Parvoviruses cause nuclear envelope breakdown by activating key enzymes of mitosis

Genome-wide CRISPR screenings identified SMCHD1 as a host-restricting factor for AAV transduction

AAV-mediated gene therapy typically requires a high dose of viral transduction, risking acute immune responses and patient safety, part of which is due to limited understanding of the host-viral interactions, especially post-transduction viral genome processing. Here, through a genome-wide CRISPR screen, we identified SMCHD1 (Structural Maintenance of Chromosomes Hinge Domain 1), an epigenetic modifier, as a critical broad-spectrum restricting host factor for post-entry AAV transgene expression. SMCHD1 knock-down by RNAi and CRISPRi or knock-out by CRISPR all resulted in significantly enhanced transgene expression across multiple viral serotypes, as well as for both single-strand and self-complementary AAV genome types. Mechanistically, upon viral transduction, SMCHD1 effectively repressed AAV transcription by the formation of an LRIF1-HP1-containing protein complex and directly binding with the AAV genome to maintain a heterochromatin-like state. SMCHD1-KO or LRIF1-KD could disrupt such a complex and thus result in AAV transcriptional activation. Together, our results highlight the host factor-induced chromatin remodeling as a critical inhibitory mechanism for AAV transduction and may shed light on further improvement in AAV-based gene therapy.

Lamin B1 curtails early human papillomavirus infection by safeguarding nuclear compartmentalization and autophagic capacity

Human papillomavirus (HPV) infection is a primary cause of cervical and head-and-neck cancers. The HPV genome enters the nucleus during mitosis when the nuclear envelope disassembles. Given that lamins maintain nuclear integrity during interphase, we asked to what extent their loss would affect early HPV infection. To address this question, we infected human cervical cancer cells and keratinocytes lacking the major lamins with a HPV16 pseudovirus (HP-PsV) encoding an EGFP reporter. We found that a sustained reduction or complete loss of lamin B1 significantly increased HP-PsV infection rate. A corresponding greater nuclear HP-PsV load in LMNB1 knockout cells was directly related to their prolonged mitotic window and extensive nuclear rupture propensity. Despite the increased HP-PsV presence, EGFP transcript levels remained virtually unchanged, indicating an additional defect in protein turnover. Further investigation revealed that LMNB1 knockout led to a substantial decrease in autophagic capacity, possibly linked to the persistent activation of cGAS by cytoplasmic chromatin exposure. Thus, the attrition of lamin B1 increases nuclear perviousness and attenuates autophagic capacity, creating an environment conducive to unrestrained accumulation of HPV capsids. Our identification of lower lamin B1 levels and nuclear BAF foci in the basal epithelial layer of several human cervix samples suggests that this pathway may contribute to an increased individual susceptibility to HPV infection.

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.

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.

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.

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 LEM-ESCRT toolkit: Repair and maintenance of the nucleus

The eukaryotic genome is enclosed in a nuclear envelope that protects it from potentially damaging cellular activities and physically segregates transcription and translation.Transport across the NE is highly regulated and occurs primarily via the macromolecular nuclear pore complexes.Loss of nuclear compartmentalization due to defects in NPC function and NE integrity are tied to neurological and ageing disorders like Alzheimer’s, viral pathogenesis, immune disorders, and cancer progression.Recent work implicates inner-nuclear membrane proteins of the conserved LEM domain family and the ESCRT machinery in NE reformation during cell division and NE repair upon rupture in migrating cancer cells, and generating seals over defective NPCs. In this review, we discuss the recent in-roads made into defining the molecular mechanisms and biochemical networks engaged by LEM and many other integral inner nuclear membrane proteins to preserve the nuclear barrier.

Workshop on RanBP2/Nup358 and acute necrotizing encephalopathy

Dominant missense mutations in RanBP2/Nup358 cause Acute Necrotizing Encephalopathy (ANE), a pediatric disease where seemingly healthy individuals develop a cytokine storm that is restricted to the central nervous system in response to viral infection. Untreated, this condition leads to seizures, coma, long-term neurological damage and a high rate of mortality. The exact mechanism by which RanBP2 mutations contribute to the development of ANE remains elusive. In November 2021, a number of clinicians and basic scientists presented their work on this disease and on the interactions between RanBP2/Nup358, viral infections, the innate immune response and other cellular processes.

Nuclear Cytoskeleton in Virus Infection

The nuclear lamina is the main component of the nuclear cytoskeleton that maintains the integrity of the nucleus. However, it represents a natural barrier for viruses replicating in the cell nucleus. The lamina blocks viruses from being trafficked to the nucleus for replication, but it also impedes the nuclear egress of the progeny of viral particles. Thus, viruses have evolved mechanisms to overcome this obstacle. Large viruses induce the assembly of multiprotein complexes that are anchored to the inner nuclear membrane. Important components of these complexes are the viral and cellular kinases phosphorylating the lamina and promoting its disaggregation, therefore allowing virus egress. Small viruses also use cellular kinases to induce lamina phosphorylation and the subsequent disruption in order to facilitate the import of viral particles during the early stages of infection or during their nuclear egress. Another component of the nuclear cytoskeleton, nuclear actin, is exploited by viruses for the intranuclear movement of their particles from the replication sites to the nuclear periphery. This study focuses on exploitation of the nuclear cytoskeleton by viruses, although this is just the beginning for many viruses, and promises to reveal the mechanisms and dynamic of physiological and pathological processes in the nucleus.

AAV9 Structural Rearrangements Induced by Endosomal Trafficking pH and Glycan Attachment

Adeno-associated viruses (AAVs) are small non-enveloped ssDNA viruses, that are currently being developed as gene therapy biologics. After cell entry, AAVs traffic to the nucleus using the endo-lysosomal pathway. The subsequent decrease in pH triggers conformational changes to the capsid that enables the externalization of the capsid protein (VP) N-termini, including the unique domain of the minor capsid protein VP1 (VP1u), which permits phospholipase activity required for the capsid lysosomal egress. Here, we report the AAV9 capsid structure, determined at the endosomal pHs (7.4, 6.0, 5.5, and 4.0) and terminal galactose-bound AAV9 capsids at pHs 7.4 and 5.5 using cryo-electron microscopy and three-dimensional image reconstruction. Taken together these studies provide insight into AAV9 capsid conformational changes at the 5-fold pore during endosomal trafficking, both in the presence and absence of its cellular glycan receptor. We visualized, for the first time, that acidification induces the externalization of the VP3 and possibly VP2 N-termini, presumably in prelude to the externalization of VP1u at pH 4.0, that is essential for lysosomal membrane disruption. In addition, the structural study of AAV9-galactose interactions demonstrates AAV9 remains attached to its glycan receptor at the late endosome pH 5.5. This interaction significantly alters the conformational stability of the variable region I of the VPs, as well as the dynamics associated with VP N-terminus externalization. Importance There are 13 distinct Adeno-associated virus (AAV) serotypes that are structurally homologous and whose capsid proteins (VP1-3) are similar in amino acid sequence. However, AAV9 is one of the most commonly studied and used as gene therapy vector. This is part because, AAV9 is capable of crossing the blood brain barrier as well as readily transduces a wide array of tissues, including the central nervous system. In this study we provide AAV9 capsid structural insight during intracellular trafficking. Although the AAV capsid has been shown to externalize the N-termini of its VPs, to enzymatically disrupt the lysosome membrane at low pH, there was no structural evidence to confirm this. By utilizing AAV9 as our model, we provide the first structural evidence that the externalization process occurs at the protein interface at the icosahedral 5-fold symmetry axis and can be triggered by lowering pH.

