Parvovirus minute virus of mice induces a DNA damage response that facilitates viral replication

Minute virus of mice NS1 redirects casein kinase 2 specificity to suppress the ATR DNA damage response pathway during infection

During infection the autonomous parvovirus minute virus of mice (MVM) generates extensive DNA damage which facilitates virus replication and induces a cellular DNA damage response (DDR) driven by the ataxia telangiectasia mutated (ATM) kinase. Atypically, the ataxia telangiectasia and Rad-3-related (ATR) DDR pathway remains inactive. Upon DNA damage ATR is normally recruited to single-stranded DNA sequences formed at genomic DNA damage sites, and while within a multiprotein complex activates, via phosphorylation, the key DDR regulator checkpoint kinase 1 (Chk1). Inactivation of ATR during MVM infection leads to the accumulation of damaged DNA and enhancement of virus replication. Although ATR is inactivated, we show that during infection, the Chk1 activation pathway downstream of the initial ATR activating events remained functional. Activation of ATR, and consequently of Chk1, requires interaction with TopBP1, which itself is maintained in proximity to ATR by interaction with the phosphorylated S387 residue of Rad9, part of the Rad9-Hus1-Rad1 (911) complex. Both MVM infection and MVM NS1 overexpression inhibited Rad9 S387 phosphorylation and subsequent ATR activation. ATR inactivation during infection was suppressed by expression of Rad9 bearing a phosphomimetic 387 residue, indicating that this site, and the function it served, was the target of NS1 inhibition. NS1 interaction with CK2α and CK2α enzymatic activity was both required to prevent ATR activation, indicating MVM retargeted this kinase's activity during infection. Inhibition of the protein phosphatase 2C (PP2C) prevented Rad9 S387 dephosphorylation and Chk1 inactivation during MVM infection and NS1 overexpression revealing its role in the pathway's suppression.

IMPORTANCE

Infection by the parvovirus minute virus of mice (MVM) causes significant DNA damage and induces a potent DNA damage response (DDR) which the virus exploits to further its replication. The cell responds to infection with an ATM-regulated DDR; however, atypically, the ATR-regulated DDR pathway is disabled during infection. This prevents Chk1 activation, thus allowing the accumulation of damaged DNA which facilitates virus replication. We describe here how MVM, and specifically the main viral replication protein NS1, inhibits ATR activation. Activation of ATR, and consequently Chk1, requires TopBP1 localization into the activating complex via its interaction with a phosphorylated residue of Rad9. We show that NS1 redirects casein kinase 2 to activate a phosphatase in the PP2C family which causes dephosphorylation of this critical residue, thus inhibiting ATR activation. This work provides mechanistic insight into one of the ways by which parvoviruses modify the host DDR response to facilitate their replication.

Inactivation of checkpoint kinase 1 (Chk1) during parvovirus minute virus of mice (MVM) infection inhibits cellular homologous recombination repair and facilitates viral genome replication

During infection, the autonomous parvovirus minute virus of mice (MVM) induces cellular DNA breaks and localizes to such sites, which presumably affords an environment beneficial for genome replication. MVM replication also benefits from the DNA damage response (DDR) mediated by the ataxia-telangiectasia mutated (ATM) kinase, while the ataxia telangiectasia and Rad-3 related (ATR) arm of the DDR is disabled, which prevents activation of its primary target, checkpoint kinase 1 (Chk1). We find here that Chk1 inactivation strongly correlates with dephosphorylation of one of its targets, RAD51, known to play a pivotal role in homologous recombination repair (HRR), thus leading to substantial inhibition of DNA repair in infected cells. We demonstrate colocalization of replicating MVM DNA with cellular double-strand breaks (DSBs) during infection, and show that an agent that exogenously induces cellular DSBs significantly increases viral DNA replication levels, establishing a role for cellular genome damage in facilitating virus DNA replication. Additionally, overexpression of active Chk1 during MVM infection was found to re-establish the activating phosphorylation of RAD51 Thr 309, significantly suppress infection-induced reduction of HRR efficiency with a concomitant increase in cellular genome DSBs, and reduce viral DNA replication levels. Thus, we conclude that during infection, MVM inhibition of Chk1 activation enhances viral replication, at least in part, by inhibiting cellular HRR.IMPORTANCEThe autonomous parvovirus minute virus of mice (MVM) has a compact DNA genome encoding a minimum number of proteins. During infection, it induces cellular DNA damage and both utilizes and modifies the subsequent cellular DNA damage response (DDR) in various ways to facilitate its replication. One of MVM's activities in this regard is to inhibit one of the primary arms of the DDR, the ataxia telangiectasia and Rad-3 related (ATR) pathway, which prevents activation of checkpoint kinase 1 (Chk1), a key protein involved in controlling the cellular DDR and preserving genome integrity. We show that prevention by MVM of Chk1 activation leads to inhibition of homologous recombination repair (HRR) of cellular DNA, which helps sustain viral replication. This work illuminates another way in which autonomous parvoviruses adjust the cellular environment for their replicative advantage.

DNA-PK and ATM drive phosphorylation signatures that antagonistically regulate cytokine responses to herpesvirus infection or DNA damage

The DNA-dependent protein kinase, DNA-PK, is an essential regulator of DNA damage repair. DNA-PK-driven phosphorylation events and the activated DNA damage response (DDR) pathways are also components of antiviral intrinsic and innate immune responses. Yet, it is not clear whether and how the DNA-PK response differs between these two forms of nucleic acid stress-DNA damage and DNA virus infection. Here, we define DNA-PK substrates and the signature cellular phosphoproteome response to DNA damage or infection with the nuclear-replicating DNA herpesvirus, HSV-1. We establish that DNA-PK negatively regulates the ataxia-telangiectasia-mutated (ATM) DDR kinase during viral infection. In turn, ATM blocks the binding of DNA-PK and the nuclear DNA sensor IFI16 to viral DNA, thereby inhibiting cytokine responses. However, following DNA damage, DNA-PK enhances ATM activity, which is required for IFN-β expression. These findings demonstrate that the DDR autoregulates cytokine expression through the opposing modulation of DDR kinases.

For better or worse: crosstalk of parvovirus and host DNA damage response

Parvoviruses are a group of non-enveloped DNA viruses that have a broad spectrum of natural infections, making them important in public health. NS1 is the largest and most complex non-structural protein in the parvovirus genome, which is indispensable in the life cycle of parvovirus and is closely related to viral replication, induction of host cell apoptosis, cycle arrest, DNA damage response (DDR), and other processes. Parvovirus activates and utilizes the DDR pathway to promote viral replication through NS1, thereby increasing pathogenicity to the host cells. Here, we review the latest progress of parvovirus in regulating host cell DDR during the parvovirus lifecycle and discuss the potential of cellular consequences of regulating the DDR pathway, targeting to provide the theoretical basis for further elucidation of the pathogenesis of parvovirus and development of new antiviral drugs.

Chaperonin TRiC/CCT subunit CCT7 is involved in the replication of canine parvovirus in F81 cells

Canine parvovirus (CPV) is one of the most common lethal viruses in canines. The virus disease is prevalent throughout the year, with high morbidity and mortality rate, causing serious harm to dogs and the dog industry. Previously, yeast two hybrid method was used to screen the protein chaperonin containing TCP-1 (CCT7) that interacts with VP2. However, the mechanism of interactions between CCT7 and VP2 on CPV replication remains unclear. In this study, we first verified the interaction between CCT7 and viral VP2 proteins using yeast one-to-one experiment and co-immunoprecipitation (CoIP) experiment. Laser confocal microscopy observation showed that CCT7 and VP2 were able to co-localize and were mostly localized in the cytoplasm. In addition, the study of VP2 truncated mutant found that the interaction region of VP2 with CCT7 was located between amino acids 231 and 320. Cycloheximide (CHX) chase experiments showed that CCT7 can improve the stability of VP2 protein. After further regulation of CCT7 expression in F81 cells, it was found that the expression level of VP2 protein was significantly reduced after knocking down CCT7 expression by RNA interference (RNAi) or HSF1A inhibitor, and increased after overexpressing host CCT7. The study reveals the role of VP2 interacting protein CCT7 in the replication process of CPV, which could provide a potential target for the prevention and control of CPV.

Antiviral Activity of an Endogenous Parvoviral Element

Endogenous viral elements (EVEs) are genomic DNA sequences derived from viruses. Some EVEs have open reading frames (ORFs) that can express proteins with physiological roles in their host. Furthermore, some EVEs exhibit a protective role against exogenous viral infection in their host. Endogenous parvoviral elements (EPVs) are highly represented in mammalian genomes, and although some of them contain ORFs, their function is unknown. We have shown that the locus EPV-Dependo.43-ODegus, an EPV with an intact ORF, is transcribed in Octodon degus (degu). Here we examine the antiviral activity of the protein encoded in this EPV, named DeRep. DeRep was produced in bacteria and used to generate antibodies that recognize DeRep in western blots of degu tissue. To test if DeRep could protect against exogenous parvovirus, we challenged cells with the minute virus of mice (MVM), a model autonomous parvovirus. We observed that MVM protein expression, DNA damage induced by replication, viral DNA, and cytopathic effects are reduced when DeRep is expressed in cells. The results of this study demonstrate that DeRep is expressed in degu and can inhibit parvovirus replication. This is the first time that an EPV has been shown to have antiviral activity against an exogenous virus.

