An Intrinsically Disordered Region of the DNA Repair Protein Nbs1 Is a Species-Specific Barrier to Herpes Simplex Virus 1 in Primates

Evolutionary analysis of ZAP and its cofactors identifies intrinsically disordered regions as central elements in host-pathogen interactions

The zinc-finger antiviral protein (ZAP) is an innate immunity sensor of non-self nucleic acids. Its antiviral activity is exerted through the physical interaction with different cofactors, including TRIM25, Riplet and KHNYN. Cellular proteins that interact with infectious agents are expected to be engaged in genetic conflicts that often result in their rapid evolution. To test this possibility and to identify the regions most strongly targeted by natural selection, we applied in silico molecular evolution tools to analyze the evolutionary history of ZAP and cofactors in four mammalian groups. We report evidence of positive selection in all genes and in most mammalian groups. On average, the intrinsically disordered regions (IDRs) embedded in the four proteins evolve significantly faster than folded domains and most positively selected sites fall within IDRs. In ZAP, the PARP domain also shows abundant signals of selection, and independent evolution in different mammalian groups suggests modulation of its ADP-ribose binding ability. Detailed analyses of the biophysical properties of IDRs revealed that chain compaction and conformational entropy are conserved across mammals. The IDRs in ZAP and KHNYN are particularly compact, indicating that they may promote phase separation (PS). In line with this hypothesis, we predicted several PS-promoting regions in ZAP and KHNYN, as well as in TRIM25. Positively selected sites are abundant in these regions, suggesting that PS may be important for the antiviral functions of these proteins and the evolutionary arms race with viruses. Our data shed light into the evolution of ZAP and cofactors and indicate that IDRs represent central elements in host-pathogen interactions.

Recurrent Duplication and Diversification of a Vital DNA Repair Gene Family Across Drosophila

Maintaining genome integrity is vital for organismal survival and reproduction. Essential, broadly conserved DNA repair pathways actively preserve genome integrity. However, many DNA repair proteins evolve adaptively. Ecological forces like UV exposure are classically cited drivers of DNA repair evolution. Intrinsic forces like repetitive DNA, which also imperil genome integrity, have received less attention. We recently reported that a Drosophila melanogaster-specific DNA satellite array triggered species-specific, adaptive evolution of a DNA repair protein called Spartan/MH. The Spartan family of proteases cleave hazardous, covalent crosslinks that form between DNA and proteins ("DNA-protein crosslink repair"). Appreciating that DNA satellites are both ubiquitous and universally fast-evolving, we hypothesized that satellite DNA turnover spurs adaptive evolution of DNA-protein crosslink repair beyond a single gene and beyond the D. melanogaster lineage. This hypothesis predicts pervasive Spartan gene family diversification across Drosophila species. To study the evolutionary history of the Drosophila Spartan gene family, we conducted population genetic, molecular evolution, phylogenomic, and tissue-specific expression analyses. We uncovered widespread signals of positive selection across multiple Spartan family genes and across multiple evolutionary timescales. We also detected recurrent Spartan family gene duplication, divergence, and gene loss. Finally, we found that ovary-enriched parent genes consistently birthed functionally diverged, testis-enriched daughter genes. To account for Spartan family diversification, we introduce a novel mechanistic model of antagonistic coevolution that links DNA satellite evolution and adaptive regulation of Spartan protease activity. This framework promises to accelerate our understanding of how DNA repeats drive recurrent evolutionary innovation to preserve genome integrity.

The immediate-early protein 1 of human herpesvirus 6B interacts with NBS1 and inhibits ATM signaling

Viral infection often trigger an ATM serine/threonine kinase (ATM)-dependent DNA damage response in host cells that suppresses viral replication. Viruses evolved different strategies to counteract this antiviral surveillance system. Here, we report that human herpesvirus 6B (HHV-6B) infection causes genomic instability by suppressing ATM signaling in host cells. Expression of immediate-early protein 1 (IE1) phenocopies this phenotype and blocks homology-directed double-strand break repair. Mechanistically, IE1 interacts with NBS1, and inhibits ATM signaling through two distinct domains. HHV-6B seems to efficiently inhibit ATM signaling as further depletion of either NBS1 or ATM do not significantly boost viral replication in infected cells. Interestingly, viral integration of HHV-6B into the host's telomeres is not strictly dependent on NBS1, challenging current models where integration occurs through homology-directed repair. Given that spontaneous IE1 expression has been detected in cells of subjects with inherited chromosomally-integrated form of HHV-6B (iciHHV-6B), a condition associated with several health conditions, our results raise the possibility of a link between genomic instability and the development of iciHHV-6-associated diseases.