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.

Confined no more: Viral mechanisms of nuclear entry and egress

Viruses are obligatory intracellular parasites. For their efficient replication, many require access to the nuclear interior. Yet, only few viral particles are small enough to passively diffuse through the nuclear pore complexes, calling for alternative strategies to bypass the nuclear envelope barrier. Some viruses will await mitotic nuclear envelope breakdown to gain access, whereas others will exploit more active means, for instance by hijacking nuclear pore transport or by directly targeting constituents of the nuclear envelope so as to remodel and temporarily perturb its integrity. After replication, newly produced viral DNA complexes need to cross the same barrier to exit the nucleus and enter the cytoplasm, where the final stages of virion maturation take place. There are also different flavours to the feat of nuclear egress that vary in delicacy and intensity. In this review, we define the major entry and egress strategies that are exploited by different viruses and describe the molecular details thereof. Ultimately, a deeper understanding of these pathways may help identifying molecular targets for blocking viral reproduction or spreading.

Journey to the Center of the Cell: Tracing the Path of AAV Transduction

As adeno-associated virus (AAV)-based gene therapies are being increasingly approved for use in humans, it is important that we understand vector-host interactions in detail. With the advances in genome-wide genetic screening tools, a clear picture of AAV-host interactions is beginning to emerge. Understanding these interactions can provide insights into the viral life cycle. Accordingly, novel strategies to circumvent the current limitations of AAV-based vectors may be explored. Here, we summarize our current understanding of the various stages in the journey of the vector from the cell surface to the nucleus and contextualize the roles of recently identified host factors.

Nuclear Membrane Rupture and Its Consequences

The nuclear envelope is often depicted as a static barrier that regulates access between the nucleus and the cytosol. However, recent research has identified many conditions in cultured cells and in vivo in which nuclear membrane ruptures cause the loss of nuclear compartmentalization. These conditions include some that are commonly associated with human disease, such as migration of cancer cells through small spaces and expression of nuclear lamin disease mutations in both cultured cells and tissues undergoing nuclear migration. Nuclear membrane ruptures are rapidly repaired in the nucleus but persist in nuclear compartments that form around missegregated chromosomes called micronuclei. This review summarizes what is known about the mechanisms of nuclear membrane rupture and repair in both the main nucleus and micronuclei, and highlights recent work connecting the loss of nuclear integrity to genome instability and innate immune signaling. These connections link nuclear membrane rupture to complex chromosome alterations, tumorigenesis, and laminopathy etiologies.

Structural and Mechanical Aberrations of the Nuclear Lamina in Disease

The nuclear lamins are the major components of the nuclear lamina in the nuclear envelope. Lamins are involved in numerous functions, including a role in providing structural support to the cell and the mechanosensing of the cell. Mutations in the genes encoding for lamins lead to the rare diseases termed laminopathies. However, not only laminopathies show alterations in the nuclear lamina. Deregulation of lamin expression is reported in multiple cancers and several viral infections lead to a disrupted nuclear lamina. The structural and mechanical effects of alterations in the nuclear lamina can partly explain the phenotypes seen in disease, such as muscular weakness in certain laminopathies and transmigration of cancer cells. However, a lot of answers to questions about the relation between changes in the nuclear lamina and disease development remain elusive. Here, we review the current understandings of the contribution of the nuclear lamina in the structural support and mechanosensing of healthy and diseased cells.

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.

Cytoplasmic Parvovirus Capsids Recruit Importin Beta for Nuclear Delivery

Parvoviruses are an important platform for gene and cancer therapy. Their cell entry and the following steps, including nuclear import, are inefficient, limiting their use in therapeutic applications. Two models exist on parvoviral nuclear entry: the classical import of the viral capsid using nuclear transport receptors of the importin (karyopherin) family or the direct attachment of the capsid to the nuclear pore complex leading to the local disintegration of the nuclear envelope. Here, by laser scanning confocal microscopy and in situ proximity ligation analyses combined with coimmunoprecipitation, we show that infection requires importin β-mediated access to the nuclear pore complex and nucleoporin 153-mediated interactions on the nuclear side. The importin β-capsid interaction continued within the nucleoplasm, which suggests a mixed model of nuclear entry in which the classical nuclear import across the nuclear pore complex is accompanied by transient ruptures of the nuclear envelope, also allowing the passive entry of importin β-capsid complexes into the nucleus.IMPORTANCE Parvoviruses are small DNA viruses that deliver their DNA into the postmitotic nuclei, which is an important step for parvoviral gene and cancer therapies. Limitations in virus-receptor interactions or endocytic entry do not fully explain the low transduction/infection efficiency, indicating a bottleneck after virus entry into the cytoplasm. We thus investigated the transfer of parvovirus capsids from the cytoplasm to the nucleus, showing that the nuclear import of the parvovirus capsid follows a unique strategy, which differs from classical nuclear import and those of other viruses.

H-1 Parvovirus as a Cancer-Killing Agent: Past, Present, and Future

The rat protoparvovirus H-1PV is nonpathogenic in humans, replicates preferentially in cancer cells, and has natural oncolytic and oncosuppressive activities. The virus is able to kill cancer cells by activating several cell death pathways. H-1PV-mediated cancer cell death is often immunogenic and triggers anticancer immune responses. The safety and tolerability of H-1PV treatment has been demonstrated in early clinical studies in glioma and pancreatic carcinoma patients. Virus treatment was associated with surrogate signs of efficacy including immune conversion of tumor microenvironment, effective virus distribution into the tumor bed even after systemic administration, and improved patient overall survival compared with historical control. However, monotherapeutic use of the virus was unable to eradicate tumors. Thus, further studies are needed to improve H-1PV's anticancer profile. In this review, we describe H-1PV's anticancer properties and discuss recent efforts to improve the efficacy of H-1PV and, thereby, the clinical outcome of H-1PV-based therapies.