The DNA Damage Sensor MRE11 Regulates Efficient Replication of the Autonomous Parvovirus Minute Virus of Mice

Parvoviruses are single-stranded DNA viruses that utilize host proteins to vigorously replicate in the nuclei of host cells, leading to cell cycle arrest. The autonomous parvovirus, minute virus of mice (MVM), forms viral replication centers in the nucleus which are adjacent to cellular DNA damage response (DDR) sites, many of which are fragile genomic regions prone to undergoing DDR during the S phase. Since the cellular DDR machinery has evolved to transcriptionally suppress the host epigenome to maintain genomic fidelity, the successful expression and replication of MVM genomes at these cellular sites suggest that MVM interacts with DDR machinery distinctly. Here, we show that efficient replication of MVM requires binding of the host DNA repair protein MRE11 in a manner that is independent of the MRE11-RAD50-NBS1 (MRN) complex. MRE11 binds to the replicating MVM genome at the P4 promoter, remaining distinct from RAD50 and NBS1, which associate with cellular DNA break sites to generate DDR signals in the host genome. Ectopic expression of wild-type MRE11 in CRISPR knockout cells rescues virus replication, revealing a dependence on MRE11 for efficient MVM replication. Our findings suggest a new model utilized by autonomous parvoviruses to usurp local DDR proteins that are crucial for viral pathogenesis and distinct from those of dependoparvoviruses, like adeno-associated virus (AAV), which require a coinfected helper virus to inactivate the local host DDR. IMPORTANCE The cellular DNA damage response (DDR) machinery protects the host genome from the deleterious consequences of DNA breaks and recognizes invading viral pathogens. DNA viruses that replicate in the nucleus have evolved distinct strategies to evade or usurp these DDR proteins. We have discovered that the autonomous parvovirus, MVM, which is used to target cancer cells as an oncolytic agent, depends on the initial DDR sensor protein MRE11 to express and replicate efficiently in host cells. Our studies reveal that the host DDR interacts with replicating MVM molecules in ways that are distinct from viral genomes being recognized as simple broken DNA molecules. These findings suggest that autonomous parvoviruses have evolved distinct mechanisms to usurp DDR proteins, which can be used to design potent DDR-dependent oncolytic agents.

The Autonomous Parvovirus Minute Virus of Mice Localizes to Cellular Sites of DNA Damage Using ATR Signaling

Minute Virus of Mice (MVM) is an autonomous parvovirus of the Parvoviridae family that replicates in mouse cells and transformed human cells. MVM genomes localize to cellular sites of DNA damage with the help of their essential non-structural phosphoprotein NS1 to establish viral replication centers. MVM replication induces a cellular DNA damage response that is mediated by signaling through the ATM kinase pathway, while inhibiting induction of the ATR kinase signaling pathway. However, the cellular signals regulating virus localization to cellular DNA damage response sites has remained unknown. Using chemical inhibitors to DNA damage response proteins, we have discovered that NS1 localization to cellular DDR sites is independent of ATM or DNA-PK signaling but is dependent on ATR signaling. Pulsing cells with an ATR inhibitor after S-phase entry leads to attenuated MVM replication. These observations suggest that the initial localization of MVM to cellular DDR sites depends on ATR signaling before it is inactivated by vigorous virus replication.

Genomes of the autonomous parvovirus minute virus of mice induce replication stress through RPA exhaustion

The oncolytic autonomous parvovirus Minute Virus of Mice (MVM) establishes infection in the nuclear environment by usurping host DNA damage signaling proteins in the vicinity of cellular DNA break sites. MVM replication induces a global cellular DNA Damage Response (DDR) that is dependent on signaling by the ATM kinase and inactivates the cellular ATR-kinase pathway. However, the mechanism of how MVM generates cellular DNA breaks remains unknown. Using single molecule DNA Fiber Analysis, we have discovered that MVM infection leads to a shortening of host replication forks as infection progresses, as well as induction of replication stress prior to the initiation of virus replication. Ectopically expressed viral non-structural proteins NS1 and NS2 are sufficient to cause host-cell replication stress, as is the presence of UV-inactivated non-replicative MVM genomes. The host single-stranded DNA binding protein Replication Protein A (RPA) associates with the UV-inactivated MVM genomes, suggesting MVM genomes might serve as a sink for cellular stores of RPA. Overexpressing RPA in host cells prior to UV-MVM infection rescues DNA fiber lengths and increases MVM replication, confirming that MVM genomes deplete RPA stores to cause replication stress. Together, these results indicate that parvovirus genomes induce replication stress through RPA exhaustion, rendering the host genome vulnerable to additional DNA breaks.

Structure and function of the parvoviral NS1 protein: a review

Parvoviruses possess a single-stranded DNA genome of about 5 kb, which contains two open reading frames (ORFs), one encoding nonstructural (NS) proteins, the other capsid proteins. The NS1 protein contains an N-terminal origin-binding domain, a helicase domain, and a C-terminal transactive domain, and is essential for effective viral replication and production of infectious virus. We first summarize the developments in the structure of NS1 protein, including the original binding domain and the helicase domain. We discuss the role of different DNA substrates in the oligomerization of these two domains of NS1. During the parvovirus life cycle, the NS1 protein is closely related to the viral gene expression, viral replication, and infection. We provide the current understanding of the impact of parvovirus NS1 protein mutations on its biological properties. Overall, in this review, we focus on the structure and function of the parvoviral NS1 protein.

Tracking of Human Parvovirus B19 Virus-Like Particles Using Short Peptide Tags Reveals a Membrane-Associated Extracellular Release of These Particles

B19 virus (B19V) is a pathogenic human parvovirus that infects erythroid progenitor cells. Because there are limited in vitro culture systems to propagate this virus, little is known about the molecular mechanisms by which it propagates in cells. In this study, we introduced a HiBiT peptide tag into various loops of VP2 located on the surface of B19V particles and evaluated their ability to form virus-like particles (VLPs). Three independent sites were identified as permissive sites for peptide tag insertion without affecting VLP formation. When the HiBiT tag was introduced into B19V clones (pB19-M20) and transfected into a semipermissive erythroleukemia cell line (UT7/Epo-S1), HiBiT-dependent luciferase activities (HiBiT activities) increased depending on helicase activity of viral NS1. Furthermore, we used a GFP11 tag-split system to visualize VLPs in the GFP1-10-expressing live cells. Time-lapse imaging of green fluorescent protein (GFP)-labeled VLPs revealed that nuclear VLPs were translocated into the cytoplasm only after cell division, suggesting that the breakdown of the nuclear envelope during mitosis contributes to VLP nuclear export. Moreover, HiBiT activities of culture supernatants were dependent on the presence of a detergent, and the released VLPs were associated with extracellular vesicles, as observed under electron microscopy. Treatment with an antimitotic agent (nocodazole) enhanced the release of VLPs. These results suggest that the virions accumulated in the cytoplasm are constitutively released from the cell as membrane-coated vesicles. These properties are likely responsible for viral escape from host immune responses and enhance membrane fusion-mediated transmission. IMPORTANCE Parvovirus particles are expected to be applied as nanoparticles in drug delivery systems. However, little is known about how nuclear-assembled B19 virus (B19V) virions are released from host cells. This study provides evidence of mitosis-dependent nuclear export of B19V and extracellular vesicle-mediated virion release. Moreover, this study provides methods for modifying particle surfaces with various exogenous factors and contributes to the development of fine nanoparticles with novel valuable functions. The pB19-M20 plasmid expressing HiBiT-tagged VP2 is a novel tool to easily quantify VP2 expression. Furthermore, this system can be applied in high-throughput screening of reagents that affect VP2 expression, which might be associated with viral propagation.

