A quantitative assay to monitor HSV-1 ICP0 ubiquitin ligase activity in vitro

A single polymorphic residue in humans underlies species-specific restriction of HSV-1 by the antiviral protein MxB

IMPORTANCE

Herpesviruses present a major global disease burden. Understanding the host cell mechanisms that block viral infections, as well as how viruses can evolve to counteract these host defenses, is critically important for understanding viral disease pathogenesis. This study reveals that the major human variant of the antiviral protein myxovirus resistance protein B (MxB) inhibits the human pathogen herpes simplex virus (HSV-1), whereas a minor human variant and orthologous MxB genes from even closely related primates do not. Thus, in contrast to the many antagonistic virus-host interactions in which the virus is successful in thwarting the host's defense systems, here the human gene appears to be at least temporarily winning at this interface of the primate-herpesvirus evolutionary arms race. Our findings further show that a polymorphism at amino acid 83 in a small fraction of the human population is sufficient to abrogate MxB's ability to inhibit HSV-1, which could have important implications for human susceptibility to HSV-1 pathogenesis.

A single polymorphic residue in humans underlies species-specific restriction of HSV-1 by the antiviral protein MxB

Myxovirus resistance proteins (MxA and MxB) are interferon-induced proteins that exert antiviral activity against a diverse range of RNA and DNA viruses. In primates, MxA has been shown to inhibit myxoviruses, bunyaviruses, and hepatitis B virus, whereas MxB restricts retroviruses and herpesviruses. As a result of their conflicts with viruses, both genes have been undergoing diversifying selection during primate evolution. Here, we investigate how MxB evolution in primates has affected its restriction of herpesviruses. In contrast to human MxB, we find that most primate orthologs, including the closely related chimpanzee MxB, do not inhibit HSV-1 replication. However, all primate MxB orthologs tested restrict human cytomegalovirus. Through the generation of human and chimpanzee MxB chimeras we show that a single residue, M83, is the key determinant of restriction of HSV-1 replication. Humans are the only primate species known to encode a methionine at this position, whereas most other primate species encode a lysine. Residue 83 is also the most polymorphic residue in MxB in human populations, with M83 being the most common variant. However, ∼2.5% of human MxB alleles encode a threonine at this position, which does not restrict HSV-1. Thus, a single amino acid variant in MxB, which has recently risen to high frequency in humans, has endowed humans with HSV-1 antiviral activity.

Importance

Herpesviruses present a major global disease burden. Understanding the host cell mechanisms that block viral infections as well as how viruses can evolve to counteract these host defenses is critically important for understanding viral disease pathogenesis, and for developing therapeutic tools aimed at treating or preventing viral infections. Additionally, understanding how these host and viral mechanisms adapt to counter one another can aid in identifying the risks of, and barriers to, cross-species transmission events. As highlighted by the recent SARS-CoV-2 pandemic, episodic transmission events can have severe consequences for human health. This study reveals that the major human variant of the antiviral protein MxB inhibits the human pathogen HSV-1, whereas human minor variants and orthologous MxB genes from even closely related primates do not. Thus, in contrast to the many antagonistic virus-host interactions in which the virus is successful in thwarting the defense systems of their native hosts, in this case the human gene appears to be at least temporarily winning at this interface of the primate-herpesviral evolutionary arms race. Our findings further show that a polymorphism at amino acid 83 in a small fraction of the human population is sufficient to abrogate MxB's ability to inhibit HSV-1, which could have important implications for human susceptibility to HSV-1 pathogenesis.

The HSV-1 ubiquitin ligase ICP0: Modifying the cellular proteome to promote infection

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ICP0 is a viral E3 ubiquitin ligase that promotes HSV-1 infection.

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ICP0 interacts with multiple component proteins of the ubiquitin pathway.

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ICP0 disrupts multiple cellular processes activated in response to infection

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ICP0 remodels the SUMO proteome to counteract host immune defences to infection.

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ICP0 is an attractive drug target for the development of antiviral HSV-1 therapeutics.

Herpes simplex virus 1 (HSV-1) hijacks ubiquitination machinery to modify the cellular proteome to create an environment permissive for virus replication. HSV-1 encodes its own RING-finger E3 ubiquitin (Ub) ligase, Infected Cell Protein 0 (ICP0), that directly interfaces with component proteins of the Ub pathway to inactivate host immune defences and cellular processes that restrict the progression of HSV-1 infection. Consequently, ICP0 plays a critical role in the infectious cycle of HSV-1 that is required to promote the efficient onset of lytic infection and productive reactivation of viral genomes from latency. This review will describe the current knowledge regarding the biochemical properties and known substrates of ICP0 during HSV-1 infection. We will highlight the gaps in the characterization of ICP0 function and propose future areas of research required to understand fully the biological properties of this important HSV-1 regulatory protein.