Infection and nuclear interaction in mammalian cells by 'Candidatus Berkiella cookevillensis', a novel bacterium isolated from amoebae

BACKGROUND

'Candidatus Berkiella cookevillensis' and 'Ca. Berkiella aquae' have previously been described as intranuclear bacteria of amoebae. Both bacteria were isolated from amoebae and were described as appearing within the nuclei of Acanthamoeba polyphaga and ultimately lysing their host cells within 4 days. Both bacteria are Gammaproteobacteria in the order Legionellales with the greatest similarity to Coxiella burnetii. Neither bacterium grows axenically in artificial culture media. In this study, we further characterized 'Ca. B. cookevillensis' by demonstrating association with nuclei of human phagocytic and nonphagocytic cell lines.

RESULTS

Transmission electron microscopy (TEM) and confocal microscopy were used to confirm nuclear co-localization of 'Ca. B. cookevillensis' in the amoeba host A. polyphaga with 100% of cells having bacteria co-localized with host nuclei by 48 h. TEM and confocal microscopy demonstrated that the bacterium was also observed to be closely associated with nuclei of human U937 and THP-1 differentiated macrophage cell lines and nonphagocytic HeLa human epithelial-like cells. Immunofluorescent staining revealed that the bacteria-containing vacuole invaginates the nuclear membranes and appears to cross from the cytoplasm into the nucleus as an intact vacuole.

CONCLUSION

Results of this study indicate that a novel coccoid bacterium isolated from amoebae can infect human cell lines by associating with the host cell nuclei, either by crossing the nuclear membranes or by deeply invaginating the nuclear membranes. When associated with the nuclei, the bacteria appear to be bound within a vacuole and replicate to high numbers by 48 h. We believe this is the first report of such a process involving bacteria and human cell lines.

Adeno-Associated Virus VP1u Exhibits Protease Activity

Adeno-associated viruses (AAVs) are being developed for gene delivery applications, with more than 100 ongoing clinical trials aimed at the treatment of monogenic diseases. In this study, the unique N-terminus of AAV capsid viral protein 1 (VP1u), containing a canonical group XIII PLA2 enzyme domain, was observed to also exhibit proteolytic activity. This protease activity can target casein and gelatin, two standard substrates used for testing protease function but does not self-cleave in the context of the capsid or target globular proteins, for example, bovine serum albumin (BSA). However, heated BSA is susceptible to VP1u-mediated cleavage, suggesting that disordered proteins are substrates for this protease function. The protease activity is partially inhibited by divalent cation chelators ethylenediaminetetraacetic acid (EDTA) and ethylene-bis(oxyethylenenitrilo)tetraacetic acid (EGTA), and human alpha-2-macroglobulin (A2M), a non-specific protease inhibitor. Interestingly, both the bovine pancreatic (group VIIA) and bee venom (group III) PLA2 enzymes also exhibit protease function against casein. This indicates that PLA2 groups, including VP1u, have a protease function. Amino acid substitution of the PLA2 catalytic motif (76HD/AN) in the AAV2 VP1u resulted in attenuation of protease activity, suggesting that the protease and PLA2 active sites are related. However, the amino acid substitution of histidine H38, which is not involved in PLA2 function, to alanine, also affects protease activity, suggesting that the active site/mechanism of the PLA2 and protease function are not identical.

In Vivo Labelling of Adenovirus DNA Identifies Chromatin Anchoring and Biphasic Genome Replication

Viruses must deliver their genomes to host cells to ensure replication and propagation. Characterizing the fate of viral genomes is crucial to understand the viral life cycle and the fate of virus-derived vector tools. Here, we integrated the ANCHOR3 system, an in vivo DNA-tagging technology, into the adenoviral genome for real-time genome detection. ANCHOR3 tagging permitted the in vivo visualization of incoming genomes at the onset of infection and of replicated genomes at late phases of infection. Using this system, we show viral genome attachment to condensed host chromosomes during mitosis, identifying this mechanism as a mode of cell-to-cell transfer. We characterize the spatiotemporal organization of adenovirus replication and identify two kinetically distinct phases of viral genome replication. The ANCHOR3 system is the first technique that allows the continuous visualization of adenoviral genomes during the entire virus life cycle, opening the way for further in-depth study.

Adenoviruses are DNA viruses with a lytic infection cycle. Following the fate of incoming as well as recently replicated genomes during infections is a challenge. In this study, we used the ANCHOR3 technology based on a bacterial partitioning system to establish a versatile in vivo imaging system for adenoviral genomes. The system allows the visualization of both individual incoming and newly replicated genomes in real time in living cells. We demonstrate that incoming adenoviral genomes are attached to condensed cellular chromatin during mitosis, facilitating the equal distribution of viral genomes in daughter cells after cell division. We show that the formation of replication centers occurs in conjunction with in vivo genome replication and determine replication rates. Visualization of adenoviral DNA revealed that adenoviruses exhibit two kinetically distinct phases of genome replication. Low-level replication occurred during early replication, while high-level replication was associated with late replication phases. The transition between these phases occurred concomitantly with morphological changes of viral replication compartments and with the appearance of virus-induced postreplication (ViPR) bodies, identified by the nucleolar protein Mybbp1A. Taken together, our real-time genome imaging system revealed hitherto uncharacterized features of adenoviral genomes in vivo. The system is able to identify novel spatiotemporal aspects of the adenovirus life cycle and is potentially transferable to other viral systems with a double-stranded DNA phase.

IMPORTANCE Viruses must deliver their genomes to host cells to ensure replication and propagation. Characterizing the fate of viral genomes is crucial to understand the viral life cycle and the fate of virus-derived vector tools. Here, we integrated the ANCHOR3 system, an in vivo DNA-tagging technology, into the adenoviral genome for real-time genome detection. ANCHOR3 tagging permitted the in vivo visualization of incoming genomes at the onset of infection and of replicated genomes at late phases of infection. Using this system, we show viral genome attachment to condensed host chromosomes during mitosis, identifying this mechanism as a mode of cell-to-cell transfer. We characterize the spatiotemporal organization of adenovirus replication and identify two kinetically distinct phases of viral genome replication. The ANCHOR3 system is the first technique that allows the continuous visualization of adenoviral genomes during the entire virus life cycle, opening the way for further in-depth study.