Adeno-Associated Virus Monoinfection Induces a DNA Damage Response and DNA Repair That Contributes to Viral DNA Replication

Adeno-associated virus (AAV) belongs to the Dependoparvovirus genus of the Parvoviridae family. AAV replication relies on a helper virus, such as adenovirus (Ad). Co-infection of AAV and Ad induces a DNA damage response (DDR), although its function in AAV DNA replication remains unknown. In this study, monoinfection of AAV2 in HEK293T cells expressing a minimal set of Ad helper genes was used to investigate the role of the DDR solely induced by AAV. We found that AAV2 DNA replication, but not single stranded (ss)DNA genome accumulation and Rep expression only, induced a robust DDR in HEK293T cells. The induced DDR featured the phosphorylation of replication protein A32 (RPA32), histone variant H2AX (H2A histone family member X), and all 3 phosphatidylinositol 3-kinase-related kinases (PIKKs). We also found that the kinase ataxia telangiectasia and Rad3-related protein (ATR) plays a major role in AAV2 DNA replication and that Y family DNA repair DNA polymerases η (Pol η) and Pol κ contribute to AAV2 DNA replication both in vitro and in HEK293T cells. Knockout of Pol η and Pol κ in HEK293T cells significantly decreased wild-type AAV2 replication and recombinant AAV2 production. Thus, our study has proven that AAV2 DNA replication induces a DDR, which in turn initiates a DNA repairing process that partially contributes to the viral genome amplification in HEK293T cells. IMPORTANCE Recombinant AAV (rAAV) has emerged as one of the preferred delivery vectors for clinical gene therapy. rAAV production in HEK293 cells by transfection of a rAAV transgene plasmid, an AAV Rep and Cap expression packaging plasmid, and an Ad helper plasmid remains the popular method. Here, we demonstrated that the high fidelity Y family DNA repair DNA polymerase, Pol η, and Pol κ, plays a significant role in AAV DNA replication and rAAV production in HEK293T cells. Understanding the AAV DNA replication mechanism in HEK293T cells could provide clues to increase rAAV vector yield produced from the transfection method. We also provide evidence that the ATR-mediated DNA repair process through Pol η and Pol κ is one of the mechanisms to amplify AAV genome, which could explain AAV replication and rAAV ssDNA genome conversion in mitotic quiescent cells.

Design and Characterization of Mutated Variants of the Oncotoxic Parvoviral Protein NS1

Oncotoxic proteins such as the non-structural protein 1 (NS1), a constituent of the rodent parvovirus H1 (H1-PV), offer a novel approach for treatment of tumors that are refractory to other treatments. In the present study, mutated NS1 variants were designed and tested with respect to their oncotoxic potential in human hepatocellular carcinoma cell lines. We introduced single point mutations of previously described important residues of the wild-type NS1 protein and a deletion of 114 base pairs localized within the N-terminal domain of NS1. Cell-viability screening with HepG2 and Hep3B hepatocarcinoma cells transfected with the constructed NS1-mutants led to identification of the single-amino acid NS1-mutant NS1-T585E, which led to a 30% decrease in cell viability as compared to NS1 wildtype. Using proteomics analysis, we could identify new interaction partners and signaling pathways of NS1. We could thus identify new oncotoxic NS1 variants and gain insight into the modes of action of NS1, which is exclusively toxic to human cancer cells. Our in-vitro studies provide mechanistic explanations for the observed oncolytic effects. Expression of NS1 variants had no effect on cell viability in NS1 unresponsive control HepG2 cells or primary mouse hepatocytes. The availability of new NS1 variants in combination with a better understanding of their modes of action offers new possibilities for the design of innovative cancer treatment strategies.

Adding recombinant AAVs to the cancer therapeutics mix

Gene therapy is a powerful biological tool that is reshaping therapeutic landscapes for several diseases. Researchers are using both non-viral and viral-based gene therapy methods with success in the lab and the clinic. In the cancer biology field, gene therapies are expanding treatment options and the possibility of favorable outcomes for patients. While cellular immunotherapies and oncolytic virotherapies have paved the way in cancer treatments based on genetic engineering, recombinant adeno-associated virus (rAAV), a viral-based module, is also emerging as a potential cancer therapeutic through its malleability, specificity, and broad application to common as well as rare tumor types, tumor microenvironments, and metastatic disease. A wide range of AAV serotypes, promoters, and transgenes have been successful at reducing tumor growth and burden in preclinical studies, suggesting more groundbreaking advances using rAAVs in cancer are on the horizon.

G2/M checkpoint regulation and apoptosis facilitate the nuclear egress of parvoviral capsids

The nuclear export factor CRM1-mediated pathway is known to be important for the nuclear egress of progeny parvovirus capsids in the host cells with virus-mediated cell cycle arrest at G2/M. However, it is still unclear whether this is the only pathway by which capsids exit the nucleus. Our studies show that the nuclear egress of DNA-containing full canine parvovirus. capsids was reduced but not fully inhibited when CRM1-mediated nuclear export was prevented by leptomycin B. This suggests that canine parvovirus capsids might use additional routes for nuclear escape. This hypothesis was further supported by our findings that nuclear envelope (NE) permeability was increased at the late stages of infection. Inhibitors of cell cycle regulatory protein cyclin-dependent kinase 1 (Cdk1) and pro-apoptotic caspase 3 prevented the NE leakage. The change in NE permeability could be explained by the regulation of the G2/M checkpoint which is accompanied by early mitotic and apoptotic events. The model of G2/M checkpoint activation was supported by infection-induced nuclear accumulation of cyclin B1 and Cdk1. Both NE permeability and nuclear egress of capsids were reduced by the inhibition of Cdk1. Additional proof of checkpoint function regulation and promotion of apoptotic events was the nucleocytoplasmic redistribution of nuclear transport factors, importins, and Ran, in late infection. Consistent with our findings, post-translational histone acetylation that promotes the regulation of several genes related to cell cycle transition and arrest was detected. In conclusion, the model we propose implies that parvoviral capsid egress partially depends on infection-induced G2/M checkpoint regulation involving early mitotic and apoptotic events.

Ataxia Telangiectasia-Mutated Is Activated but Not Required for Productive Autographa californica Multiple Nucleopolyhedrovirus Infection

Multiplication of the invertebrate DNA baculoviruses activates the host DNA damage response (DDR), which promotes virus DNA replication. DDR signaling is initiated by the host insect's phosphatidylinositol-3 kinase-related kinases (PIKKs), including ataxia telangiectasia-mutated kinase (ATM). Like other PIKKs, ATM phosphorylates an array of host DDR proteins at serine/threonine glutamine (S/TQ) motifs, the result of which leads to cell cycle arrest, DNA repair, or apoptosis. To define the role of host PIKKs in baculovirus replication, we compared replication levels of the baculovirus prototype species Autographa californica multiple nucleopolyhedrovirus in permissive Spodoptera frugiperda (SF21) cells with and without ATM function. Caffeine, which inhibits multiple DDR kinases, and the ATM-specific inhibitors KU-55933 and KU-60019 each prevented phosphorylation of Spodoptera histone H2AX (SfH2AX), a recognized indicator of ATM activity. However, only caffeine reduced autographa californica multiple nucleopolyhedrovirus (AcMNPV)-induced bulk phosphorylation of S/TQ protein motifs. Furthermore, only caffeine, not KU-55933 or KU-60019, reduced AcMNPV yields, suggesting a limited role for ATM. To investigate further, we identified and edited the Spodoptera ATM gene (sfatm). Consistent with ATM's known functions, CRISPR/Cas9-mediated knockout of sfatm eliminated DNA damage-induced phosphorylation of DDR marker SfH2AX in SF21 cells. However, loss of sfatm failed to affect the levels of AcMNPV multiplication. These findings suggested that in the absence of the kinase SfATM, another caffeine-sensitive host DDR kinase promotes S/TQ phosphorylation and baculovirus multiplication. Thus, baculoviruses activate and utilize the host insect DDR in an ATM-independent manner. IMPORTANCE The DDR, while necessary for the maintenance and fidelity of the host genome, represents an important cellular response to viral infection. The prolific DNA baculoviruses activate and manipulate the invertebrate DDR by using mechanisms that positively impact virus multiplication, including virus DNA replication. As the key DDR initiator kinase, ATM was suspected to play a critical role in this host response. However, we show here that baculovirus AcMNPV activates an ATM-independent DDR. By identifying the insect host ATM ortholog (Spodoptera frugiperda SfATM) and evaluating genetic knockouts, we show that SfATM is dispensable for AcMNPV activation of the DDR and for virus replication. Thus, another PIKK, possibly the closely related kinase ATR (ATM- and Rad3-related kinase), is responsible for efficient baculovirus multiplication. These findings better define the host pathways used by invertebrates to engage viral pathogens, including DNA viruses.

Nuclear entry and egress of parvoviruses

Parvoviruses are small non-enveloped single-stranded DNA viruses, which depend on host cell nuclear transcriptional and replication machinery. After endosomal exposure of nuclear localization sequence and a phospholipase A₂ domain on the capsid surface, and escape into the cytosol, parvovirus capsids enter the nucleus. Due to the small capsid diameter of 18-26 nm, intact capsids can potentially pass into the nucleus through nuclear pore complexes (NPCs). This might be facilitated by active nuclear import, but capsids may also follow an alternative entry pathway that includes activation of mitotic factors and local transient disruption of the nuclear envelope. The nuclear entry is followed by currently undefined events of viral genome uncoating. After genome release, viral replication compartments are initiated and infection proceeds. Parvoviral genomes replicate during cellular S phase followed by nuclear capsid assembly during virus-induced S/G2 cell cycle arrest. Nuclear egress of capsids occurs upon nuclear envelope degradation during apoptosis and cell lysis. An alternative pathway for nuclear export has been described using active transport through the NPC mediated by the chromosome region maintenance 1 protein, CRM1, which is enhanced by phosphorylation of the N-terminal domain of VP2. However, other alternative but not yet uncharacterized nuclear export pathways cannot be excluded.