Characterization of Intrinsically Disordered Proteins in Healthy and Diseased States by Nuclear Magnetic Resonance

INTRODUCTION

Intrinsically Disordered Proteins (IDPs) are active in different cellular procedures like ordered assembly of chromatin and ribosomes, interaction with membrane, protein, and ligand binding, molecular recognition, binding, and transportation via nuclear pores, microfilaments and microtubules process and disassembly, protein functions, RNA chaperone, and nucleic acid binding, modulation of the central dogma, cell cycle, and other cellular activities, post-translational qualification and substitute splicing, and flexible entropic linker and management of signaling pathways.

METHODS

The intrinsic disorder is a precise structural characteristic that permits IDPs/IDPRs to be involved in both one-to-many and many-to-one signaling. IDPs/IDPRs also exert some dynamical and structural ordering, being much less constrained in their activities than folded proteins. Nuclear magnetic resonance (NMR) spectroscopy is a major technique for the characterization of IDPs, and it can be used for dynamic and structural studies of IDPs.

RESULTS AND CONCLUSION

This review was carried out to discuss intrinsically disordered proteins and their different goals, as well as the importance and effectiveness of NMR in characterizing intrinsically disordered proteins in healthy and diseased states.

Spermine enhances antiviral and anticancer responses by stabilizing DNA binding with the DNA sensor cGAS

Self-nonself discrimination is vital for the immune system to mount responses against pathogens while maintaining tolerance toward the host and innocuous commensals during homeostasis. Here, we investigated how indiscriminate DNA sensors, such as cyclic GMP-AMP synthase (cGAS), make this self-nonself distinction. Screening of a small-molecule library revealed that spermine, a well-known DNA condenser associated with viral DNA, markedly elevates cGAS activation. Mechanistically, spermine condenses DNA to enhance and stabilize cGAS-DNA binding, optimizing cGAS and downstream antiviral signaling. Spermine promotes condensation of viral, but not host nucleosome, DNA. Deletion of viral DNA-associated spermine, by propagating virus in spermine-deficient cells, reduced cGAS activation. Spermine depletion subsequently attenuated cGAS-mediated antiviral and anticancer immunity. Collectively, our results reveal a pathogenic DNA-associated molecular pattern that facilitates nonself recognition, linking metabolism and pathogen recognition.

Fatal Human Alphaherpesvirus 1 Infection in Free-Ranging Black-Tufted Marmosets in Anthropized Environments, Brazil, 2012–2019

Human alphaherpesvirus 1 (HuAHV1) causes fatal neurologic infections in captive New World primates. To determine risks for interspecies transmission, we examined data for 13 free-ranging, black-tufted marmosets (Callithrix penicillata) that died of HuAHV1 infection and had been in close contact with humans in anthropized areas in Brazil during 2012–2019. We evaluated pathologic changes in the marmosets, localized virus and antigen, and assessed epidemiologic features. The main clinical findings were neurologic signs, necrotizing meningoencephalitis, and ulcerative glossitis; 1 animal had necrotizing hepatitis. Transmission electron microscopy revealed intranuclear herpetic inclusions, and immunostaining revealed HuAHV1 and herpesvirus particles in neurons, glial cells, tongue mucosal epithelium, and hepatocytes. PCR confirmed HuAHV1 infection. These findings illustrate how disruption of the One Health equilibrium in anthropized environments poses risks for interspecies virus transmission with potential spillover not only from animals to humans but also from humans to free-ranging nonhuman primates or other animals.

Schlafens Can Put Viruses to Sleep

The Schlafen gene family encodes for proteins involved in various biological tasks, including cell proliferation, differentiation, and T cell development. Schlafens were initially discovered in mice, and have been studied in the context of cancer biology, as well as their role in protecting cells during viral infection. This protein family provides antiviral barriers via direct and indirect effects on virus infection. Schlafens can inhibit the replication of viruses with both RNA and DNA genomes. In this review, we summarize the cellular functions and the emerging relationship between Schlafens and innate immunity. We also discuss the functions and distinctions of this emerging family of proteins as host restriction factors against viral infection. Further research into Schlafen protein function will provide insight into their mechanisms that contribute to intrinsic and innate host immunity.

Positive natural selection in primate genes of the type I interferon response

Background

The Type I interferon response is an important first-line defense against viruses. In turn, viruses antagonize (i.e., degrade, mis-localize, etc.) many proteins in interferon pathways. Thus, hosts and viruses are locked in an evolutionary arms race for dominance of the Type I interferon pathway. As a result, many genes in interferon pathways have experienced positive natural selection in favor of new allelic forms that can better recognize viruses or escape viral antagonists. Here, we performed a holistic analysis of selective pressures acting on genes in the Type I interferon family. We initially hypothesized that the genes responsible for inducing the production of interferon would be antagonized more heavily by viruses than genes that are turned on as a result of interferon. Our logic was that viruses would have greater effect if they worked upstream of the production of interferon molecules because, once interferon is produced, hundreds of interferon-stimulated proteins would activate and the virus would need to counteract them one-by-one.

