Nuclear import and export of viruses and virus genomes

Novel Nucleus-Oriented Quenched Activity-Based Probes Link Cathepsin Nuclear Localization with Mitosis

Cysteine cathepsins are important proteases that are highly upregulated in cancers and other diseases. While their reported location is mostly endolysosomal, some evidence shows their nuclear localization and involvement in the cell cycle. We aim to generate tools to investigate the involvement of cathepsins in the cell cycle progression. To investigate nuclear cathepsin activity, we designed nucleus-directed quenched activity-based probes (qABPs) by attaching cell-penetrating peptides (CPPs). qABPs are active-site-directed compounds that enable direct real-time monitoring of enzyme activity by the covalent linkage between the probe and the enzyme's active site. Biochemical evaluation of the CPP-qABPs showed potent and selective probes; cell fractionation, multimodal flow cytometry-imaging, and time-lapse movies demonstrated nuclear cathepsin activity in living cells. Interestingly, these probes reveal a spatiotemporal pattern, a surge of nuclear cathepsin just before mitosis, suggesting yet unrevealed roles of cathepsin in cell division. In summary, these nuclear-directed qABPs serve as unique scientific tools to unlock the hidden features of cysteine proteases and to understand their involvement in cell division and cancer.

Temporal insights into molecular and cellular responses during rAAV production in HEK293T cells

The gene therapy field seeks cost-effective, large-scale production of recombinant adeno-associated virus (rAAV) vectors for high-dosage therapeutic applications. Although strategies like suspension cell culture and transfection optimization have shown moderate success, challenges persist for large-scale applications. To unravel molecular and cellular mechanisms influencing rAAV production, we conducted an SWATH-MS proteomic analysis of HEK293T cells transfected using standard, sub-optimal, and optimal conditions. Gene Ontology and pathway analysis revealed significant protein expression variations, particularly in processes related to cellular homeostasis, metabolic regulation, vesicular transport, ribosomal biogenesis, and cellular proliferation under optimal transfection conditions. This resulted in a 50% increase in rAAV titer compared with the standard protocol. Additionally, we identified modifications in host cell proteins crucial for AAV mRNA stability and gene translation, particularly regarding AAV capsid transcripts under optimal transfection conditions. Our study identified 124 host proteins associated with AAV replication and assembly, each exhibiting distinct expression pattern throughout rAAV production stages in optimal transfection condition. This investigation sheds light on the cellular mechanisms involved in rAAV production in HEK293T cells and proposes promising avenues for further enhancing rAAV titer during production.

Interferon-induced MXB protein restricts vimentin-dependent viral infection

Type I interferon (IFN) inhibits a wide spectrum of viruses through stimulating the expression of antiviral proteins. As an IFN-induced protein, myxovirus resistance B (MXB) protein was reported to inhibit multiple highly pathogenic human viruses. It remains to be determined whether MXB employs a common mechanism to restrict different viruses. Here, we find that IFN alters the subcellular localization of hundreds of host proteins, and this IFN effect is partially lost upon MXB depletion. The results of our mechanistic study reveal that MXB recognizes vimentin (VIM) and recruits protein kinase B (AKT) to phosphorylate VIM at amino acid S38, which leads to reorganization of the VIM network and impairment of intracellular trafficking of virus protein complexes, hence causing a restriction of virus infection. These results highlight a new function of MXB in modulating VIM-mediated trafficking, which may lead towards a novel broad-spectrum antiviral strategy to control a large group of viruses that depend on VIM for successful replication.

Retroviral hijacking of host transport pathways for genome nuclear export

Recent advances in the study of virus-cell interactions have improved our understanding of how viruses that replicate their genomes in the nucleus (e.g., retroviruses, hepadnaviruses, herpesviruses, and a subset of RNA viruses) hijack cellular pathways to export these genomes to the cytoplasm where they access virion egress pathways. These findings shed light on novel aspects of viral life cycles relevant to the development of new antiviral strategies and can yield new tractable, virus-based tools for exposing additional secrets of the cell. The goal of this review is to summarize defined and emerging modes of virus-host interactions that drive the transit of viral genomes out of the nucleus across the nuclear envelope barrier, with an emphasis on retroviruses that are most extensively studied. In this context, we prioritize discussion of recent progress in understanding the trafficking and function of the human immunodeficiency virus type 1 Rev protein, exemplifying a relatively refined example of stepwise, cooperativity-driven viral subversion of multi-subunit host transport receptor complexes.

Mechanistic inhibition of Monkeypox and Marburg virus infection by O-rhamnosides and Kaempferol-o-rhamnosides derivatives: a new-fangled computational approach

The increasing incidence of Monkeypox virus (Mpox) and Marburg virus (MARV) infections worldwide presents a significant challenge to global health, as limited treatment options are currently available. This study investigates the potential of several O-rhamnosides and Kaempferol-O-rhamnosides as Mpox and MARV inhibitors using molecular modeling methods, including ADMET, molecular docking, and molecular dynamics/MD simulation. The effectiveness of these compounds against the viruses was assessed using the Prediction of Activity Spectra for Substances (PASS) prediction. The study's primary focus is molecular docking prediction, which demonstrated that ligands (L07, L08, and L09) bind to Mpox (PDB ID: 4QWO) and MARV (PDB ID: 4OR8) with binding affinities ranging from -8.00 kcal/mol to -9.5 kcal/mol. HOMO-LUMO based quantum calculations were employed to determine the HOMO-LUMO gap of frontier molecular orbitals (FMOs) and to estimate chemical potential, electronegativity, hardness, and softness. Drug similarity and ADMET prediction assessments of pharmacokinetic properties revealed that the compounds were likely non-carcinogenic, non-hepatotoxic, and rapidly soluble. Molecular dynamic (MD) modeling was used to identify the most favorable docked complexes involving bioactive chemicals. MD simulations indicate that varying types of kaempferol-O-rhamnoside are necessary for successful docking validation and maintaining the stability of the docked complex. These findings could facilitate the discovery of novel therapeutic agents for treating illnesses caused by the Mpox and MARV viruses.