Differential phosphorylation and n-terminal configuration of capsid subunits in parvovirus assembly and viral trafficking

The T1 parvovirus Minute Virus of Mice (MVM) was used to study the roles that phosphorylation and N-terminal domains (Nt) configuration of capsid subunits may play in icosahedral nuclear viruses assembly. In synchronous MVM infection, capsid subunits newly assembled as two types of cytoplasmic trimeric intermediates (3VP2, and 1VP1:2VP2) harbored a VP1 phosphorylation level fivefold higher than that of VP2, and hidden Nt. Upon nuclear translocation at S phase, VP1-Nt became exposed in the heterotrimer and subsequent subviral assembly intermediates. Empty capsid subunits showed a phosphorylation level restored to VP1:VP2 stoichiometry, and the Nt concealed in their interior. However ssDNA-filled virus maturing at S/G2 lacked VP1 phosphorylation and one major VP2 phosphopeptide, and exposed VP2-Nt. Endosomal VP2-Nt cleavage resulted in VP3 subunits devoid of any phospholabel, implying that incoming viral particles specifically harbor a low phosphorylation status. Phosphorylation provides a mechanistic coupling of parvovirus nuclear assembly to the cell cycle.

Viral highway to nucleus exposed by image correlation analyses

Parvoviral genome translocation from the plasma membrane into the nucleus is a coordinated multistep process mediated by capsid proteins. We used fast confocal microscopy line scan imaging combined with image correlation methods including auto-, pair- and cross-correlation, and number and brightness analysis, to study the parvovirus entry pathway at the single-particle level in living cells. Our results show that the endosome-associated movement of virus particles fluctuates from fast to slow. Fast transit of single cytoplasmic capsids to the nuclear envelope is followed by slow movement of capsids and fast diffusion of capsid fragments in the nucleoplasm. The unique combination of image analyses allowed us to follow the fate of intracellular single virus particles and their interactions with importin β revealing previously unknown dynamics of the entry pathway.

Bypassing Border Control: Nuclear Envelope Rupture in Disease

Recent observations in laminopathy patient cells and cancer cells have revealed that the nuclear envelope (NE) can transiently rupture during interphase. NE rupture leads to an uncoordinated exchange of nuclear and cytoplasmic material, thereby deregulating cellular homeostasis. Moreover, concurrently inflicted DNA damage could prime rupture-prone cells for genome instability. Thus, NE rupture may represent a novel pathogenic mechanism that has far-reaching consequences for cell and organism physiology.

Protoparvovirus Cell Entry

The Protoparvovirus (PtPV) genus of the Parvoviridae family of viruses includes important animal pathogens and reference molecular models for the entire family. Some virus members of the PtPV genus have arisen as promising tools to treat tumoral processes, as they exhibit marked oncotropism and oncolytic activities while being nonpathogenic for humans. The PtPVs invade and replicate within the nucleus making extensive use of the transport, transcription and replication machineries of the host cells. In order to reach the nucleus, PtPVs need to cross over several intracellular barriers and traffic through different cell compartments, which limit their infection efficiency. In this review we summarize molecular interactions, capsid structural transitions and hijacking of cellular processes, by which the PtPVs enter and deliver their single-stranded DNA genome into the host cell nucleus. Understanding mechanisms that govern the complex PtPV entry will be instrumental in developing approaches to boost their anticancer therapeutic potential and improving their safety profile.

Protoparvovirus Knocking at the Nuclear Door

Protoparvoviruses target the nucleus due to their dependence on the cellular reproduction machinery during the replication and expression of their single-stranded DNA genome. In recent years, our understanding of the multistep process of the capsid nuclear import has improved, and led to the discovery of unique viral nuclear entry strategies. Preceded by endosomal transport, endosomal escape and microtubule-mediated movement to the vicinity of the nuclear envelope, the protoparvoviruses interact with the nuclear pore complexes. The capsids are transported actively across the nuclear pore complexes using nuclear import receptors. The nuclear import is sometimes accompanied by structural changes in the nuclear envelope, and is completed by intranuclear disassembly of capsids and chromatinization of the viral genome. This review discusses the nuclear import strategies of protoparvoviruses and describes its dynamics comprising active and passive movement, and directed and diffusive motion of capsids in the molecularly crowded environment of the cell.

Nuclear Envelope Rupture Is Enhanced by Loss of p53 or Rb

The mammalian nuclear envelope (NE) forms a stable physical barrier between the nucleus and the cytoplasm, normally breaking down only during mitosis. However, spontaneous transient NE rupture in interphase can occur when NE integrity is compromised, such as when the nucleus experiences mechanical stress. For instance, deficiencies in the nuclear lamins and their associated proteins can cause NE rupture that is promoted by forces exerted by actin filaments. NE rupture can allow cytoplasmic nucleases to access chromatin, potentially compromising genome integrity. Importantly, spontaneous NE rupture was noted in several human cancer cell lines, but the cause of this defect is not known. Here, we investigated the mechanistic contributions of two major tumor suppressors, p53 (TP53) and Rb (RB1), to the repression of NE rupture. NE rupture was induced in normal human epithelial RPE-1 cells upon impairment of either Rb or p53 achieved by shRNA knockdown and CRISPR/Cas9 gene editing. NE rupture did not involve diminished expression of NE components or greater cell motility. However, cells that underwent NE rupture displayed a larger nuclear projection area. In conclusion, the data indicate that NE rupture in cancer cells is likely due to loss of either the Rb or the p53 pathway.Implications: These findings imply that tumor suppression by Rb and p53 includes the ability to prevent NE rupture, thereby protecting against genome alterations. Mol Cancer Res; 15(11); 1579-86. ©2017 AACR.

Bursting the Bubble - Nuclear Envelope Rupture as a Path to Genomic Instability?

The nuclear envelope safeguards the genetic material inside the nucleus by separating it from the cytoplasm. Until recently, it was assumed that nuclear envelope (NE) breakdown occurs only in a highly controlled fashion during mitosis when the chromatin is condensed and divided between the daughter cells. However, recent studies have demonstrated that adherent and migrating cells exhibit transient NE rupture during interphase caused by compression from cytoskeletal or external forces. NE rupture results in uncontrolled exchange between the nuclear interior and cytoplasm and leads to DNA damage. In this review, we discuss the causes and consequences of NE rupture, and how NE rupture could contribute to genomic instability.

Nuclear Import of Hepatitis B Virus Capsids and Genome

Hepatitis B virus (HBV) is an enveloped pararetrovirus with a DNA genome, which is found in an up to 36 nm-measuring capsid. Replication of the genome occurs via an RNA intermediate, which is synthesized in the nucleus. The virus must have thus ways of transporting its DNA genome into this compartment. This review summarizes the data on hepatitis B virus genome transport and correlates the finding to those from other viruses.

