cellular iron distribution during BoHV-1 infection, as revealed by Perls' Prussian blue staining. However, cellular iron content, evaluated by Atomic Absorption Spectrophotometry, resulted essentially unaltered. These findings reveal that MG-132 may contribute to limit cellular iron availability for virus replication thereby enhancing cell survival.

MG-132 reduces virus release in Bovine herpesvirus-1 infection

Bovine herpesvirus 1 (BoHV-1) can provoke conjunctivitis, abortions and shipping fever. BoHV-1 infection can also cause immunosuppression and increased susceptibility to secondary bacterial infections, leading to pneumonia and occasionally to death. Herein, we investigated the influence of MG-132, a proteasome inhibitor, on BoHV-1 infection in bovine kidney (MDBK) cells. Infection of MDBK cells with BoHV-1 induces apoptotic cell death that enhances virus release. Whereas, MG-132 inhibited virus-induced apoptosis and stimulated autophagy. Protein expression of viral infected cell protein 0 (bICP0), which is constitutively expressed during infection and is able to stimulate Nuclear factor kappa B (NF-κB), was completely inhibited by MG-132. These results were accompanied by a significant delay in the NF-κB activation. Interestingly, the efficient virus release provoked by BoHV-1-induced apoptosis was significantly reduced by MG-132. Overall, this study suggests that MG-132, through the activation of autophagy, may limit BoHV-1 replication during productive infection, by providing an antiviral defense mechanism.

Replication and recombination of herpes simplex virus DNA

Replication of herpes simplex virus takes place in the cell nucleus and is carried out by a replisome composed of six viral proteins: the UL30-UL42 DNA polymerase, the UL5-UL8-UL52 helicase-primase, and the UL29 single-stranded DNA-binding protein ICP8. The replisome is loaded on origins of replication by the UL9 initiator origin-binding protein. Virus replication is intimately coupled to recombination and repair, often performed by cellular proteins. Here, we review new significant developments: the three-dimensional structures for the DNA polymerase, the polymerase accessory factor, and the single-stranded DNA-binding protein; the reconstitution of a functional replisome in vitro; the elucidation of the mechanism for activation of origins of DNA replication; the identification of cellular proteins actively involved in or responding to viral DNA replication; and the elucidation of requirements for formation of replication foci in the nucleus and effects on protein localization.

Cellular proteasome activity facilitates herpes simplex virus entry at a postpenetration step

Herpes simplex virus (HSV) entry into cells is a multistep process that engages the host cell machinery. The proteasome is a large, ATP-dependent, multisubunit protease that plays a critical role in the maintenance of cell homeostasis. A battery of assays were used to demonstrate that proteasome inhibitors blocked an early step in HSV entry that occurred after capsid penetration into the cytosol but prior to capsid arrival at the nuclear periphery. Proteasome-dependent viral entry was not reliant on host or viral protein synthesis. MG132, a peptide aldehyde that competitively inhibits the degradative activity of the proteasome, had a reversible inhibitory effect on HSV entry. HSV can use endocytic or nonendocytic pathways to enter cells. These distinct entry routes were both dependent on proteasome-mediated proteolysis. In addition, HSV successfully entered cells in the absence of a functional host ubiquitin-activating enzyme, suggesting that viral entry is ubiquitin independent. We propose that proteasomal degradation of virion and/or host proteins is required for efficient delivery of incoming HSV capsids to the nucleus.

Direct interaction between the N- and C-terminal portions of the herpes simplex virus type 1 origin binding protein UL9 implies the formation of a head-to-tail dimer

UL9, a superfamily II helicase, is a multifunctional protein required for herpes simplex virus type 1 replication in vivo. Although the C-terminal 317-amino-acid DNA binding domain of UL9 exists as a monomer, the full-length protein behaves as a dimer in solution. Thus, it has been assumed that the N-terminal 534 residues contain a region necessary for efficient dimerization and that UL9 dimers are in a head-to-head configuration. We recently showed, however, that residues in the N terminus could modulate the inhibitory properties of UL9 by decreasing the DNA binding ability of the C terminus (S. Chattopadhyay and S. K. Weller, J. Virol. 80:4491-4500, 2006). We suggested that a direct interaction between the N- and C-terminal portions of UL9 might exist and serve to modulate the DNA binding activities of the C terminus. In this study, we used a coimmunoprecipitation assay to show that the N-terminal portion of UL9 can indeed directly interact with the C terminus. A series of truncation mutant proteins were used to show that a region in the N terminus between residues 293 and 321 is necessary for efficient interaction. Similarly, a region in the C terminus between residues 600 and 800 is required for this interaction. The simplest model to explain these data is that UL9 dimers are oriented in a head-to-tail arrangement in which the N terminus is in contact with the C terminus.