Results

We curated multiple sequence alignments of primate orthologs for 131 genes active in interferon production and signaling (herein, “induction” genes), 100 interferon-stimulated genes, and 100 randomly chosen genes. We analyzed each multiple sequence alignment for the signatures of recurrent positive selection. Counter to our hypothesis, we found the interferon-stimulated genes, and not interferon induction genes, are evolving significantly more rapidly than a random set of genes. Interferon induction genes evolve in a way that is indistinguishable from a matched set of random genes (22% and 18% of genes bear signatures of positive selection, respectively). In contrast, interferon-stimulated genes evolve differently, with 33% of genes evolving under positive selection and containing a significantly higher fraction of codons that have experienced selection for recurrent replacement of the encoded amino acid.

Conclusion

Viruses may antagonize individual products of the interferon response more often than trying to neutralize the system altogether.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01783-z.

Comparative proteomics identifies Schlafen 5 (SLFN5) as a herpes simplex virus restriction factor that suppresses viral transcription

Intrinsic antiviral host factors confer cellular defence by limiting virus replication and are often counteracted by viral countermeasures. We reasoned that host factors that inhibit viral gene expression could be identified by determining proteins bound to viral DNA (vDNA) in the absence of key viral antagonists. Herpes simplex virus 1 (HSV-1) expresses E3 ubiquitin-protein ligase ICP0 (ICP0), which functions as an E3 ubiquitin ligase required to promote infection. Cellular substrates of ICP0 have been discovered as host barriers to infection but the mechanisms for inhibition of viral gene expression are not fully understood. To identify restriction factors antagonized by ICP0, we compared proteomes associated with vDNA during HSV-1 infection with wild-type virus and a mutant lacking functional ICP0 (ΔICP0). We identified the cellular protein Schlafen family member 5 (SLFN5) as an ICP0 target that binds vDNA during HSV-1 ΔICP0 infection. We demonstrated that ICP0 mediates ubiquitination of SLFN5, which leads to its proteasomal degradation. In the absence of ICP0, SLFN5 binds vDNA to repress HSV-1 transcription by limiting accessibility of RNA polymerase II to viral promoters. These results highlight how comparative proteomics of proteins associated with viral genomes can identify host restriction factors and reveal that viral countermeasures can overcome SLFN antiviral activity.

En Guard! The Interactions between Adenoviruses and the DNA Damage Response

Virus–host cell interactions include several skirmishes between the virus and its host, and the DNA damage response (DDR) network is one of their important battlegrounds. Although some aspects of the DDR are exploited by adenovirus (Ad) to improve virus replication, especially at the early phase of infection, a large body of evidence demonstrates that Ad devotes many of its proteins, including E1B-55K, E4orf3, E4orf4, E4orf6, and core protein VII, and utilizes varied mechanisms to inhibit the DDR. These findings indicate that the DDR would strongly restrict Ad replication if allowed to function efficiently. Various Ad serotypes inactivate DNA damage sensors, including the Mre11-Rad50-Nbs1 (MRN) complex, DNA-dependent protein kinase (DNA-PK), and Poly (ADP-ribose) polymerase 1 (PARP-1). As a result, these viruses inhibit signaling via DDR transducers, such as the ataxia-telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR) kinases, to downstream effectors. The different Ad serotypes utilize both shared and distinct mechanisms to inhibit various branches of the DDR. The aim of this review is to understand the interactions between Ad proteins and the DDR and to appreciate how these interactions contribute to viral replication.

Intrinsically disordered regions are abundant in simplexvirus proteomes and display signatures of positive selection

Whereas the majority of herpesviruses co-speciated with their mammalian hosts, human herpes simplex virus 2 (HSV-2, genus Simplexvirus) most likely originated from the cross-species transmission of chimpanzee herpesvirus 1 to an ancestor of modern humans. We exploited the peculiar evolutionary history of HSV-2 to investigate the selective events that drove herpesvirus adaptation to a new host. We show that HSV-2 intrinsically disordered regions (IDRs)-that is, protein domains that do not adopt compact three-dimensional structures-are strongly enriched in positive selection signals. Analysis of viral proteomes indicated that a significantly higher portion of simplexvirus proteins is disordered compared with the proteins of other human herpesviruses. IDR abundance in simplexvirus proteomes was not a consequence of the base composition of their genomes (high G + C content). Conversely, protein function determines the IDR fraction, which is significantly higher in viral proteins that interact with human factors. We also found that the average extent of disorder in herpesvirus proteins tends to parallel that of their human interactors. These data suggest that viruses that interact with fast-evolving, disordered human proteins, in turn, evolve disordered viral interactors poised for innovation. We propose that the high IDR fraction present in simplexvirus proteomes contributes to their wider host range compared with other herpesviruses.