Alterations in nuclear structure promote lupus autoimmunity in a mouse model

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by the development of autoantibodies that recognize components of the cell nucleus. The vast majority of lupus research has focused on either the contributions of immune cell dysfunction or the genetics of the disease. Because granulocytes isolated from human SLE patients had alterations in neutrophil nuclear morphology that resembled the Pelger-Huet anomaly, and had prominent mis-splicing of mRNA encoding the nuclear membrane protein lamin B receptor (LBR), consistent with their Pelger-Huet-like nuclear morphology, we used a novel mouse model system to test the hypothesis that a disruption in the structure of the nucleus itself also contributes to the development of lupus autoimmunity. The lupus-prone mouse strain New Zealand White (NZW) was crossed with c57Bl/6 mice harboring a heterozygous autosomal dominant mutation in Lbr (B6.Lbr(ic/+)), and the (NZW×B6.Lbr(ic))F1 offspring were evaluated for induction of lupus autoimmunity. Only female (NZW×B6.Lbr(ic))F1 mice developed lupus autoimmunity, which included splenomegaly, kidney damage and autoantibodies. Kidney damage was accompanied by immune complex deposition, and perivascular and tubule infiltration of mononuclear cells. The titers of anti-chromatin antibodies exceeded those of aged female MRL-Fas(lpr) mice, and were predominantly of the IgG2 subclasses. The anti-nuclear antibody staining profile of female (NZW×B6.Lbr(ic))F1 sera was complex, and consisted of an anti-nuclear membrane reactivity that colocalized with the A-type lamina, in combination with a homogeneous pattern that was related to the recognition of histones with covalent modifications that are associated with gene activation. An anti-neutrophil IgM recognizing calreticulin, but not myeloperoxidase (MPO) or proteinase 3 (PR3), was also identified. Thus, alterations in nuclear structure contribute to lupus autoimmunity when expressed in the context of a lupus-prone genetic background, suggesting a mechanism for the development of lupus autoimmunity in genetically predisposed individuals that is induced by the disruption of nuclear architecture.

Nuclear Egress of Herpesviruses: The Prototypic Vesicular Nucleocytoplasmic Transport

Herpesvirus particles mature in two different cellular compartments. While capsid assembly and packaging of the genomic linear double-stranded DNA occur in the nucleus, virion formation takes place in the cytoplasm by the addition of numerous tegument proteins as well as acquisition of the viral envelope by budding into cellular vesicles derived from the trans-Golgi network containing virally encoded glycoproteins. To gain access to the final maturation compartment, herpesvirus nucleocapsids have to cross a formidable barrier, the nuclear envelope (NE). Since the ca. 120 nm diameter capsids are unable to traverse via nuclear pores, herpesviruses employ a vesicular transport through both leaflets of the NE. This process involves proteins which support local dissolution of the nuclear lamina to allow access of capsids to the inner nuclear membrane (INM), drive vesicle formation from the INM and mediate inclusion of the capsid as well as scission of the capsid-containing vesicle (also designated as "primary virion"). Fusion of the vesicle membrane (i.e., the "primary envelope") with the outer nuclear membrane subsequently results in release of the nucleocapsid into the cytoplasm for continuing virion morphogenesis. While this process has long been thought to be unique for herpesviruses, a similar pathway for nuclear egress of macromolecular complexes has recently been observed in Drosophila. Thus, herpesviruses may have coopted a hitherto unrecognized cellular mechanism of vesicle-mediated nucleocytoplasmic transport. This could have far reaching consequences for our understanding of cellular functions as again unraveled by the study of viruses.

Breaching the Barrier-The Nuclear Envelope in Virus Infection

Many DNA and a few RNA viruses use the host cell nucleus for virion formation and/or genome replication. To this end, the nuclear envelope (NE) barrier has to be overcome for entry into and egress from the intranuclear replication compartment. Different virus families have devised ingenious ways of entering and leaving the nucleus usurping cellular transport pathways through the nuclear pore complex but also translocating directly through both membranes of the NE. This intriguing diversity in nuclear entry and egress of viruses also highlights different ways nucleocytoplasmic transport can occur. Thus, the study of interactions between viruses and the NE also helps to unravel hitherto unknown cellular pathways such as vesicular nucleocytoplasmic transfer.

Alterations of Nuclear Architecture and Epigenetic Signatures during African Swine Fever Virus Infection

Viral interactions with host nucleus have been thoroughly studied, clarifying molecular mechanisms and providing new antiviral targets. Considering that African swine fever virus (ASFV) intranuclear phase of infection is poorly understood, viral interplay with subnuclear domains and chromatin architecture were addressed. Nuclear speckles, Cajal bodies, and promyelocytic leukaemia nuclear bodies (PML-NBs) were evaluated by immunofluorescence microscopy and Western blot. Further, efficient PML protein knockdown by shRNA lentiviral transduction was used to determine PML-NBs relevance during infection. Nuclear distribution of different histone H3 methylation marks at lysine’s 9, 27 and 36, heterochromatin protein 1 isoforms (HP1α, HPβ and HPγ) and several histone deacetylases (HDACs) were also evaluated to assess chromatin status of the host. Our results reveal morphological disruption of all studied subnuclear domains and severe reduction of viral progeny in PML-knockdown cells. ASFV promotes H3K9me3 and HP1β foci formation from early infection, followed by HP1α and HDAC2 nuclear enrichment, suggesting heterochromatinization of host genome. Finally, closeness between DNA damage response factors, disrupted PML-NBs, and virus-induced heterochromatic regions were identified. In sum, our results demonstrate that ASFV orchestrates spatio-temporal nuclear rearrangements, changing subnuclear domains, relocating Ataxia Telangiectasia Mutated Rad-3 related (ATR)-related factors and promoting heterochromatinization, probably controlling transcription, repressing host gene expression, and favouring viral replication.

Viruses and the nuclear envelope

Viruses encounter and manipulate almost all aspects of cell structure and metabolism. The nuclear envelope (NE), with central roles in cell structure and genome function, acts and is usurped in diverse ways by different viruses. It can act as a physical barrier to infection that must be overcome, as a functional barrier that restricts infection by various mechanisms and must be counteracted or indeed as a positive niche, important or even essential for virus infection or production of progeny virions. This review summarizes virus-host interactions at the NE, highlighting progress in understanding the replication of viruses including HIV-1, Influenza, Herpes Simplex, Adenovirus and Ebola, and molecular insights into hitherto unknown functional pathways at the NE.