A Novel Luminescence-Based High-Throughput Approach for Cellular Resolution of Protein Ubiquitination Using Tandem Ubiquitin Binding Entities (TUBEs)

Protein turnover is highly regulated by the posttranslational process of ubiquitination. Deregulation of the ubiquitin proteasome system (UPS) has been implicated in cancer and neurodegenerative diseases, and modulating this system has proven to be a viable approach for therapeutic intervention. The development of novel technologies that enable high-throughput studies of substrate protein ubiquitination is key for UPS drug discovery. Conventional approaches for studying ubiquitination either have high protein requirements or rely on exogenous or modified ubiquitin moieties, thus limiting their utility. In order to circumvent these issues, we developed a high-throughput live-cell assay that combines the NanoBiT luminescence-based technology with tandem ubiquitin binding entities (TUBEs) to resolve substrate ubiquitination. To demonstrate the effectiveness and utility of this assay, we studied compound-induced ubiquitination of the G to S Phase Transition 1 (GSPT1) protein. Using this assay, we characterized compounds with varying levels of GSPT1 ubiquitination activity. This method provides a live-cell-based approach for assaying substrate ubiquitination that can be adapted to study the kinetics of ubiquitin transfer onto a substrate protein of interest. In addition, our results show that this approach is portable for studying the ubiquitination of target proteins with diverse functions.

Discovery of Small-Molecule Inhibitors Targeting the E3 Ubiquitin Ligase Activity of the Herpes Simplex Virus 1 ICP0 Protein Using an In Vitro High-Throughput Screening Assay

Herpes simplex virus 1 (HSV-1) has infected more than 80% of the population. Reactivation of the virus causes diseases ranging in severity from benign cold sores to fatal encephalitis. Current treatments involve viral DNA replication inhibitors, but the emergence of drug-resistant mutants is observed frequently, highlighting the need for novel antiviral therapies. Infected cell protein 0 (ICP0) of HSV-1 is encoded by an immediate early gene and plays a fundamental role during infection, because it enables viral gene expression and blocks antiviral responses. One mechanism by which ICP0 functions is through an E3 ubiquitin ligase activity that induces the degradation of targeted proteins. A ΔICP0 virus or mutants with deficiencies in E3 ligase activity cannot counteract beta interferon (IFN-β)-induced restriction of viral infection, are highly immunogenic, are avirulent, and fail to spread. Thus, small molecules interfering with essential and conserved ICP0 functions are expected to compromise HSV-1 infection. We have developed a high-throughput screening assay, based on the autoubiquitination properties of ICP0, to identify small-molecule inhibitors of ICP0 E3 ubiquitin ligase activity. Through a pilot screening procedure, we identified nine compounds that displayed dose-dependent inhibitory effects on ICP0 but not on Mdm2, a control E3 ubiquitin ligase. Following validation, one compound displayed ICP0-dependent inhibition of HSV-1 infection. This compound appeared to bind ICP0 in a cellular thermal shift assay, it blocked ICP0 self-elimination, and it blocked wild-type but not ICP0-null virus gene expression. This scaffold displays specificity and could be used to develop optimized ICP0 E3 ligase inhibitors.IMPORTANCE Since acyclovir and its derivatives were launched for herpesviruses control almost four decades ago, the search for novel antivirals has waned. However, as human life expectancy has increased, so has the number of immunocompromised individuals who receive prolonged treatment for HSV recurrences. This has led to an increase in unresponsive patients due to acquired viral drug resistance. Thus, novel treatments need to be explored. Here we explored the HSV-1 ICP0 E3 ligase as a potential antiviral target because (i) ICP0 is expressed before virus replication, (ii) it is essential for infection in vivo, (iii) it is required for efficient reactivation of the virus from latency, (iv) inhibition of its E3 ligase activity would sustain host immune responses, and (v) it is shared by other herpesviruses. We report a compound that inhibits HSV-1 infection in an ICP0-dependent manner by inhibiting ICP0 E3 ligase activity.

Phosphorylation of Tudor-SN, a novel substrate of JNK, is involved in the efficient recruitment of Tudor-SN into stress granules

Posttranslational modifications of certain stress granule (SG) proteins are closely related to the assembly of SGs, a type of cytoplasmic foci structure. Our previous studies revealed that the Tudor staphylococcal nuclease (Tudor-SN) protein participates in the formation of SGs. However, the functional significance of potential Tudor-SN modifications during stress has not been reported. In this study, we demonstrated that the Tudor-SN protein was phosphorylated at threonine 103 (T103) upon stimulation with arsenite. In addition, c-Jun N-terminal kinase (JNK) was found to be responsible for Tudor-SN phosphorylation at the T103 site. We further illustrated that either a T103A mutation or the suppression of phosphorylation of T103 by the JNK inhibitor SP600125 inhibited the efficient recruitment of Tudor-SN into SGs. In addition, the T103A mutation could affect the physical binding of Tudor-SN with the G3BP (Ras-GAP SH3 domain-binding protein) protein but not with the HuR (Hu antigen R) protein and AGTR1-3'UTR (3'-untranslated region of angiotensin II receptor, type 1) mRNA cargo. These data suggested that JNK-enhanced Tudor-SN phosphorylation promotes the interaction between Tudor-SN and G3BP and facilitates the efficient recruitment of Tudor-SN into SGs under conditions of sodium arsenite-induced oxidative stress. This finding provides novel insights into the physiological function of Tudor-SN modification.