DNA Damage Response Differentially Affects BoHV-1 Gene Transcription in Cell Type-Dependent Manners

Bovine herpesvirus 1 (BoHV-1), an important pathogen of cattle, is also a promising oncolytic virus. Recent studies have demonstrated that the virus infection induces DNA damage and DNA damage response (DDR), potentially accounting for virus infection-induced cell death and oncolytic effects. However, whether the global DDR network affects BoHV-1 productive infection remains to be elucidated. In this study, we show that global DDR induced by ultraviolet (UV) irradiation prior to BoHV-1 infection differentially affected transcription of immediate early (IE) genes, such as infected cell protein 0 (bICP0) and bICP22, in a cell-type-dependent manner. In addition, UV-induced DDR may affect the stabilization of viral protein levels, such as glycoprotein C (gC) and gD, because the variation in mRNA levels of gC and gD as a consequence of UV treatment were not in line with the variation in individual protein levels. The virus productive infection also affects UV-primed DDR signaling, as demonstrated by the alteration of phosphorylated histone H2AX (γH2AX) protein levels and γH2AX formation following virus infection. Taken together, for the first time, we evidenced the interplay between UV-primed global DDR and BoHV-1 productive infection. UV-primed global DDR differentially modulates the transcription of virus genes and stabilization of virus protein. Vice versa, the virus infection may affect UV-primed DDR signaling.

Protein Myristoylation Plays a Role in the Nuclear Entry of the Parvovirus Minute Virus of Mice

Being nonpathogenic to humans, rodent parvoviruses (PVs) are naturally oncolytic viruses with great potential as anti-cancer agents. As these viruses replicate in the host cell nucleus, they must gain access to the nucleus during infection. The PV minute virus of mice (MVM) and several other PVs transiently disrupt the nuclear envelope (NE) and enter the nucleus through the resulting breaks. However, the molecular basis of this unique nuclear entry pathway remains uncharacterized. In this study, we used MVM as a model to investigate the molecular mechanism by which PVs induce NE disruption during viral nuclear entry. By combining bioinformatics analyses, metabolic labeling assays, mutagenesis, and pharmacological inhibition, we identified a functional myristoylation site at the sequence ⁷⁸GGKVGH⁸³ of the unique portion of the capsid protein VP1 (VP1u) of MVM. Performing proteolytic cleavage studies with a peptide containing this myristoylation site or with purified virions, we found tryptophan at position 77 of MVM VP1u is susceptible to chymotrypsin cleavage, implying this cleavage exposes G (glycine) 78 at the N-terminus of VP1u for myristoylation. Subsequent experiments using inhibitors of myristoylation and cellular proteases with MVM-infected cells, or an imaging-based quantitative NE permeabilization assay, further indicate protein myristoylation and a chymotrypsin-like activity are essential for MVM to locally disrupt the NE during viral nuclear entry. We thus propose a model for the nuclear entry of MVM in which NE disruption is mediated by VP1u myristoylation after the intact capsid undergoes proteolytic processing to expose the required N-terminal G for myristoylation. IMPORTANCE Rodent parvoviruses (PVs), including minute virus of mice (MVM), have the ability to infect and kill cancer cells and thereby possess great potential in anti-cancer therapy. In fact, some of these viruses are currently being investigated in both preclinical studies and clinical trials to treat a wide variety of cancers. However, the detailed mechanism of how PVs enter the cell nucleus remains unknown. In this study, we for the first time demonstrated a chemical modification called "myristoylation" of a MVM protein plays an essential role in the nuclear entry of the virus. We also showed, in addition to protein myristoylation, a chymotrypsin-like activity, which may come from cellular proteasomes, is required for MVM to get myristoylated and enter the nucleus. These findings deepen our understanding on how MVM and other related PVs infect host cells and provide new insights for the development of PV-based anti-cancer therapies.

The small nonstructural protein NP1 of human bocavirus 1 directly interacts with Ku70 and RPA70 and facilitates viral DNA replication

Human bocavirus 1 (HBoV1), a member of the genus Bocaparvovirus of the family Parvoviridae, causes acute respiratory tract infections in young children. Well-differentiated pseudostratified human airway epithelium cultured at an air-liquid interface (HAE-ALI) is an ideal in vitro culture model to study HBoV1 infection. Unique to other parvoviruses, bocaparvoviruses express a small nonstructured protein NP1 of ~25 kDa from an open reading frame (ORF) in the center of the viral genome. NP1 plays an important role in viral DNA replication and pre-mRNA processing. In this study, we performed an affinity purification assay to identify HBoV1 NP1-inteacting proteins. We identified that Ku70 and RPA70 directly interact with the NP1 at a high binding affinity, characterized with an equilibrium dissociation constant (KD) of 95 nM and 122 nM, respectively. Furthermore, we mapped the key NP1-interacting domains of Ku70 at aa266-439 and of RPA70 at aa181-422. Following a dominant negative strategy, we revealed that the interactions of Ku70 and RPA70 with NP1 play a significant role in HBoV1 DNA replication not only in an in vitro viral DNA replication assay but also in HBoV1-infected HAE-ALI cultures. Collectively, our study revealed a novel mechanism by which HBoV1 NP1 enhances viral DNA replication through its direct interactions with Ku70 and RPA70.

Parvovirus nonstructural protein 2 interacts with chromatin-regulating cellular proteins

Autonomous parvoviruses encode at least two nonstructural proteins, NS1 and NS2. While NS1 is linked to important nuclear processes required for viral replication, much less is known about the role of NS2. Specifically, the function of canine parvovirus (CPV) NS2 has remained undefined. Here we have used proximity-dependent biotin identification (BioID) to screen for nuclear proteins that associate with CPV NS2. Many of these associations were seen both in noninfected and infected cells, however, the major type of interacting proteins shifted from nuclear envelope proteins to chromatin-associated proteins in infected cells. BioID interactions revealed a potential role for NS2 in DNA remodeling and damage response. Studies of mutant viral genomes with truncated forms of the NS2 protein suggested a change in host chromatin accessibility. Moreover, further studies with NS2 mutants indicated that NS2 performs functions that affect the quantity and distribution of proteins linked to DNA damage response. Notably, mutation in the splice donor site of the NS2 led to a preferred formation of small viral replication center foci instead of the large coalescent centers seen in wild-type infection. Collectively, our results provide insights into potential roles of CPV NS2 in controlling chromatin remodeling and DNA damage response during parvoviral replication.

Parvoviruses are small, nonenveloped DNA viruses, that besides being noteworthy pathogens in many animal species, including humans, are also being developed as vectors for gene and cancer therapy. Canine parvovirus is an autonomously replicating parvovirus that encodes two nonstructural proteins, NS1 and NS2. NS1 is required for viral DNA replication and packaging, as well as gene expression. However, very little is known about the function of NS2. Our studies indicate that NS2 serves a previously undefined important function in chromatin modification and DNA damage responses. Therefore, it appears that although both NS1 and NS2 are needed for a productive infection they play very different roles in the process.

The large nonstructural protein (NS1) of the human bocavirus 1 (HBoV1) directly interacts with Ku70, which plays an important role in virus replication in human airway epithelia

Human bocavirus 1 (HBoV1), an autonomous human parvovirus, causes acute respiratory tract infections in young children. HBoV1 infects well-differentiated (polarized) human airway epithelium cultured at an air-liquid interface (HAE-ALI). HBoV1 expresses a large nonstructural protein, NS1, that is essential for viral DNA replication. HBoV1 infection of polarized human airway epithelial cells induces a DNA damage response (DDR) that is critical to viral DNA replication involving DNA repair with error-free Y-family DNA polymerases. HBoV1 NS1 or the isoform NS1-70 per se induces a DDR. In this study, using the second-generation proximity-dependent biotin identification (BioID2) approach, we identified that Ku70 is associated with the NS1-BioID2 pulldown complex through a direct interaction with NS1. Biolayer interferometry (BLI) assay determined a high binding affinity of NS1 with Ku70, which has an equilibrium dissociation constant (K_D) value of 0.16 μM and processes the strongest interaction at the C-terminal domain. The association of Ku70 with NS1 was also revealed during HBoV1 infection of HAE-ALI. Knockdown of Ku70 and overexpression of the C-terminal domain of Ku70 significantly decreased HBoV1 replication in HAE-ALI. Thus, our study provides, for the first time, a direct interaction of parvovirus large nonstructural protein NS1 with Ku70.