The Mammalian Cell Cycle Regulates Parvovirus Nuclear Capsid Assembly

It is unknown whether the mammalian cell cycle could impact the assembly of viruses maturing in the nucleus. We addressed this question using MVM, a reference member of the icosahedral ssDNA nuclear parvoviruses, which requires cell proliferation to infect by mechanisms partly understood. Constitutively expressed MVM capsid subunits (VPs) accumulated in the cytoplasm of mouse and human fibroblasts synchronized at G0, G1, and G1/S transition. Upon arrest release, VPs translocated to the nucleus as cells entered S phase, at efficiencies relying on cell origin and arrest method, and immediately assembled into capsids. In synchronously infected cells, the consecutive virus life cycle steps (gene expression, proteins nuclear translocation, capsid assembly, genome replication and encapsidation) proceeded tightly coupled to cell cycle progression from G0/G1 through S into G2 phase. However, a DNA synthesis stress caused by thymidine irreversibly disrupted virus life cycle, as VPs became increasingly retained in the cytoplasm hours post-stress, forming empty capsids in mouse fibroblasts, thereby impairing encapsidation of the nuclear viral DNA replicative intermediates. Synchronously infected cells subjected to density-arrest signals while traversing early S phase also blocked VPs transport, resulting in a similar misplaced cytoplasmic capsid assembly in mouse fibroblasts. In contrast, thymidine and density arrest signals deregulating virus assembly neither perturbed nuclear translocation of the NS1 protein nor viral genome replication occurring under S/G2 cycle arrest. An underlying mechanism of cell cycle control was identified in the nuclear translocation of phosphorylated VPs trimeric assembly intermediates, which accessed a non-conserved route distinct from the importin α2/β1 and transportin pathways. The exquisite cell cycle-dependence of parvovirus nuclear capsid assembly conforms a novel paradigm of time and functional coupling between cellular and virus life cycles. This junction may determine the characteristic parvovirus tropism for proliferative and cancer cells, and its disturbance could critically contribute to persistence in host tissues.

Cellular and viral life cycles are connected through multiple, though poorly understood, mechanisms. Parvoviruses infect humans and a broad spectrum of animals, causing a variety of diseases, but they are also used in experimental cancer therapy and serve as vectors for gene therapy. Parvoviruses can only multiply in proliferating cells providing essential replicative and transcriptional functions. However, it is unknown whether the cell cycle regulatory machinery may also control parvovirus assembly. We found that the nuclear translocation of parvovirus MVM capsid subunits (VPs) was highly dependent on physiological cell cycle regulations in mammalian fibroblasts, including: quiescence, progression through G1/S boundary, DNA synthesis, and cell to cell contacts. VPs nuclear translocation was significantly more sensitive to cell cycle controls than viral genome replication and gene expression. The results support nuclear capsid assembly as the major driving process of parvoviruses biological hallmarks, such as pathogenesis in proliferative tissues and tropism for cancer cells. In addition, disturbing the tight coupling of parvovirus assembly with the cell cycle may determine viral persistence in quiescent and post-mitotic host tissues. These findings may contribute to understand cellular regulations on the assembly of other nuclear eukaryotic viruses, and to develop cell cycle-based avenues for antiviral therapy.

Nuclear entry of DNA viruses

DNA viruses undertake their replication within the cell nucleus, and therefore they must first deliver their genome into the nucleus of their host cells. Thus, trafficking across the nuclear envelope is at the basis of DNA virus infections. Nuclear transport of molecules with diameters up to 39 nm is a tightly regulated process that occurs through the nuclear pore complex (NPC). Due to the enormous diversity of virus size and structure, each virus has developed its own strategy for entering the nucleus of their host cells, with no two strategies alike. For example, baculoviruses target their DNA-containing capsid to the NPC and subsequently enter the nucleus intact, while the hepatitis B virus capsid crosses the NPC but disassembles at the nuclear side of the NPC. For other viruses such as herpes simplex virus and adenovirus, although both dock at the NPC, they have each developed a distinct mechanism for the subsequent delivery of their genome into the nucleus. Remarkably, other DNA viruses, such as parvoviruses and human papillomaviruses, access the nucleus through an NPC-independent mechanism. This review discusses our current understanding of the mechanisms used by DNA viruses to deliver their genome into the nucleus, and further presents the experimental evidence for such mechanisms.

Tumor Selectivity of Oncolytic Parvoviruses: From in vitro and Animal Models to Cancer Patients

Oncolytic virotherapy of cancer is among the innovative modalities being under development and especially promising for targeting tumors, which are resistant to conventional treatments. Presently, at least a dozen of viruses, belonging to nine different virus families, are being tested within the frames of various clinical studies in cancer patients. Continuously growing preclinical evidence showing that the autonomous rat parvovirus H-1 (H-1PV) is able to kill tumor cells that resist conventional treatments and to achieve a complete cure of various human tumors in animal models argues for its inclusion in the arsenal of oncolytic viruses with an especially promising bench to bedside translation potential. Oncolytic parvovirus safe administration to humans relies on the intrinsic preference of these agents for quickly proliferating, metabolically, and biochemically disturbed tumor versus normal cells (tumor selectivity or oncotropism). The present review summarizes and discusses (i) preclinical evidence of H-1PV innocuousness for normal cells and healthy tissues in vitro and in animals, respectively, (ii) toxicological assessments of H-1PV mono- or combined therapy in tumor-bearing virus-permissive animal models, as well as (iii) historical results of experimental infection of human cancer patients with H-1PV. Altogether, these data argue against a risk of H-1PV inducing significant toxic effects in human patients. This highly favorable safety profile allowed the translation of H-1PV preclinical research into a Phase I/IIa clinical trial being currently in progress.

Old foes, new understandings: nuclear entry of small non-enveloped DNA viruses

The nuclear import of viral genomes is an important step of the infectious cycle for viruses that replicate in the nucleus of their host cells. Although most viruses use the cellular nuclear import machinery or some components of this machinery, others have developed sophisticated ways to reach the nucleus. Some of these have been known for some time; however, recent studies have changed our understanding of how some non-enveloped DNA viruses access the nucleus. For example, parvoviruses enter the nucleus through small disruptions of the nuclear membranes and nuclear lamina, and adenovirus tugs at the nuclear pore complex, using kinesin-1, to disassemble their capsids and deliver viral proteins and genomes into the nucleus. Here we review recent findings of the nuclear import strategies of three small non-enveloped DNA viruses, including adenovirus, parvovirus, and the polyomavirus simian virus 40.

Oncolytic parvoviruses: from basic virology to clinical applications

Accumulated evidence gathered over recent decades demonstrated that some members of the Parvoviridae family, in particular the rodent protoparvoviruses H-1PV, the minute virus of mice and LuIII have natural anticancer activity while being nonpathogenic to humans. These studies have laid the foundations for the launch of a first phase I/IIa clinical trial, in which the rat H-1 parvovirus is presently undergoing evaluation for its safety and first signs of efficacy in patients with glioblastoma multiforme. After a brief overview of the biology of parvoviruses, this review focuses on the studies which unraveled the antineoplastic properties of these agents and supported their clinical use as anticancer therapeutics. Furthermore, the development of novel parvovirus-based anticancer strategies with enhanced specificity and efficacy is discussed, in particular the development of second and third generation vectors and the combination of parvoviruses with other anticancer agents. Lastly, we address the key challenges that remain towards a more rational and efficient use of oncolytic parvoviruses in clinical settings, and discuss how a better understanding of the virus life-cycle and of the cellular factors involved in virus infection, replication and cytotoxicity may promote the further development of parvovirus-based anticancer therapies, open new prospects for treatment and hopefully improve clinical outcome.

cPKC regulates interphase nuclear size during Xenopus development

During Xenopus development, increased nuclear cPKC activity and decreased nuclear association of lamins mediate nuclear scaling.