Reduction of infectious bursal disease virus replication in cultured cells by proteasome inhibitors

Infectious bursal disease virus (IBDV) is the etiological agent of a highly contagious disease in chickens. In a recent report, proteasome inhibitor MG132 has been shown to completely inhibit IBDV-induced apoptosis. This raises the possibility that the ubiquitin–proteasome pathway may be used by the virus to promote viral replication. In this study, we examined the interplay between IBDV replication and the ubiquitin–proteasome pathway in cultured cells. Treatment of DF-1 cells with the proteasome inhibitors MG132 or lactacystin significantly decreased virus release in the supernatant and prevented virus-induced cytopathic effect. Inhibition of the ubiquitin–proteasome pathway did reduce markedly viral RNA transcription and protein translation but not affect virus internalization. We also demonstrated that IBDV activates caspase pathway via triggering the efflux of cytochrome c in mitochondria into cytosol of infected cells. This activity was dose-dependently reduced by proteasome inhibitor treatment. Taken together, our data suggest that proteasome inhibitor reduces IBDV replication through inhibition of viral RNA transcription and protein synthesis, and thus preventing IBDV-induced apoptosis.

Cathepsin B mediates cleavage of herpes simplex virus type 1 origin binding protein (OBP) to yield OBPC-1, and cleavage is dependent upon viral DNA replication

Although the seven viral proteins required for herpes simplex virus type 1 (HSV-1) DNA replication have been identified, the mechanism by which viral DNA synthesis is regulated is unclear. HSV-1 DNA replication is thought to occur in two stages: origin-dependent DNA replication (stage I) mediated by the origin binding protein (OBP), followed by origin- and OBP-independent DNA replication (stage II). The mechanism that facilitates the switch from stage I to stage II is unknown; however, it must involve the loss of OBP function or OBP itself from the replication initiation complex. Previous studies from this laboratory identified a transcript (UL8.5) and protein (OBPC) that are in frame with and comprise the C terminus of the gene specifying OBP. Because of its DNA binding ability, OBPC has been hypothesized to mediate the switch from stage I to stage II. Here, we identify a second protein (OBPC-2) that is also in frame with the C terminus of OBP but comprises a smaller portion of the protein. We demonstrate that the protein originally identified (OBPC-1) is a cathepsin B-mediated cleavage product of OBP, while OBPC-2 may be the product of the UL8.5 transcript. We further demonstrate that the cleavage of OBP to yield OBPC-1 is dependent upon viral DNA replication. These results suggest that cleavage may be a mechanism by which OBP levels and/or activity are regulated during infection.

Wanderings of a DNA enzymologist: from DNA polymerase to viral latency

I am a member of what has been called, perhaps too grandiosely, "The Greatest Generation." I grew up during the Great Depression and served in the U.S. Army during World War II. Because of my military service and the benefits of the GI Bill, I was able to attend college and, later, graduate school. Early in my graduate studies, I became fascinated with enzymes and the biochemical reactions that they catalyze. This fascination has never left me during the 50 years I have been a "DNA enzymologist." I was fortunate to have had as a mentor Arthur Kornberg, one of the great biochemists of the twentieth century, and a splendid group of postdocs and graduate students. I have studied DNA polymerases, DNA nucleases, DNA ligases, and DNA recombinases, enzymes that are critical to our understanding of DNA replication, repair, and recombination. Most recently, I have been studying herpes virus replication and inadvertently wandered into an entirely new area-viral latency.