IMPORTANCE Parvovirus infection induces a DNA damage response (DDR) that plays a pivotal role in viral DNA replication. The DDR includes activation of ATM (ataxia telangiectasia mutated), ATR (ATM- and RAD3-related), and DNA-PKcs (DNA-dependent protein kinase catalytic subunit). The large nonstructural protein (NS1) often plays a role in the induction of DDR; however, how the DDR is induced during parvovirus infection or simply by the NS1 is not well studied. Activation of DNA-PKcs has been shown as one of the key DDR pathways in DNA replication of HBoV1. We identified that HBoV1 NS1 directly interacts with Ku70, but not Ku80, of the Ku70/Ku80 heterodimer at high affinity. This interaction is also important for HBoV1 replication in HAE-ALI. We propose that the interaction of NS1 with Ku70 recruits the Ku70/Ku80 complex to the viral DNA replication center, which activates DNA-PKcs and facilitates viral DNA replication.

Small but mighty: old and new parvoviruses of veterinary significance

In line with the Latin expression "sed parva forti" meaning "small but mighty," the family Parvoviridae contains many of the smallest known viruses, some of which result in fatal or debilitating infections. In recent years, advances in metagenomic viral discovery techniques have dramatically increased the identification of novel parvoviruses in both diseased and healthy individuals. While some of these discoveries have solved etiologic mysteries of well-described diseases in animals, many of the newly discovered parvoviruses appear to cause mild or no disease, or disease associations remain to be established. With the increased use of animal parvoviruses as vectors for gene therapy and oncolytic treatments in humans, it becomes all the more important to understand the diversity, pathogenic potential, and evolution of this diverse family of viruses. In this review, we discuss parvoviruses infecting vertebrate animals, with a special focus on pathogens of veterinary significance and viruses discovered within the last four years.

The adeno-associated virus 2 (AAV2) genome and rep 68/78 proteins interact with cellular sites of DNA damage

Nuclear DNA viruses simultaneously access cellular factors that aid their life cycle while evading inhibitory factors by localizing to distinct nuclear sites. Adeno-Associated Viruses (AAVs), which are Dependoviruses in the family Parvovirinae, are non-enveloped icosahedral viruses, that have been developed as recombinant AAV vectors (rAAV) to express transgenes. AAV2 expression and replication occur in nuclear viral replication centers (VRCs), which relies on cellular replication machinery as well as coinfection by helper viruses such as adenoviruses or herpesviruses, or exogenous DNA damage to host cells. AAV2 infection induces a complex cellular DNA damage response (DDR), either in response to viral DNA or viral proteins expressed in the host nucleus during infection, where VRCs colocalize with DDR proteins. We have previously developed a modified iteration of a viral chromosome conformation capture (V3C-seq) assay to show that the autonomous parvovirus Minute Virus of Mice (MVM) localizes to cellular sites of DNA damage to establish and amplify its replication. Similar V3C-seq assays to map AAV2 show that the AAV2 genome colocalized with cellular sites of DNA damage under both non-replicating and replicating conditions. The AAV2 non-structural protein Rep 68/78, also localized to cellular DDR sites during both non-replicating and replicating infections, and also when ectopically expressed. Ectopically expressed Rep could be efficiently re-localized to DDR sites induced by micro-irradiation. Recombinant AAV2 gene therapy vector genomes derived from AAV2 localized to sites of cellular DNA damage to a lesser degree, suggesting that the Inverted Terminal Repeat (ITR) origins of replication were insufficient for targeting.

Molecular and Pathological Investigations of Selected Viral Neuropathogens in Rabies-Negative Brains of Cats and Dogs Revealed Neurotropism of Carnivore Protoparvovirus-1

Throughout the year, the Thai Red Cross Society (TRCS), Bangkok, Thailand, received more than 100 animals that died of suspected rabies due to neurological clinical signs. Concerning the role of viral infection in the brain in the outcome of neurological diseases in cats and dogs, a comprehensive study was conducted of 107 brain samples of cats and dogs submitted to the TRCS from August 2019 to August 2020. Selective molecular screening using conventional polymerase chain reaction (PCR) and reverse transcription PCR targeting nine viral pathogens was employed in addition to histopathological investigations. The results showed that carnivore protoparvovirus-1 (CPPV-1) was detected in 18.69% of the cats and dogs sampled (20/107). These results were found in young and old animals; the brain tissue did not show any pathological changes suggesting encephalitis or cerebellar hypoplasia. In addition, feline calicivirus, feline alphaherpesvirus-1, feline coronavirus, and canine distemper virus were also detected, providing a broader range of potential viral infections to consider in the clinical manifestation of neurological disorders in companion animals. The detection of all pathogens was confirmed by the localization of each viral antigen in various resident brain cells using immunohistochemistry. A unique L582S amino acid substitution of the non-structural protein 1 gene coding sequence, speculated to be associated with the neurotropism of CPPV-1 in cats and dogs, was not evident. In conclusion, this study revealed a noteworthy neurotropism of CPPV-1 in both cats and dogs without neurological lesions.

Viral targeting of glioblastoma stem cells with patient-specific genetic and post-translational p53 deregulations

Cancer therapy urges targeting of malignant subsets within self-renewing heterogeneous stem cell populations. We dissect the genetic and functional heterogeneity of human glioblastoma stem cells (GSCs) within patients by their innate responses to non-pathogenic mouse parvoviruses that are tightly restrained by cellular physiology. GSC neurospheres accumulate assembled capsids but restrict viral NS1 cytotoxic protein expression by an innate PKR/eIF2α-P response counteractable by electric pulses. NS1 triggers a comprehensive DNA damage response involving cell-cycle arrest, neurosphere disorganization, and bystander disruption of GSC-derived brain tumor architecture in rodent models. GSCs and cancer cell lines permissive to parvovirus genome replication require p53-Ser15 phosphorylation (Pp53S15). NS1 expression is enhanced by exogeneous Pp53S15 induction but repressed by wtp53. Consistently, patient-specific GSC subpopulations harboring p53 gain-of-function mutants and/or Pp53S15 are selective viral targets. This study provides a molecular foundation for personalized biosafe viral therapies against devastating glioblastoma and other cancers with deregulated p53 signaling.

Concepts to Reveal Parvovirus-Nucleus Interactions

Parvoviruses are small single-stranded (ss) DNA viruses, which replicate in the nucleoplasm and affect both the structure and function of the nucleus. The nuclear stage of the parvovirus life cycle starts at the nuclear entry of incoming capsids and culminates in the successful passage of progeny capsids out of the nucleus. In this review, we will present past, current, and future microscopy and biochemical techniques and demonstrate their potential in revealing the dynamics and molecular interactions in the intranuclear processes of parvovirus infection. In particular, a number of advanced techniques will be presented for the detection of infection-induced changes, such as DNA modification and damage, as well as protein-chromatin interactions.

Recent Advances in Molecular Biology of Human Bocavirus 1 and Its Applications

Human bocavirus 1 (HBoV1) was discovered in human nasopharyngeal specimens in 2005. It is an autonomous human parvovirus and causes acute respiratory tract infections in young children. HBoV1 infects well differentiated or polarized human airway epithelial cells in vitro. Unique among all parvoviruses, HBoV1 expresses 6 non-structural proteins, NS1, NS1-70, NS2, NS3, NS4, and NP1, and a viral non-coding RNA (BocaSR), and three structural proteins VP1, VP2, and VP3. The BocaSR is the first identified RNA polymerase III (Pol III) transcribed viral non-coding RNA in small DNA viruses. It plays an important role in regulation of viral gene expression and a direct role in viral DNA replication in the nucleus. HBoV1 genome replication in the polarized/non-dividing airway epithelial cells depends on the DNA damage and DNA repair pathways and involves error-free Y-family DNA repair DNA polymerase (Pol) η and Pol κ. Importantly, HBoV1 is a helper virus for the replication of dependoparvovirus, adeno-associated virus (AAV), in polarized human airway epithelial cells, and HBoV1 gene products support wild-type AAV replication and recombinant AAV (rAAV) production in human embryonic kidney (HEK) 293 cells. More importantly, the HBoV1 capsid is able to pseudopackage an rAAV2 or rHBoV1 genome, producing the rAAV2/HBoV1 or rHBoV1 vector. The HBoV1 capsid based rAAV vector has a high tropism for human airway epithelia. A deeper understanding in HBoV1 replication and gene expression will help find a better way to produce the rAAV vector and to increase the efficacy of gene delivery using the rAAV2/HBoV1 or rHBoV1 vector, in particular, to human airways. This review summarizes the recent advances in gene expression and replication of HBoV1, as well as the use of HBoV1 as a parvoviral vector for gene delivery.