Dramatic changes in cell and nuclear size occur during development and differentiation, and aberrant nuclear size is associated with many disease states. However, the mechanisms that regulate nuclear size are largely unknown. A robust system for investigating nuclear size is early Xenopus laevis development, during which reductions in nuclear size occur without changes in DNA content. To identify cellular factors that regulate nuclear size during development, we developed a novel nuclear resizing assay wherein nuclei assembled in Xenopus egg extract become smaller in the presence of cytoplasmic interphase extract isolated from post-gastrula Xenopus embryos. We show that nuclear shrinkage depends on conventional protein kinase C (cPKC). Increased nuclear cPKC localization and activity and decreased nuclear association of lamins mediate nuclear size reductions during development, and manipulating cPKC activity in vivo during interphase alters nuclear size in the embryo. We propose a model of steady-state nuclear size regulation whereby nuclear expansion is balanced by an active cPKC-dependent mechanism that reduces nuclear size.

Targeting of viral capsids to nuclear pores in a cell-free reconstitution system

Many viruses deliver their genomes into the nucleoplasm for viral transcription and replication. Here, we describe a novel cell-free system to elucidate specific interactions between viruses and nuclear pore complexes (NPCs). Nuclei reconstituted in vitro from egg extracts of Xenopus laevis, an established biochemical system to decipher nuclear functions, were incubated with GFP-tagged capsids of herpes simplex virus, an alphaherpesvirus replicating in the nucleus. Capsid binding to NPCs was analyzed using fluorescence and field emission scanning electron microscopy. Tegument-free capsids or viral capsids exposing inner tegument proteins on their surface bound to nuclei, while capsids inactivated by a high-salt treatment or covered by inner and outer tegument showed less binding. There was little binding of the four different capsid types to nuclei lacking functional NPCs. This novel approach provides a powerful system to elucidate the molecular mechanisms that enable viral structures to engage with NPCs. Furthermore, this assay could be expanded to identify molecular cues triggering viral genome uncoating and nuclear import of viral genomes.

Classic nuclear localization signals and a novel nuclear localization motif are required for nuclear transport of porcine parvovirus capsid proteins

Nuclear targeting of capsid proteins (VPs) is important for genome delivery and precedes assembly in the replication cycle of porcine parvovirus (PPV). Clusters of basic amino acids, corresponding to potential nuclear localization signals (NLS), were found only in the unique region of VP1 (VP1up, for VP1 unique part). Of the five identified basic regions (BR), three were important for nuclear localization of VP1up: BR1 was a classic Pat7 NLS, and the combination of BR4 and BR5 was a classic bipartite NLS. These NLS were essential for viral replication. VP2, the major capsid protein, lacked these NLS and contained no region with more than two basic amino acids in proximity. However, three regions of basic clusters were identified in the folded protein, assembled into a trimeric structure. Mutagenesis experiments showed that only one of these three regions was involved in VP2 transport to the nucleus. This structural NLS, termed the nuclear localization motif (NLM), is located inside the assembled capsid and thus can be used to transport trimers to the nucleus in late steps of infection but not for virions in initial infection steps. The two NLS of VP1up are located in the N-terminal part of the protein, externalized from the capsid during endosomal transit, exposing them for nuclear targeting during early steps of infection. Globally, the determinants of nuclear transport of structural proteins of PPV were different from those of closely related parvoviruses. Importance: Most DNA viruses use the nucleus for their replication cycle. Thus, structural proteins need to be targeted to this cellular compartment at two distinct steps of the infection: in early steps to deliver viral genomes to the nucleus and in late steps to assemble new viruses. Nuclear targeting of proteins depends on the recognition of a stretch of basic amino acids by cellular transport proteins. This study reports the identification of two classic nuclear localization signals in the minor capsid protein (VP1) of porcine parvovirus. The major protein (VP2) nuclear localization was shown to depend on a complex structural motif. This motif can be used as a strategy by the virus to avoid transport of incorrectly folded proteins and to selectively import assembled trimers into the nucleus. Structural nuclear localization motifs can also be important for nuclear proteins without a classic basic amino acid stretch, including multimeric cellular proteins.

Parvoviruses: Small Does Not Mean Simple

Parvoviruses are small, rugged, nonenveloped protein particles containing a linear, nonpermuted, single-stranded DNA genome of ∼5 kb. Their limited coding potential requires optimal adaptation to the environment of particular host cells, where entry is mediated by a variable program of capsid dynamics, ultimately leading to genome ejection from intact particles within the host nucleus. Genomes are amplified by a continuous unidirectional strand-displacement mechanism, a linear adaptation of rolling circle replication that relies on the repeated folding and unfolding of small hairpin telomeres to reorient the advancing fork. Progeny genomes are propelled by the viral helicase into the preformed capsid via a pore at one of its icosahedral fivefold axes. Here we explore how the fine-tuning of this unique replication system and the mechanics that regulate opening and closing of the capsid fivefold portals have evolved in different viral lineages to create a remarkably complex spectrum of phenotypes.

Large scale RNAi reveals the requirement of nuclear envelope breakdown for nuclear import of human papillomaviruses

A two-step, high-throughput RNAi silencing screen was used to identify host cell factors required during human papillomavirus type 16 (HPV16) infection. Analysis of validated hits implicated a cluster of mitotic genes and revealed a previously undetermined mechanism for import of the viral DNA (vDNA) into the nucleus. In interphase cells, viruses were endocytosed, routed to the perinuclear area, and uncoated, but the vDNA failed to be imported into the nucleus. Upon nuclear envelope perforation in interphase cells HPV16 infection occured. During mitosis, the vDNA and L2 associated with host cell chromatin on the metaphase plate. Hence, we propose that HPV16 requires nuclear envelope breakdown during mitosis for access of the vDNA to the nucleoplasm. The results accentuate the value of genes found by RNAi screens for investigation of viral infections. The list of cell functions required during HPV16 infection will, moreover, provide a resource for future virus-host cell interaction studies.