DNA binding activity of the herpes simplex virus type 1 origin binding protein, UL9, can be modulated by sequences in the N terminus: correlation between transdominance and DNA binding

UL9, the origin binding protein of herpes simplex virus type 1, is a member of the SF2 family of helicases. Cotransfection of cells with infectious viral DNA and plasmids expressing either full-length UL9 or the C-terminal DNA binding domain alone results in the drastic inhibition of plaque formation which can be partially relieved by an insertion mutant lacking DNA binding activity. In this work, C-terminally truncated mutants which terminate at or near residue 359 were shown to potentiate plaque formation, while other C-terminal truncations were inhibitory. Thus, residues in the N-terminal region appear to regulate the inhibitory properties of UL9. To identify which residues were involved in this regulation, a series of N-terminally truncated mutants were constructed which contain the DNA binding domain and various N-terminal extensions. Mutants whose N terminus is either at residue 494 or 535 were able to bind the origin efficiently and were inhibitory to plaque formation, whereas constructs whose N terminus is at residue 304 or 394 were defective in origin binding activity and were able to relieve inhibition. Since UL9 is required for viral infection at early but not late times and is inhibitory to infection when overexpressed, we propose that the DNA binding activities of UL9 are regulated during infection. For infection to proceed, UL9 may need to switch from a DNA binding to a non-DNA binding mode, and we suggest that sequences residing in the N terminus play a role in this switch.

The cellular localization pattern of Varicella-Zoster virus ORF29p is influenced by proteasome-mediated degradation

Varicella-zoster virus (VZV) open reading frame 29 (ORF29) encodes a single-stranded DNA binding protein. During lytic infection, ORF29p is localized primarily to infected-cell nuclei, whereas during latency it appears in the cytoplasm of infected neurons. Following reactivation, ORF29p accumulates in the nucleus. In this report, we analyze the cellular localization patterns of ORF29p during VZV infection and during autonomous expression. Our results demonstrate that ORF29p is excluded from the nucleus in a cell-type-specific manner and that its cellular localization pattern may be altered by subsequent expression of VZV ORF61p or herpes simplex virus type 1 ICP0. In these cases, ORF61p and ICP0 induce nuclear accumulation of ORF29p in cell lines where it normally remains cytoplasmic. One cellular system utilized by ICP0 to influence protein abundance is the proteasome degradation pathway. Inhibition of the 26S proteasome, but not heat shock treatment, resulted in accumulation of ORF29p in the nucleus, similar to the effect of ICP0 expression. Immunofluorescence microscopy and pulse-chase experiments reveal that stabilization of ORF29p correlates with its nuclear accumulation and is dependent on a functional nuclear localization signal. ORF29p nuclear translocation in cultured enteric neurons and cells derived from an astrocytoma is reversible, as the protein's distribution and stability revert to the previous states when the proteasomal activity is restored. Thus, stabilization of ORF29p leads to its nuclear accumulation. Although proteasome inhibition induces ORF29p nuclear accumulation, this is not sufficient to reactivate latent VZV or target the immediate-early protein ORF62p to the nucleus in cultured guinea pig enteric neurons.

Pyrrolidine dithiocarbamate reduces coxsackievirus B3 replication through inhibition of the ubiquitin-proteasome pathway

Coxsackievirus B3 (CVB3) is one of the most common pathogens for viral myocarditis. The lack of effective therapeutics for CVB3-caused viral diseases underscores the importance of searching for antiviral compounds. Pyrrolidine dithiocarbamate (PDTC) is an antioxidant and is recently reported to inhibit ubiquitin-proteasome-mediated proteolysis. Previous studies have shown that PDTC inhibits replication of rhinovirus, influenza virus, and poliovirus. In the present study, we report that PDTC is a potent inhibitor of CVB3. Coxsackievirus-infected HeLa cells treated with PDTC showed a significant reduction of CVB3 viral RNA synthesis, viral protein VP1 expression, and viral progeny release. Similar to previous observation that divalent ions mediate the function of PDTC, we further report that serum-containing copper and zinc are required for its antiviral activity. CVB3 infection resulted in massive generation of reactive oxygen species (ROS). Although PDTC alleviated ROS generation, the antiviral activity was unlikely dependent on its antioxidant effect because the potent antioxidant, N-acetyl-L-cysteine, failed to inhibit CVB3 replication. Consistent with previous reports that PDTC inhibits ubiquitin-proteasome-mediated protein degradation, we found that PDTC treatment led to the accumulation of several short-lived proteins in infected cells. We further provide evidence that the inhibitory effect of PDTC on protein degradation was not due to inhibition of proteasome activity but likely modulation of ubiquitination. Together with our previous findings that proteasome inhibition reduces CVB3 replication (H. Luo, J. Zhang, C. Cheung, A. Suarez, B. M. McManus, and D. Yang, Am. J. Pathol. 163:381-385, 2003), results in this study suggest a strong antiviral effect of PDTC on coxsackievirus, likely through inhibition of the ubiquitin-proteasome pathway.