Recruitment of DNA Repair MRN Complex by Intrinsically Disordered Protein Domain Fused to Cas9 Improves Efficiency of CRISPR-Mediated Genome Editing

CRISPR/Cas9 is a powerful tool for genome editing in cells and organisms. Nevertheless, introducing directed templated changes by homology-directed repair (HDR) requires the cellular DNA repair machinery, such as the MRN complex (Mre11/Rad50/Nbs1). To improve the process, we tailored chimeric constructs of Cas9, in which SpCas9 was fused at its N- or C-terminus to a 126aa intrinsically disordered domain from HSV-1 alkaline nuclease (UL12) that recruits the MRN complex. The chimeric Cas9 constructs were two times more efficient in homology-directed editing of endogenous loci in tissue culture cells. This effect was dependent upon the MRN-recruiting activity of the domain and required lower amounts of the chimeric Cas9 in comparison with unmodified Cas9. The new constructs improved the yield of edited cells when making endogenous point mutations or inserting small tags encoded by oligonucleotide donor DNA (ssODN), and also with larger insertions encoded by plasmid DNA donor templates. Improved editing was achieved with both transfected plasmid-encoded Cas9 constructs as well as recombinant Cas9 protein transfected as ribonucleoprotein complexes. Our strategy was highly efficient in restoring a genetic defect in a cell line, exemplifying the possible implementation of our strategy in gene therapy. These constructs provide a simple approach to improve directed editing.

Selective use of primate CD4 receptors by HIV-1

Individuals chronically infected with HIV-1 harbor complex viral populations within their bloodstreams. Recently, it has come to light that when these people infect others, the new infection is typically established by only one or a small number of virions from within this complex viral swarm. An important goal is to characterize the biological properties of HIV-1 virions that seed and exist early in new human infections because these are potentially the only viruses against which a prophylactic HIV-1 vaccine would need to elicit protection. This includes understanding how the Envelope (Env) protein of these virions interacts with the T-cell receptor CD4, which supports attachment and entry of HIV-1 into target cells. We examined early HIV-1 isolates for their ability to infect cells via the CD4 receptor of 15 different primate species. Primates were the original source of HIV-1 and now serve as valuable animal models for studying HIV-1. We find that most primary isolates of HIV-1 from the blood, including early isolates, are highly selective and enter cells through some primate CD4 receptor orthologs but not others. This phenotype is remarkably consistent, regardless of route of transmission, viral subtype, or time of isolation post infection. We show that the weak CD4 binding affinity of blood-derived HIV-1 isolates is what makes them sensitive to the small sequence differences in CD4 from one primate species to the next. To substantiate this, we engineered an early HIV-1 Env to have high, medium, or low binding affinity to CD4, and we show that it loses the ability to enter cells via the CD4 receptor of many primate species as the binding affinity gets weaker. Based on the phenotype of selective use of primate CD4, we find that weak CD4 binding appears to be a nearly universal property of HIV-1 circulating in the bloodstream. Therefore, weak binding to CD4 must be a selected and important property in the biology of HIV-1 in the body. We identify six primate species that encode CD4 receptors that fully support the entry of early HIV-1 isolates despite their low binding affinity for CD4. These findings will help inform long-standing efforts to model HIV-1 transmission and early disease in primates.

The current animal model for HIV, the macaque, encodes a CD4 receptor that is non-permissive for HIV entry. This paper reveals that six primate species encode CD4 receptors compatible with HIV infection, potentially making them powerful tools for the study of HIV biology. Furthermore, weak CD4 binding is a nearly constant, and apparently selected, property of HIV circulating in the human bloodstream.

How host genetics dictates successful viral zoonosis

Viruses of wild and domestic animals can infect humans in a process called zoonosis, and these events can give rise to explosive epidemics such as those caused by the HIV and Ebola viruses. While humans are constantly exposed to animal viruses, those that can successfully infect and transmit between humans are exceedingly rare. The key event in zoonosis is when an animal virus begins to replicate (one virion making many) in the first human subject. Only at this point will the animal virus first experience the selective environment of the human body, rendering possible viral adaptation and refinement for humans. In addition, appreciable viral titers in this first human may enable infection of a second, thus initiating selection for viral variants with increased capacity for spread. We assert that host genetics plays a critical role in defining which animal viruses in nature will achieve this key event of replication in a first human host. This is because animal viruses that pose the greatest risk to humans will have few (or no) genetic barriers to replicating themselves in human cells, thus requiring minimal mutations to make this jump. Only experimental virology provides a path to identifying animal viruses with the potential to replicate themselves in humans because this information will not be evident from viral sequencing data alone.