Certain human papillomaviruses (HPV) are the etiological cause of cervical cancers and other epithelial tumors. Recent advances in the development of anti-HPV vaccines and their increasing deployment provide hope for a significant decrease of these cancers in the future. However, many details of the transmission of HPV between infected and uninfected individuals are still not well understood. In particular, the multistep process of entry into host cells is only partially characterized. Here, we used a systematic RNAi silencing approach to identify host cell proteins required during HPV16 infection, the most prevalent cancer-causing HPV. Our results indicated that HPV entry requires cells to divide. Cell division is important, since it removes the barrier between the nucleoplasm and the cytosol, so that the virus can access the nucleus for transcription and replication. Our study provides new mechanistic insights into the entry process of an important pathogen, and explains why it can only infect the stem cells or transiently amplifying cells of human skin or mucosa. This work also highlights a novel nuclear import strategy for DNA viruses, which typically use the nuclear import machinery to access the nucleoplasm for infection, Thus, HPV entry displays another unique aspect uncommon to most other viruses.

Parvovirus particles and movement in the cellular cytoplasm and effects of the cytoskeleton

Cell infection by parvoviruses requires that capsids be delivered from outside the cell to the cytoplasm, followed by genome trafficking to the nucleus. Here we microinject capsids into cells that lack receptors and followed their movements within the cell over time. In general the capsids remained close to the positions where they were injected, and most particles did not move to the vicinity of or enter the nucleus. When 70 kDa-dextran was injected along with the capsids that did not enter the nucleus in significant amounts. Capsids conjugated to peptides containing the SV40 large T-antigen nuclear localization signal remained in the cytoplasm, although bovine serum albumen conjugated to the same peptide entered the nucleus rapidly. No effects of disruption of microfilaments, intermediate filaments, or microtubules on the distribution of the capsids were observed. These results suggest that movement of intact capsids within cells is primarily associated with passive processes.

Breaching the nuclear envelope in development and disease

In eukaryotic cells the nuclear genome is enclosed by the nuclear envelope (NE). In metazoans, the NE breaks down in mitosis and it has been assumed that the physical barrier separating nucleoplasm and cytoplasm remains intact during the rest of the cell cycle and cell differentiation. However, recent studies suggest that nonmitotic NE remodeling plays a critical role in development, virus infection, laminopathies, and cancer. Although the mechanisms underlying these NE restructuring events are currently being defined, one common theme is activation of protein kinase C family members in the interphase nucleus to disrupt the nuclear lamina, demonstrating the importance of the lamina in maintaining nuclear integrity.

Differential replication of two porcine parvovirus strains in bovine cell lines ensues from initial DNA processing and NS1 expression

Porcine parvovirus (PPV) is a small DNA virus with restricted coding capacity. The 5 kb genome expresses three major non-structural proteins (NS1, NS2 and SAT), and two structural proteins (VP1 and VP2). These few viral proteins are pleiotropic and interact with cellular components throughout viral replication. In this regard, very few cell lines have been shown to replicate the virus efficiently. Cell lines were established from a primary culture of bovine cells that allowed allotropic variants of PPV to be distinguished. Three cell lines were differentially sensitive to infection by two prototype PPV strains, NADL-2 and Kresse. In the first cell line (D10), infection was restricted early in the infectious cycle and was not productive. Infection of the second cell line (G11) was 1000 times less efficient with the NADL-2 strain compared with porcine cells, while production of infectious virus of the Kresse strain was barely detectable. Restriction points in these cells were the initial generation of DNA replication intermediates and NS1 production. Infection with chimeras between NADL-2 and Kresse showed that residues outside the previously described allotropic determinant were also partially responsible for the restriction to Kresse replication in G11 cells. F4 cells were permissive to both strains, although genome replication and infectious virus production were lower than in the porcine cells used for comparison. These results highlight the dependent nature of parvovirus tropism on host factors and suggest that cells from a non-host origin can fully support a productive infection by both strains.

Recombinant adeno-associated virus utilizes host cell nuclear import machinery to enter the nucleus

Recombinant adeno-associated viral (rAAV) vectors have garnered much promise in gene therapy applications. However, widespread clinical use has been limited by transduction efficiency. Previous studies suggested that the majority of rAAV accumulates in the perinuclear region of cells, presumably unable to traffic into the nucleus. rAAV nuclear translocation remains ill-defined; therefore, we performed microscopy, genetic, and biochemical analyses in vitro in order to understand this mechanism. Lectin blockade of the nuclear pore complex (NPC) resulted in inhibition of nuclear rAAV2. Visualization of fluorescently labeled particles revealed that rAAV2 localized to importin-β-dense regions of cells in late trafficking steps. Additionally, small interfering RNA (siRNA) knockdown of importin-β partially inhibited rAAV2 nuclear translocation and inhibited transduction by 50 to 70%. Furthermore, coimmunopreciptation (co-IP) analysis revealed that capsid proteins from rAAV2 could interact with importin-β and that this interaction was sensitive to the small GTPase Ran. More importantly, mutations to key basic regions in the rAAV2 capsid severely inhibited interactions with importin-β. We tested several other serotypes and found that the extent of importin-β interaction varied, suggesting that different serotypes may utilize alternative import proteins for nuclear translocation. Co-IP and siRNA analyses were used to investigate the role of other karyopherins, and the results suggested that rAAV2 may utilize multiple import proteins for nuclear entry. Taken together, our results suggest that rAAV2 interacts with importin-β alone or in complex with other karyopherins and enters the nucleus via the NPC. These results may lend insight into the design of novel AAV vectors that have an enhanced nuclear entry capability and transduction potential.

IMPORTANCE

Use of recombinant adeno-associated viral (rAAV) vectors for gene therapy applications is limited by relatively low transduction efficiency, in part due to cellular barriers that hinder successful subcellular trafficking to the nucleus, where uncoating and subsequent gene expression occur. Nuclear translocation of rAAV has been regarded as a limiting step for successful transduction but it remains ill-defined. We explored potential nuclear entry mechanisms for rAAV2 and found that rAAV2 can utilize the classical nuclear import pathway, involving the nuclear pore complex, the small GTPase Ran, and cellular karyopherins. These results could lend insight into the rational design of novel rAAV vectors that can more efficiently translocate to the nucleus, which may lead to more efficient transduction.

Interaction of parvoviruses with the nuclear envelope

Parvoviruses are serious pathogens but also serve as platforms for gene therapy or for using their lytic activity in experimental cancer treatment. Despite of their growing importance during the last decade little is known on how the viral genome is transported into the nucleus of the infected cell, which is crucial for replication. As nucleic acids are not karyophilic per se nuclear import must be driven by proteins attached to the viral genome. In turn, presence and conformation of these proteins depend upon the entry pathway of the virus into the cell. This review focuses on the trafficking of the parvoviral genome from the cellular periphery to nucleus. Despite of the uncertainties in knowledge about the entry pathway we show that parvoviruses developed a unique strategy to pass the nuclear envelope by hijacking enzymes involved in mitosis.