The ubiquitin-proteasome machinery is essential for nuclear translocation of incoming minute virus of mice

Minute virus of mice (MVM) infection is disrupted by proteasome inhibitors. Here, we show that inhibition of the ubiquitin-proteasome pathway did not affect viral entry and had influence neither on the natural proteolytic cleavage of VP2 to VP3 nor on the externalization of the N terminal of VP1. In both MG132-treated and untreated cells, MVM particles accumulated progressively in the perinuclear region. However, in MG132-treated cells, MVM was not able to penetrate into the nuclei, remaining blocked in the perinuclear region without capsid disassembly. MVM was similarly sensitive to MG132 in the two cell lines tested, A9 and NB324K. After releasing from the reversible MG132 block, MVM recovered the ability to translocate to the nuclei and replicate. Analysis of viral capsid proteins during internalization showed no evidence of capsid ubiquitination or degradation. We examined the effect of MG132 on two other parvoviruses, canine (CPV) and bovine parvovirus (BPV). Similarly to MVM, CPV infection was sensitive to MG132; however, BPV infection, as previously shown for adeno-associated viruses (AAVs), was not disturbed. These findings suggest that parvoviruses follow divergent strategies for nuclear transport, some of them requiring active proteasomes.

The neural F-box protein NFB42 mediates the nuclear export of the herpes simplex virus type 1 replication initiator protein (UL9 protein) after viral infection

The neural F-box 42-kDa protein (NFB42) is a component of the SCF(NFB42) E3 ubiquitin ligase that is expressed in all major areas of the brain; it is not detected in nonneuronal tissues. We previously identified NFB42 as a binding partner for the herpes simplex virus 1 (HSV-1) UL9 protein, the viral replication-initiator, and showed that coexpression of NFB42 and UL9 in human embryonic kidney (293T) cells led to a significant decrease in the level of UL9 protein. We have now found that HSV-1 infection promotes the shuttling of NFB42 between the cytosol and the nucleus in both 293T cells and primary hippocampal neurons, permitting NFB42 to bind to the phosphorylated UL9 protein, which is localized in the nucleus. This interaction mediates the export of the UL9 protein from the nucleus to the cytosol, leading to its ubiquitination and degradation via the 26S proteasome. Because the intranuclear localization of the UL9 protein, along with other viral and cellular factors, is an essential step in viral DNA replication, degradation of the UL9 protein in neurons by means of nuclear export through its specific interaction with NFB42 may prevent active replication and promote neuronal latency of HSV-1.

Herpes simplex virus infections are arrested in Oct-1-deficient cells

Expression of the herpes simplex virus (HSV) immediate early (IE) genes is regulated by a multiprotein complex that is assembled on the TAATGARAT enhancer core element. The complex contains the cellular POU domain protein Oct-1, the viral transactivator VP16, and the cellular cofactor host cell factor 1. The current model suggests that the assembly depends on recognition of the core element by Oct-1. Here, HSV infection of Oct-1-deficient mouse embryonic fibroblast cells demonstrates that Oct-1 is critical for IE gene expression at low multiplicities of infection (moi). However, the protein is not essential for IE gene expression at high moi, indicating that VP16-mediated transcriptional induction through other IE regulatory elements is also important. This induction depends, at least in part, on the GA-binding protein binding elements that are present in each IE enhancer domain. Surprisingly, whereas the viral IE genes are expressed after high moi infection of Oct-1-deficient cells, the assembly of viral replication factories is severely impaired, revealing a second critical role for Oct-1 in HSV replication. The results have implications for both the HSV lytic and latency-reactivation cycles.