TOP2β-Dependent Nuclear DNA Damage Shapes Extracellular Growth Factor Responses via Dynamic AKT Phosphorylation to Control Virus Latency

The mTOR pathway integrates both extracellular and intracellular signals and serves as a central regulator of cell metabolism, growth, survival, and stress responses. Neurotropic viruses, such as herpes simplex virus-1 (HSV-1), also rely on cellular AKT-mTORC1 signaling to achieve viral latency. Here, we define a novel genotoxic response whereby spatially separated signals initiated by extracellular neurotrophic factors and nuclear DNA damage are integrated by the AKT-mTORC1 pathway. We demonstrate that endogenous DNA double-strand breaks (DSBs) mediated by Topoisomerase 2β-DNA cleavage complex (TOP2βcc) intermediates are required to achieve AKT-mTORC1 signaling and maintain HSV-1 latency in neurons. Suppression of host DNA-repair pathways that remove TOP2βcc trigger HSV-1 reactivation. Moreover, perturbation of AKT phosphorylation dynamics by downregulating the PHLPP1 phosphatase led to AKT mis-localization and disruption of DSB-induced HSV-1 reactivation. Thus, the cellular genome integrity and environmental inputs are consolidated and co-opted by a latent virus to balance lifelong infection with transmission.

Yeast XRS2 and human NBN gene: Experimental evidence for homology using codon optimized cDNA

The genes, XRS2 in Saccharomyces cerevisiae and NBN in mammals, have little sequence identity at the amino acid level. Nevertheless, they are both found together with MRE11 and RAD50 in a highly conserved protein complex which functions in the repair of DNA double-strand breaks. Here, we have examined the evolutionary and functional relationship of these two genes by cross-complementation experiments. These experiments necessitated sequence correction for specific codon usage before they could be successfully conducted. We present evidence that despite extreme sequence divergence nibrin can, at least partially, replace Xrs2 in the cellular DNA damage response, and Xrs2 is able to promote nuclear localization of MRE11 in NBS cells. We discuss that the extreme sequence divergence reflects a unique adaptive pressure during evolution related to the specific eukaryotic role for both Xrs2 and nibrin in the subcellular localisation of the DNA repair complex. This, we suggest, is of particular relevance when cells are infected by viruses. The conflict hypothesis of co-evolution of DNA repair genes and DNA viruses may thus explain the very low sequence identity of these two homologous genes.

Virus DNA Replication and the Host DNA Damage Response

Viral DNA genomes have limited coding capacity and therefore harness cellular factors to facilitate replication of their genomes and generate progeny virions. Studies of viruses and how they interact with cellular processes have historically provided seminal insights into basic biology and disease mechanisms. The replicative life cycles of many DNA viruses have been shown to engage components of the host DNA damage and repair machinery. Viruses have evolved numerous strategies to navigate the cellular DNA damage response. By hijacking and manipulating cellular replication and repair processes, DNA viruses can selectively harness or abrogate distinct components of the cellular machinery to complete their life cycles. Here, we highlight consequences for viral replication and host genome integrity during the dynamic interactions between virus and host.

Species-Specific Deamidation of cGAS by Herpes Simplex Virus UL37 Protein Facilitates Viral Replication

Herpes simplex virus 1 (HSV-1) establishes infections in humans and mice, but some non-human primates exhibit resistance via unknown mechanisms. Innate immune recognition pathways are highly conserved but are pivotal in determining susceptibility to DNA virus infections. We report that variation of a single amino acid residue in the innate immune sensor cGAS determines species-specific inactivation by HSV-1. The HSV-1 UL37 tegument protein deamidates human and mouse cGAS. Deamidation impairs the ability of cGAS to catalyze cGAMP synthesis, which activates innate immunity. HSV-1 with deamidase-deficient UL37 promotes robust antiviral responses and is attenuated in mice in a cGAS- and STING-dependent manner. Mutational analyses identified a single asparagine in human and mouse cGAS that is not conserved in many non-human primates. This residue underpins UL37-mediated cGAS deamidation and species permissiveness of HSV-1. Thus, HSV-1 mediates cGAS deamidation for immune evasion and exploits species sequence variation to disarm host defenses.

Smc5/6 Antagonism by HBx Is an Evolutionarily Conserved Function of Hepatitis B Virus Infection in Mammals

Chronic infection with hepatitis B virus (HBV) is a major cause of liver disease and cancer in humans. HBVs (family Hepadnaviridae) have been associated with mammals for millions of years. Recently, the Smc5/6 complex, known for its essential housekeeping functions in genome maintenance, was identified as an antiviral restriction factor of human HBV. The virus has, however, evolved to counteract this defense mechanism by degrading the complex via its regulatory HBx protein. Whether the antiviral activity of the Smc5/6 complex against hepadnaviruses is an important and evolutionarily conserved function is unknown. In this study, we used an evolutionary and functional approach to address this question. We first performed phylogenetic and positive selection analyses of the Smc5/6 complex subunits and found that they have been conserved in primates and mammals. Yet, Smc6 showed marks of adaptive evolution, potentially reminiscent of a virus-host "arms race." We then functionally tested the HBx proteins from six divergent hepadnaviruses naturally infecting primates, rodents, and bats. We demonstrate that despite little sequence homology, these HBx proteins efficiently degraded mammalian Smc5/6 complexes, independently of the host species and of the sites under positive selection. Importantly, all HBx proteins also rescued the replication of an HBx-deficient HBV in primary human hepatocytes. These findings point to an evolutionarily conserved requirement for Smc5/6 inactivation by HBx, showing that Smc5/6 antiviral activity has been an important defense mechanism against hepadnaviruses in mammals. It will be interesting to investigate whether Smc5/6 may further be a restriction factor of other, yet-unidentified viruses that may have driven some of its adaptation.IMPORTANCE Infection with hepatitis B virus (HBV) led to 887,000 human deaths in 2015. HBV has been coevolving with mammals for millions of years. Recently, the Smc5/6 complex, which has essential housekeeping functions, was identified as a restriction factor of human HBV antagonized by the regulatory HBx protein. Here we address whether the antiviral activity of Smc5/6 is an important evolutionarily conserved function. We found that all six subunits of Smc5/6 have been conserved in primates, with only Smc6 showing signatures of an "evolutionary arms race." Using evolution-guided functional analyses that included infections of primary human hepatocytes, we demonstrated that HBx proteins from very divergent mammalian HBVs could all efficiently antagonize Smc5/6, independently of the host species and sites under positive selection. These findings show that Smc5/6 antiviral activity against HBV is an important function in mammals. They also raise the intriguing possibility that Smc5/6 may restrict other, yet-unidentified viruses.