Utilization of Host Cell Chromosome Conformation by Viral Pathogens: Knowing When to Hold and When to Fold

Though viruses have their own genomes, many depend on the nuclear environment of their hosts for replication and survival. A substantial body of work has therefore been devoted to understanding how viral and eukaryotic genomes interact. Recent advances in chromosome conformation capture technologies have provided unprecedented opportunities to visualize how mammalian genomes are organized and, by extension, how packaging of nuclear DNA impacts cellular processes. Recent studies have indicated that some viruses, upon entry into host cell nuclei, produce factors that alter host chromatin topology, and thus, impact the 3D organization of the host genome. Additionally, a variety of distinct viruses utilize host genome architectural factors to advance various aspects of their life cycles. Indeed, human gammaherpesviruses, known for establishing long-term reservoirs of latent infection in B lymphocytes, utilize 3D principles of genome folding to package their DNA and establish latency in host cells. This manipulation of host epigenetic machinery by latent viral genomes is etiologically linked to the onset of B cell oncogenesis. Small DNA viruses, by contrast, are tethered to distinct cellular sites that support virus expression and replication. Here, we briefly review the recent findings on how viruses and host genomes spatially communicate, and how this impacts virus-induced pathology.

Antiviral immunity and nucleic acid sensing in haematopoietic stem cell gene engineering

The low gene manipulation efficiency of human hematopoietic stem and progenitor cells (HSPC) remains a major hurdle for sustainable and broad clinical application of innovative therapies for a wide range of disorders. Given that all current and emerging gene transfer and editing technologies are bound to expose HSPC to exogenous nucleic acids and most often also to viral vectors, we reason that host antiviral factors and nucleic acid sensors play a pivotal role in the efficacy of HSPC genetic manipulation. Here, we review recent progress in our understanding of vector-host interactions and innate immunity in HSPC upon gene engineering and discuss how dissecting this crosstalk can guide the development of more stealth and efficient gene therapy approaches in the future.

Ursolic Acid Regulates Cell Cycle and Proliferation in Colon Adenocarcinoma by Suppressing Cyclin B1

Aims: The biological functions of cyclin B1 (CCNB1) in colon adenocarcinoma (COAD) will be explored in this study. Furthermore, the therapeutic effects and potential molecular mechanisms of ursolic acid (UA) in COAD cells will also be investigated in vitro.

Methods: COAD data were obtained from Gene Expression Omnibus (GEO) and The Cancer Genome Atlas (TCGA) databases. Differentially expressed genes (DEGs) were determined with differential analysis. The biological functions of CCNB1 were analyzed through the GeneCards, the Search Tool for the Retrieval of Interacting Genes (STRING), and the Database for Annotation, Visualization, and Integrated Discovery (DAVID) databases. Therapeutic effects of UA on COAD cell lines HCT-116 and SW-480 were analyzed by CCK-8 and high-content screening (HCS) imaging assay. Flow cytometry was utilized to detect cell cycle changes of SW-480 and HCT-116 cells. Levels of mRNA and expression proteins of HCT-116, SW-480, and normal colon epithelial cells NCM-460 were determined by qRT-PCR and western blot.

Results: CCNB1 was highly expressed and acted as an oncogene in COAD patients. CCNB1 and its interacting genes were significantly enriched in the cell cycle pathway. UA effectively inhibited the proliferation and injured COAD cells. In addition, UA arrested cell cycle of COAD cells in S phase. With regard to the molecular mechanisms of UA, we demonstrated that UA can significantly downregulate CCNB1 and its interacting genes and proteins, including CDK1, CDC20, CCND1, and CCNA2, which contributed to cell cycle blocking and COAD treatment.

Conclusion: Results from this study revealed that UA possesses therapeutic effects on COAD. The anti-COAD activities of UA are tightly related to suppression of CCNB1 and its interacting targets, which is crucial in abnormal cell cycle process.

Binding of CCCTC-Binding Factor (CTCF) to the Minute Virus of Mice Genome Is Important for Proper Processing of Viral P4-Generated Pre-mRNAs

Specific chromatin immunoprecipitation of salt-fractionated infected cell extracts has demonstrated that the CCCTC-binding factor (CTCF), a highly conserved, 11-zinc-finger DNA-binding protein with known roles in cellular and viral genome organization and gene expression, specifically binds the genome of Minute Virus of Mice (MVM). Mutations that diminish binding of CTCF to MVM affect processing of the P4-generated pre-mRNAs. These RNAs are spliced less efficiently to generate the R1 mRNA, and definition of the NS2-specific exon upstream of the small intron is reduced, leading to relatively less R2 and the generation of a novel exon-skipped product. These results suggest a model in which CTCF is required for proper engagement of the spliceosome at the MVM small intron and for the first steps of processing of the P4-generated pre-mRNA.

The NS1 protein of the parvovirus MVM Aids in the localization of the viral genome to cellular sites of DNA damage

The autonomous parvovirus Minute Virus of Mice (MVM) localizes to cellular DNA damage sites to establish and sustain viral replication centers, which can be visualized by focal deposition of the essential MVM non-structural phosphoprotein NS1. How such foci are established remains unknown. Here, we show that NS1 localized to cellular sites of DNA damage independently of its ability to covalently bind the 5’ end of the viral genome, or its consensus DNA binding sequence. Many of these sites were identical to those occupied by virus during infection. However, localization of the MVM genome to DNA damage sites occurred only when wild-type NS1, but not its DNA-binding mutant was expressed. Additionally, wild-type NS1, but not its DNA binding mutant, could localize a heterologous DNA molecule containing the NS1 binding sequence to DNA damage sites. These findings suggest that NS1 may function as a bridging molecule, helping the MVM genome localize to cellular DNA damage sites to facilitate ongoing virus replication.

Parvoviruses are among the simplest of viruses, depending almost exclusively on host cell factors to successfully replicate. We have previously shown that the parvovirus Minute Virus of Mice (MVM) establishes replication centers at sites that are associated with cellular regions of DNA damage. These sites are primed to contain factors necessary to efficiently initiate vigorous virus lytic infection. The process by which viral proteins and viral DNA specifically localize to these sites has previously remained unknown. In this study we show that the essential viral protein NS1 possesses the intrinsic ability to localize to cellular sites of DNA damage. Additionally, wild-type NS1, but not its DNA binding mutant, could localize to sites of DNA damage both the MVM genome, or a heterologous DNA molecule engineered to contain NS1 binding sites. This work provides the first evidence that NS1 may function as a bridging molecule to localize the MVM genome to cellular sites of DNA damage to facilitate ongoing replication.

The combined usage of Matrine and Osthole inhibited endoplasmic reticulum apoptosis induced by PCV2

Background

Porcine circovirus type 2 (PCV2) is an important and common DNA virus that infect pig and can cause immunosuppression and induce apoptosis in the infected cells. To escape the host immune system, PCV2 constantly builds up complex mechanisms or mutates genes, and that is why it is difficult to eradicate complex PCV2 infection by relying on vaccines and single compound. At present, there is few literature reports on the effective prevention and treatment of PCV2 infection by a combination of two or more compounds. Previously, we have demonstrated the anti-PCV2 effect of Matrine in vitro, but its mechanism has not been further evaluated. Literatures have proven that Osthole has a variety of pharmacological activities, and we tested the ability of Osthole to inhibit PCV2 replication in cell culture. Therefore, this study explored the synergistic antiviral effect of Matrine combined with Osthole and their synergistic anti-apoptotic mechanism.

Results

Osthole alone had an anti-PCV2 effect, and then its synergistic anti-PCV2 effect of Osthole and Matrine was better than that of Matrine or Osthole alone as demonstrated by qRT-PCR, IFA and Western blotting results. The anti-apoptotic mechanism of these two compounds by inducing the PERK pathway by PCV2 was elucidated through Annexin V-FITC/PI, JC-1 and Western blotting. Matrine and Osthole combination could inhibit the expression of Cap in Cap-transfected PK-15 cells, thus inhibiting Cap-induced PERK apoptosis. Ribavirin was used as a positive control.

Conclusions

The combination of Osthole and Matrine had the synergistic effect of anti-PCV2 infection by directly inhibiting the expression of PCV2 Cap protein. The combination of these two compounds also inhibited PERK apoptosis induced by PCV2 Cap protein, possibly by regulating the level of GRP78. The results formed a base for further studies on the mechanism of anti-PCV2 in vivo using Matrine and Osthole combination and developing new anti-PCV2 compounds with Cap and GRP78 as therapeutic targets.

Modification of Nuclear Compartments and the 3D Genome in the Course of a Viral Infection

The review addresses the question of how the structural and functional compartmentalization of the cell nucleus and the 3D organization of the cellular genome are modified during the infection of cells with various viruses. Particular attention is paid to the role of the introduced changes in the implementation of the viral strategy to evade the antiviral defense systems and provide conditions for viral replication. The discussion focuses on viruses replicating in the cell nucleus. Cytoplasmic viruses are mentioned in cases when a significant reorganization of the nuclear compartments or the 3D genome structure occurs during an infection with these viruses.

Cepharanthine and Curcumin inhibited mitochondrial apoptosis induced by PCV2

Background

Porcine circovirus type 2 (PCV2) is an immunosuppressive pathogen with high prevalence rate in pig farms. It has caused serious economic losses to the global pig industry. Due to the rapid mutation of PCV2 strain and co-infection of different genotypes, vaccination could not eradicate the infection of PCV2. It is necessary to screen and develop effective new compounds and explore their anti-apoptotic mechanism. The 13 natural compounds were purchased, with a clear plant origin, chemical structure and content and specific biological activities.