The Major Tegument Protein of Bovine Herpesvirus 1, VP8, Interacts with DNA Damage Response Proteins and Induces Apoptosis

VP8, the U_L47 gene product in bovine herpesvirus-1 (BoHV-1), is a major tegument protein that is essential for virus replication in vivo The major DNA damage response protein, ataxia telangiectasia mutated (ATM), phosphorylates Nijmegen breakage syndrome (NBS1) and structural maintenance of chromosome-1 (SMC1) proteins during the DNA damage response. VP8 was found to interact with ATM and NBS1 during transfection and BoHV-1 infection. However, VP8 did not interfere with phosphorylation of ATM in transfected or BoHV-1-infected cells. In contrast, VP8 inhibited phosphorylation of both NBS1 and SMC1 in transfected cells, as well as in BoHV-1-infected cells, but not in cells infected with a VP8 deletion mutant (BoHV-1ΔU_L47). Inhibition of NBS1 and SMC1 phosphorylation was observed at 4 h postinfection by nuclear VP8. Furthermore, UV light-induced cyclobutane pyrimidine dimer (CPD) repair was reduced in the presence of VP8, and VP8 in fact enhanced etoposide or UV-induced apoptosis. This suggests that VP8 blocks the ATM/NBS1/SMC1 pathway and inhibits DNA repair. VP8 induced apoptosis in VP8-transfected cells through caspase-3 activation. The fact that BoHV-1 is known to induce apoptosis through caspase-3 activation is in agreement with this observation. The role of VP8 was confirmed by the observation that BoHV-1 induced significantly more apoptosis than BoHV-1ΔU_L47. These data reveal a potential role of VP8 in the modulation of the DNA damage response pathway and induction of apoptosis during BoHV-1 infection.IMPORTANCE To our knowledge, the effect of BoHV-1 infection on the DNA damage response has not been characterized. Since BoHV-1ΔU_L47 was previously shown to be avirulent in vivo, VP8 is critical for the progression of viral infection. We demonstrated that VP8 interacts with DNA damage response proteins and disrupts the ATM-NBS1-SMC1 pathway by inhibiting phosphorylation of DNA repair proteins NBS1 and SMC1. Furthermore, interference of VP8 with DNA repair was correlated with decreased cell viability and increased DNA damage-induced apoptosis. These data show that BoHV-1 VP8 developed a novel strategy to interrupt the ATM signaling pathway and to promote apoptosis. These results further enhance our understanding of the functions of VP8 during BoHV-1 infection and provide an additional explanation for the reduced virulence of BoHV-1ΔU_L47.

Virus DNA Replication and the Host DNA Damage Response

Viral DNA genomes have limited coding capacity and therefore harness cellular factors to facilitate replication of their genomes and generate progeny virions. Studies of viruses and how they interact with cellular processes have historically provided seminal insights into basic biology and disease mechanisms. The replicative life cycles of many DNA viruses have been shown to engage components of the host DNA damage and repair machinery. Viruses have evolved numerous strategies to navigate the cellular DNA damage response. By hijacking and manipulating cellular replication and repair processes, DNA viruses can selectively harness or abrogate distinct components of the cellular machinery to complete their life cycles. Here, we highlight consequences for viral replication and host genome integrity during the dynamic interactions between virus and host.