Results

The maximum no-cytotoxic concentration (MNTC) and 50% cytotoxic concentration (CC₅₀) of 13 tested compounds were obtained by the cytopathologic effect (CPE) assay and (3-(4,5-dimethyithiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method in PK-15 cells. The results of qPCR and Western blot showed that, compared with the PCV2 infected group, the expression of Cap in Paeonol (0.4 mg/mL and 0.2 mg/mL), Cepharanthine (0.003 mg/mL, 0.0015 mg/mL and 0.00075 mg/mL) and Curcumin (0.02 mg/mL, 0.001 mg/mL and 0.005 mg/mL) treated groups were significantly lowered in a dose-dependent manner. The results of Annexin V-FITC/PI, JC-1, Western blot and ROS analysis showed that the expression of cleaved caspase-3 and Bax were up-regulated Bcl-2 was down-regulated in Cepharanthine or Curcumin treated groups, while ROS and MMP value were decreased at different degrees and the apoptosis rate was reduced. In this study, Ribavirin was used as a positive control.

Conclusions

Paeonol, Cepharanthine and Curcumin have significant antiviral effect. And the PCV2-induced Mitochondrial apoptosis was mainly remitted by Cepharanthine and Curcumin.

A Roadmap for the Success of Oncolytic Parvovirus-Based Anticancer Therapies

Autonomous rodent protoparvoviruses (PVs) are promising anticancer agents due to their excellent safety profile, natural oncotropism, and oncosuppressive activities. Viral infection can trigger immunogenic cell death, activating the immune system against the tumor. However, the efficacy of this treatment in recent clinical trials is moderate compared with results seen in preclinical work. Various strategies have been employed to improve the anticancer activities of oncolytic PVs, including development of second-generation parvoviruses with enhanced oncolytic and immunostimulatory activities and rational combination of PVs with other therapies. Understanding the cellular factors involved in the PV life cycle is another important area of investigation. Indeed, these studies may lead to the identification of biomarkers that would allow a more personalized use of PV-based therapies. This review focuses on this work and the challenges that still need to be overcome to move PVs forward into clinical practice as an effective therapeutic option for cancer patients.

Rotavirus activates a noncanonical ATM-Chk2 branch of DNA damage response during infection to positively regulate viroplasm dynamics

Surveillance for maintaining genomic pristineness, a protective safeguard of great onco-preventive significance, has been dedicated in eukaryotic cells to a highly conserved and synchronised signalling cascade called DNA damage response (DDR). Not surprisingly, foreign genetic elements like those of viruses are often potential targets of DDR. Viruses have evolved novel ways to subvert this genome vigilance by twisting canonical DDR to a skewed, noncanonical response through selective hijacking of some DDR components while antagonising the others. Though reported for many DNA and a few RNA viruses, potential implications of DDR have not been addressed yet in case of infection with rotavirus (RV), a double-stranded RNA virus. In the present study, we aimed at the modulation of ataxia telangiectasia mutated (ATM)-checkpoint kinase 2 (Chk2) branch of DDR in response to RV infection in vitro. We found activation of the transducer kinase ATM and its downstream effector Chk2 in RV-SA11-infected cells, the activation response being maximal at 6-hr post infection. Moreover, ATM activation was found to be dependent on induction of the upstream sensor Mre11-Rad50-Nbs1 (MRN) complex. Interestingly, RV-SA11-mediated maximal induction of ATM-Chk2 pathway was revealed to be neither preceded by occurrence of nuclear DNA damage nor transduced to formation of damage-induced canonical nuclear foci. Subsequent investigations affirmed sequestration of MRN components as well as ATM-Chk2 proteins away from nucleus into cytosolic RV replication factories (viroplasms). Chemical intervention targeting ATM and Chk2 significantly inhibited fusion and maturation of viroplasms leading to attenuated viral propagation. Cumulatively, the current study describes RV-mediated activation of a noncanonical ATM-Chk2 branch of DDR skewed in favour of facilitated viroplasm fusion and productive viral perpetuation.

African swine fever virus replication events and cell nucleus: New insights and perspectives

African swine fever (ASF) is currently matter for major concerns in global swine industry as it is highly contagious and causes acute fatal haemorrhagic fever in domestic pigs and wild boar. The absence of effective vaccines and treatments pushes ASF control to relay on strict sanitary and stamping out measures with costly socio-economic impacts. The current epidemic scenario of fast spreading throughout Asiatic countries impels further studies on prevention and combat strategies against ASF. Herein we review knowledge on African Swine Fever Virus (ASFV) interactions with the host cell nucleus and on the functional properties of different viral DNA-replication related proteins. This entails, the confirmation of an intranuclear viral DNA replication phase, the characterization of cellular DNA damage responses (DDR), the subnuclear compartments disruption due to viral modulation, and the unravelling of the biological role of several viral proteins (A104R, I215 L, P1192R, QP509 L and Q706 L), so to contribute to underpin rational strategies for vaccine candidates development.

Analysis of Global Transcriptome Change in Mouse Embryonic Fibroblasts After dsDNA and dsRNA Viral Mimic Stimulation

The activation of innate immunity by viral nucleic acids present in the cytoplasm plays an essential role in controlling viral infection in both immune and non-immune cells. The dsDNA and dsRNA viral mimics can stimulate the cytosolic nucleic acids sensors and activate the antiviral innate immunity. In this study, taking advantage of dsDNA and dsRNA viral mimics, we investigated the global transcriptome changes after the antiviral immunity activation in mouse embryonic fibroblasts. Results from our data identified a positive feedback up-regulation of sensors (e.g., Tlr2, Tlr3, Ddx58, cGAS), transducers (e.g., Traf2, Tbk1) and transcription factors (e.g., Irf7, Jun, Stat1, Stat2) in multiple pathways involved in detecting viral or microbial infections upon viral mimic stimulation. A group of genes involved in DNA damage response and DNA repair such as Parp9, Dtx3l, Rad52 were also up-regulated, implying the involvement of these genes in antiviral immunity. Molecular function analysis further showed that groups of helicase genes (e.g., Dhx58, Helz2), nuclease genes (e.g., Dnase1l3, Rsph10b), methyltransferase genes (e.g., histone methyltransferase Prdm9, Setdb2; RNA methyltransferase Mettl3, Mttl14), and protein ubiquitin-ligase genes (e.g., Trim genes and Rnf genes) were up-regulated upon antiviral immunity activation. In contrast, viral mimic stimulation down-regulated genes involved in a broad range of general biological processes (e.g., cell division, metabolism), cellular components (e.g., mitochondria and ribosome), and molecular functions (e.g., cell-cell adhesion, microtubule binding). In summary, our study provides valuable information about the global transcriptome changes upon antiviral immunity activation. The identification of novel groups of genes up-regulated upon antiviral immunity activation serves as useful resource for mining new antiviral sensors and effectors.

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.

Parvovirus minute virus of mice interacts with sites of cellular DNA damage to establish and amplify its lytic infection

We have developed a generally adaptable, novel high-throughput Viral Chromosome Conformation Capture assay (V3C-seq) for use in trans that allows genome-wide identification of the direct interactions of a lytic virus genome with distinct regions of the cellular chromosome. Upon infection, we found that the parvovirus Minute Virus of Mice (MVM) genome initially associated with sites of cellular DNA damage that in mock-infected cells also exhibited DNA damage as cells progressed through S-phase. As infection proceeded, new DNA damage sites were induced, and virus subsequently also associated with these. Sites of association identified biochemically were confirmed microscopically and MVM could be targeted specifically to artificially induced sites of DNA damage. Thus, MVM established replication at cellular DNA damage sites, which provide replication and expression machinery, and as cellular DNA damage accrued, virus spread additionally to newly damaged sites to amplify infection. MVM-associated sites overlap significantly with previously identified topologically-associated domains (TADs).

Viruses are small infectious particles that can only reproduce with the help of a host. Once they are inside their victim, they hijack the cells’ genetic material and reprogram it to become a virus factory that produces more virus particles. Parvoviruses, for example, are among the simplest of viruses and need all resources a cell has to offer to successfully replicate.

This process often takes place at so-called replication centers that contain these necessary factors. It was previously thought that parvoviruses set up such centers randomly, and gather the required molecules such as proteins to these sites. However, it was not well understood how they do this.

Now, Majumder et al. have developed a new method that enabled them to study in detail how parvoviruses gain access to the resources of the cell they need to initiate and amplify replication. The results show that parvoviruses set up their replication centers at sites on the host DNA that are already rich in proteins needed to repair and then replicate damaged DNA. Some of these sites already exist in the cell’s genetic material as a consequence of naturally occurring processes, but others are created during infection by the virus. These findings may have important implications for how other viruses may establish their replication.

Viruses, including parvoviruses, are important pathogens. Like many microbes, viruses can be beneficial for our health and environment. Others, however, can be harmful. A clearer understanding of how viruses establish and amplify an infection may provide new treatment opportunities.

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.

Human Parvovirus B19 Utilizes Cellular DNA Replication Machinery for Viral DNA Replication

Human parvovirus B19 (B19V) infection of human erythroid progenitor cells (EPCs) induces a DNA damage response and cell cycle arrest at late S phase, which facilitates viral DNA replication. However, it is not clear exactly which cellular factors are employed by this single-stranded DNA virus. Here, we used microarrays to systematically analyze the dynamic transcriptome of EPCs infected with B19V. We found that DNA metabolism, DNA replication, DNA repair, DNA damage response, cell cycle, and cell cycle arrest pathways were significantly regulated after B19V infection. Confocal microscopy analyses revealed that most cellular DNA replication proteins were recruited to the centers of viral DNA replication, but not the DNA repair DNA polymerases. Our results suggest that DNA replication polymerase δ and polymerase α are responsible for B19V DNA replication by knocking down its expression in EPCs. We further showed that although RPA32 is essential for B19V DNA replication and the phosphorylated forms of RPA32 colocalized with the replicating viral genomes, RPA32 phosphorylation was not necessary for B19V DNA replication. Thus, this report provides evidence that B19V uses the cellular DNA replication machinery for viral DNA replication.IMPORTANCE Human parvovirus B19 (B19V) infection can cause transient aplastic crisis, persistent viremia, and pure red cell aplasia. In fetuses, B19V infection can result in nonimmune hydrops fetalis and fetal death. These clinical manifestations of B19V infection are a direct outcome of the death of human erythroid progenitors that host B19V replication. B19V infection induces a DNA damage response that is important for cell cycle arrest at late S phase. Here, we analyzed dynamic changes in cellular gene expression and found that DNA metabolic processes are tightly regulated during B19V infection. Although genes involved in cellular DNA replication were downregulated overall, the cellular DNA replication machinery was tightly associated with the replicating single-stranded DNA viral genome and played a critical role in viral DNA replication. In contrast, the DNA damage response-induced phosphorylated forms of RPA32 were dispensable for viral DNA replication.

Protoparvovirus Interactions with the Cellular DNA Damage Response

Protoparvoviruses are simple single-stranded DNA viruses that infect many animal species. The protoparvovirus minute virus of mice (MVM) infects murine and transformed human cells provoking a sustained DNA damage response (DDR). This DDR is dependent on signaling by the ATM kinase and leads to a prolonged pre-mitotic cell cycle block that features the inactivation of ATR-kinase mediated signaling, proteasome-targeted degradation of p21, and inhibition of cyclin B1 expression. This review explores how protoparvoviruses, and specifically MVM, co-opt the common mechanisms regulating the DDR and cell cycle progression in order to prepare the host nuclear environment for productive infection.

Hypothesis: RNA and DNA Viral Sequence Integration into the Mammalian Host Genome Supports Long-Term B Cell and T Cell Adaptive Immunity

Viral sequence integration into the mammalian genome has long been perceived as a health risk. In some cases, integration translates to chronic viral infection, and in other instances, oncogenic gene mutations occur. However, research also shows that animal cells can benefit from integrated viral sequences (e.g., to support host cell development or to silence foreign invaders). Here we propose that, comparable with the clustered regularly interspaced short palindromic repeats that provide bacteria with adaptive immunity against invasive bacteriophages, animal cells may co-opt integrated viral sequences to support immune memory. We hypothesize that host cells express viral peptides from open reading frames in integrated sequences to boost adaptive B cell and T cell responses long after replicating viruses are cleared. In support of this hypothesis, we examine previous literature describing (1) viruses that infect acutely (e.g., vaccinia viruses and orthomyxoviruses) followed by unexplained, long-term persistence of viral nucleotide sequences, viral peptides, and virus-specific adaptive immunity, (2) the high frequency of endogenous viral genetic elements found in animal genomes, and (3) mechanisms with which animal host machinery supports foreign sequence integration.

Minute Virus of Mice Inhibits Transcription of the Cyclin B1 Gene during Infection

Replication of minute virus of mice (MVM) induces a sustained cellular DNA damage response (DDR) which the virus then exploits to prepare the nuclear environment for effective parvovirus takeover. An essential aspect of the MVM-induced DDR is the establishment of a potent premitotic block, which we previously found to be independent of activated p21 and ATR/Chk1 signaling. This arrest, unlike others reported previously, depends upon a significant, specific depletion of cyclin B1 and its encoding RNA, which precludes cyclin B1/CDK1 complex function, thus preventing mitotic entry. We show here that while the stability of cyclin B1 RNA was not affected by MVM infection, the production of nascent cyclin B1 RNA was substantially diminished at late times postinfection. Ectopic expression of NS1 alone did not reduce cyclin B1 expression. MVM infection also reduced the levels of cyclin B1 protein, and RNA levels normally increased in response to DNA-damaging reagents. We demonstrated that at times of reduced cyclin B1 expression during infection, there was a significantly reduced occupancy of RNA polymerase II and the essential mitotic transcription factor FoxM1 on the cyclin B1 gene promoter. Additionally, while total FoxM1 levels remained constant, there was a significant decrease of the phosphorylated, likely active, forms of FoxM1. Targeting of a constitutively active FoxM1 construct or the activation domain of FoxM1 to the cyclin B1 gene promoter via clustered regularly interspaced short palindromic repeats (CRISPR)-enzymatically inactive Cas9 in MVM-infected cells increased both cyclin B1 protein and RNA levels, implicating FoxM1 as a critical target for cyclin B1 inhibition during MVM infection.IMPORTANCE Replication of the parvovirus minute virus of mice (MVM) induces a sustained cellular DNA damage response (DDR) which the virus exploits to prepare the nuclear environment for effective takeover. An essential aspect of the MVM-induced DDR is establishment of a potent premitotic block. This block depends upon a significant, specific depletion of cyclin B1 and its encoding RNA that precludes cyclin B1/CDK1 complex functions necessary for mitotic entry. We show that reduced cyclin B1 expression is controlled primarily at the level of transcription initiation. Additionally, the essential mitotic transcription factor FoxM1 and RNA polymerase II were found to occupy the cyclin B1 gene promoter at reduced levels during infection. Recruiting a constitutively active FoxM1 construct or the activation domain of FoxM1 to the cyclin B1 gene promoter via CRISPR-catalytically inactive Cas9 (dCas9) in MVM-infected cells increased expression of both cyclin B1 protein and RNA, implicating FoxM1 as a critical target mediating MVM-induced cyclin B1 inhibition.

Cytoplasmic ATR Activation Promotes Vaccinia Virus Genome Replication

In contrast to most DNA viruses, poxviruses replicate their genomes in the cytoplasm without host involvement. We find that vaccinia virus induces cytoplasmic activation of ATR early during infection, before genome uncoating, which is unexpected because ATR plays a fundamental nuclear role in maintaining host genome integrity. ATR, RPA, INTS7, and Chk1 are recruited to cytoplasmic DNA viral factories, suggesting canonical ATR pathway activation. Consistent with this, pharmacological and RNAi-mediated inhibition of canonical ATR signaling suppresses genome replication. RPA and the sliding clamp PCNA interact with the viral polymerase E9 and are required for DNA replication. Moreover, the ATR activator TOPBP1 promotes genome replication and associates with the viral replisome component H5. Our study suggests that, in contrast to long-held beliefs, vaccinia recruits conserved components of the eukaryote DNA replication and repair machinery to amplify its genome in the host cytoplasm.

Induction of a Cellular DNA Damage Response by Porcine Circovirus Type 2 Facilitates Viral Replication and Mediates Apoptotic Responses

Cellular DNA damage response (DDR) triggered by infection of DNA viruses mediate cell cycle checkpoint activation, DNA repair, or apoptosis induction. In the present study, infection of porcine circovirus type 2 (PCV2), which serves as a major etiological agent of PCV2-associated diseases (PCVAD), was found to elicit a DNA damage response (DDR) as observed by the phosphorylation of H2AX and RPA32 following infection. The response requires active viral replication, and all the ATM (ataxia telangiectasia-mutated kinase), ATR (ATM- and Rad3-related kinase), and DNA-PK (DNA-dependent protein kinase) are the transducers of the DDR signaling events in the PCV2-infected cells as demonstrated by the phosphorylation of ATM, ATR, and DNA-PK signalings as well as reductions in their activations after treatment with specific kinase inhibitors. Inhibitions of ATM, ATR, and DNA-PK activations block viral replication and prevent apoptotic responses as observed by decreases in cleaved poly-ADP ribose polymerase (PARP) and caspase-3 as well as fragmented DNA following PCV2 infection. These results reveal that PCV2 is able to exploit the cellular DNA damage response machinery for its own efficient replication and for apoptosis induction, further extending our understanding for the molecular mechanism of PCV2 infection.