Serotype-specific restriction of wild-type adenoviruses by the cellular Mre11-Rad50-Nbs1 complex

During viral replication in the nucleus, the DNA genomes of adenoviruses are accessible to cellular DNA-binding proteins. Human adenovirus type 5 (Ad5) targets the cellular Mre11-Rad50-Nbs1 complex (MRN) to evade detection by the DNA damage response (DDR). Ad5 mutants that cannot target MRN have reduced viral propagation. Previous studies showed that diverse adenovirus serotypes interact differently with MRN. While these studies revealed diverse MRN interactions among serotypes, it remains unclear how these differences influence viral replication. Here, we examined effects of the DDR on several adenovirus serotypes. We demonstrate that wild-type Ad9 and Ad12 do not overcome MRN impairment. We also examined viral proteins involved in targeting MRN and found that unlike Ad5-E4orf3, expression of Ad9-E4orf3 is not sufficient for MRN mislocalization observed during infection. We conclude that adenovirus serotypes target MRN in distinct ways, and the MRN complex can impair DNA replication of wild-type viruses across the adenovirus family.

Control of yeast retrotransposons mediated through nucleoporin evolution

Yeasts serve as hosts to several types of genetic parasites. Few studies have addressed the evolutionary trajectory of yeast genes that control the stable co-existence of these parasites with their host cell. In Saccharomyces yeasts, the retrovirus-like Ty retrotransposons must access the nucleus. We show that several genes encoding components of the yeast nuclear pore complex have experienced natural selection for substitutions that change the encoded protein sequence. By replacing these S. cerevisiae genes with orthologs from other Saccharomyces species, we discovered that natural sequence changes have affected the mobility of Ty retrotransposons. Specifically, changing the genetic sequence of NUP84 or NUP82 to match that of other Saccharomyces species alters the mobility of S. cerevisiae Ty1 and Ty3. Importantly, all tested housekeeping functions of NUP84 and NUP82 remained equivalent across species. Signatures of natural selection, resulting in altered interactions with viruses and parasitic genetic elements, are common in host defense proteins. Yet, few instances have been documented in essential housekeeping proteins. The nuclear pore complex is the gatekeeper of the nucleus. This study shows how the evolution of this large, ubiquitous eukaryotic complex can alter the replication of a molecular parasite, but concurrently maintain essential host functionalities regarding nucleocytoplasmic trafficking.

Dengue viruses cleave STING in humans but not in nonhuman primates, their presumed natural reservoir

Human dengue viruses emerged from primate reservoirs, yet paradoxically dengue does not reach high titers in primate models. This presents a unique opportunity to examine the genetics of spillover versus reservoir hosts. The dengue virus 2 (DENV2) - encoded protease cleaves human STING, reducing type I interferon production and boosting viral titers in humans. We find that both human and sylvatic (reservoir) dengue viruses universally cleave human STING, but not the STING of primates implicated as reservoir species. The special ability of dengue to cleave STING is thus specific to humans and a few closely related ape species. Conversion of residues 78/79 to the human-encoded 'RG' renders all primate (and mouse) STINGs sensitive to viral cleavage. Dengue viruses may have evolved to increase viral titers in the dense and vast human population, while maintaining decreased titers and pathogenicity in the more rare animals that serve as their sustaining reservoir in nature.

Species-specific vulnerability of RanBP2 shaped the evolution of SIV as it transmitted in African apes

HIV-1 arose as the result of spillover of simian immunodeficiency viruses (SIVs) from great apes in Africa, namely from chimpanzees and gorillas. Chimpanzees and gorillas were, themselves, infected with SIV after virus spillover from African monkeys. During spillover events, SIV is thought to require adaptation to the new host species. The host barriers that drive viral adaptation have predominantly been attributed to restriction factors, rather than cofactors (host proteins exploited to promote viral replication). Here, we consider the role of one cofactor, RanBP2, in providing a barrier that drove viral genome evolution during SIV spillover events. RanBP2 (also known as Nup358) is a component of the nuclear pore complex known to facilitate nuclear entry of HIV-1. Our data suggest that transmission of SIV from monkeys to chimpanzees, and then from chimpanzees to gorillas, both coincided with changes in the viral capsid that allowed interaction with RanBP2 of the new host species. However, human RanBP2 subsequently provided no barrier to the zoonotic transmission of SIV from chimpanzees or gorillas, indicating that chimpanzee- and gorilla-adapted SIVs are pre-adapted to humans in this regard. Our observations are in agreement with RanBP2 driving virus evolution during cross-species transmissions of SIV, particularly in the transmissions to and between great ape species.

Multiple times, HIV-1 has entered the human population after emerging from a viral reservoir that exists in African primates. First, simian immunodeficiency virus (SIV) made the jump from monkeys into African great apes, and then from apes (namely, chimpanzees and gorillas) into humans. It is well appreciated that restriction factors, which are specialized proteins of the innate immune system, acted as host-specific barriers that drove virus adaptation during these spillover events. Here, we present data showing that a major constituent of the nuclear pore complex, RanBP2, was also a barrier to the spillover of SIVs, particularly in great ape species. Spillover of SIV into chimpanzee and gorilla populations required that the SIV capsid mutate to establish interaction with RanBP2 in the new host species. Our study highlights how essential housekeeping proteins, despite being generally more evolutionarily conserved than restriction factors, can also drive virus evolution during spillover events.

Species-specific functions of Epstein-Barr virus nuclear antigen 2 (EBNA2) reveal dual roles for initiation and maintenance of B cell immortalization

Epstein-Barr virus (EBV) and related lymphocryptoviruses (LCV) from non-human primates infect B cells, transform their growth to facilitate life-long viral persistence in the host, and contribute to B cell oncogenesis. Co-evolution of LCV with their primate hosts has led to species-specificity so that LCVs preferentially immortalize B cells from their natural host in vitro. We investigated whether the master regulator of transcription, EBV nuclear antigen 2 (EBNA2), is involved in LCV species-specificity. Using recombinant EBVs, we show that EBNA2 orthologues of LCV isolated from chimpanzees, baboons, cynomolgus or rhesus macaques cannot replace EBV EBNA2 for the immortalization of human B cells. Thus, LCV species-specificity is functionally linked to viral proteins expressed during latent, growth-transforming infection. In addition, we identified three independent domains within EBNA2 that act through species-specific mechanisms. Importantly, the EBNA2 orthologues and species-specific EBNA2 domains separate unique roles for EBNA2 in the initiation of B cell immortalization from those responsible for maintaining the immortalized state. Investigating LCV species-specificity provides a novel approach to identify critical steps underlying EBV-induced B cell growth transformation, persistent infection, and oncogenesis.

Minute Virus of Canines NP1 Protein Governs the Expression of a Subset of Essential Nonstructural Proteins via Its Role in RNA Processing

Parvoviruses use a variety of means to control the expression of their compact genomes. The bocaparvovirus minute virus of canines (MVC) encodes a small, genus-specific protein, NP1, which governs access to the viral capsid gene via its role in alternative polyadenylation and alternative splicing of the single MVC pre-mRNA. In addition to NP1, MVC encodes five additional nonstructural proteins (NS) that share an initiation codon at the left end of the genome and which are individually encoded by alternative multiply spliced mRNAs. We found that three of these proteins were encoded by mRNAs that excise the NP1-regulated MVC intron immediately upstream of the internal polyadenylation site, (pA)p, and that generation of these proteins was thus regulated by NP1. Splicing of their progenitor mRNAs joined the amino termini of these proteins to the NP1 open reading frame, and splice site mutations that prevented their expression inhibited virus replication in a host cell-dependent manner. Thus, in addition to controlling capsid gene access, NP1 also controls the expression of three of the five identified NS proteins via its role in governing MVC pre-mRNA splicing.IMPORTANCE The Parvovirinae are small nonenveloped icosahedral viruses that are important pathogens in many animal species, including humans. Minute virus of canine (MVC) is an autonomous parvovirus in the genus Bocaparvovirus It has a single promoter that generates a single pre-mRNA. NP1, a small genus-specific MVC protein, participates in the processing of this pre-mRNA and so controls capsid gene access via its role in alternative internal polyadenylation and splicing. We show that NP1 also controls the expression of three of the five identified NS proteins via its role in governing MVC pre-mRNA splicing. These NS proteins together are required for virus replication in a host cell-dependent manner.

Histone Deacetylase Inhibitors Reduce the Number of Herpes Simplex Virus-1 Genomes Initiating Expression in Individual Cells

Although many viral particles can enter a single cell, the number of viral genomes per cell that establish infection is limited. However, mechanisms underlying this restriction were not explored in depth. For herpesviruses, one of the possible mechanisms suggested is chromatinization and silencing of the incoming genomes. To test this hypothesis, we followed infection with three herpes simplex virus 1 (HSV-1) fluorescence expressing recombinants in the presence or absence of histone deacetylases inhibitors (HDACi's). Unexpectedly, a lower number of viral genomes initiated expression in the presence of these inhibitors. This phenomenon was observed using several HDACi: Trichostatin A (TSA), Suberohydroxamic Acid, Valporic Acid, and Suberoylanilide Hydroxamic Acid. We found that HDACi presence did not change the progeny outcome from the infected cells but did alter the kinetic of the gene expression from the viral genomes. Different cell types (HFF, Vero, and U2OS), which vary in their capability to activate intrinsic and innate immunity, show a cell specific basal average number of viral genomes establishing infection. Importantly, in all cell types, treatment with TSA reduced the number of viral genomes. ND10 nuclear bodies are known to interact with the incoming herpes genomes and repress viral replication. The viral immediate early protein, ICP0, is known to disassemble the ND10 bodies and to induce degradation of some of the host proteins in these domains. HDACi treated cells expressed higher levels of some of the host ND10 proteins (promyelocytic leukemia and ATRX), which may explain the lower number of viral genomes initiating expression per cell. Corroborating this hypothesis, infection with three HSV-1 recombinants carrying a deletion in the gene coding for ICP0, show a reduction in the number of genomes being expressed in U2OS cells. We suggest that alterations in the levels of host proteins involved in intrinsic antiviral defense may result in differences in the number of genomes that initiate expression.