Nuclear translocation of spike mRNA and protein is a novel feature of SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes severe pathophysiology in vulnerable older populations and appears to be highly pathogenic and more transmissible than other coronaviruses. The spike (S) protein appears to be a major pathogenic factor that contributes to the unique pathogenesis of SARS-CoV-2. Although the S protein is a surface transmembrane type 1 glycoprotein, it has been predicted to be translocated into the nucleus due to the novel nuclear localization signal (NLS) "PRRARSV," which is absent from the S protein of other coronaviruses. Indeed, S proteins translocate into the nucleus in SARS-CoV-2-infected cells. S mRNAs also translocate into the nucleus. S mRNA colocalizes with S protein, aiding the nuclear translocation of S mRNA. While nuclear translocation of nucleoprotein (N) has been shown in many coronaviruses, the nuclear translocation of both S mRNA and S protein reveals a novel feature of SARS-CoV-2.

Nuclear translocation of spike mRNA and protein is a novel pathogenic feature of SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes severe pathophysiology in vulnerable older populations and appears to be highly pathogenic and more transmissible than SARS-CoV or MERS-CoV [1, 2]. The spike (S) protein appears to be a major pathogenic factor that contributes to the unique pathogenesis of SARS-CoV-2. Although the S protein is a surface transmembrane type 1 glycoprotein, it has been predicted to be translocated into the nucleus due to the novel nuclear localization signal (NLS) "PRRARSV", which is absent from the S protein of other coronaviruses. Indeed, S proteins translocate into the nucleus in SARS-CoV-2-infected cells. To our surprise, S mRNAs also translocate into the nucleus. S mRNA colocalizes with S protein, aiding the nuclear translocation of S mRNA. While nuclear translocation of nucleoprotein (N) has been shown in many coronaviruses, the nuclear translocation of both S mRNA and S protein reveals a novel pathogenic feature of SARS-CoV-2.

Author summary

One of the novel sequence insertions resides at the S1/S2 boundary of Spike (S) protein and constitutes a functional nuclear localization signal (NLS) motif "PRRARSV", which may supersede the importance of previously proposed polybasic furin cleavage site "RRAR". Indeed, S protein's NLS-driven nuclear translocation and its possible role in S mRNA's nuclear translocation reveal a novel pathogenic feature of SARS-CoV-2.

Characterization of the functional domains of nuclear shuttle protein (NSP) of Indian cassava mosaic virus using green fluorescent protein as reporter

Indian cassava mosaic virus (ICMV), responsible for the cassava mosaic disease in India, harbours two circular genomic components, DNA-A and DNA-B; the former being responsible for the encapsidation and replication and the latter for intra- and inter-cellular movement of the viral DNA. Two proteins, encoded by DNA-B, the movement protein (MP) and the nuclear shuttle protein (NSP), act in concert on the newly replicated viral DNA to move it from the nucleus to the cell periphery. To map the functional domains of NSP, the intra-cellular localization of its full-length protein and deletion derivatives was studied in the epidermal cells of detached leaves of the laboratory host plant, Nicotiana benthamiana, where the target proteins were transiently expressed as GFP fusions. This analysis revealed domains for nuclear localization at the N-terminus, as well as for localization towards the cell periphery both at the C-terminus and center of the NSP.

Uncovering the F-Actin-Based Nuclear Egress Mechanism of Newly Synthesized Influenza A Virus Ribonucleoprotein Complexes by Single-Particle Tracking

Nuclear trafficking of viral genome is an essential cellular process in the life cycles of viruses. Despite substantial progress in uncovering a wide variety of complicated mechanisms of virus entry, intracellular transport of viral components, virus assembly, and egress, the temporal and spatial dynamics of viral genes trafficking within the nucleus remains poorly understood. Herein, using single-particle tracking, we explored the real-time dynamic nuclear trafficking of influenza A virus (IAV) genes packaged as the viral ribonucleoprotein complexes (vRNPs) by combining a four-plasmid DNA transfection system for the reconstruction of green fluorescent protein (GFP)-labeled vRNPs and a spinning disk super-resolution fluorescence microscope. We found that IAV infection significantly induced the formation of actin microfilaments (F-actin) in the nucleus. In combination with the fluorescent protein-tagged nuclear F-actin probe, we visualized the directed movement of GFP-labeled vRNPs foci along the nuclear F-actin with a speed of 0.18 μm/s, which is similar to the microfilaments-dependent slow directed motion of IAVs in the cytoplasm. The disruption of nuclear F-actin after treatment with microfilament inhibitors caused a considerable decrease in vRNPs motility and suppressed the nuclear export of newly produced vRNPs, indicating that the slow, directed movement plays a crucial role in facilitating the nuclear egress of vRNPs. Our findings identified a nuclear F-actin-dependent pathway for IAV vRNPs transporting from the nucleus into the cytoplasm, which may in turn uncover a novel target for antiviral treatment.

Gene therapy strategies for rare monogenic disorders with nuclear or mitochondrial gene mutations

Rare monogenic disorders are a group of single-gene-mutated diseases that have a low incidence rate (less than 0.5‰) and eventually lead to patient disability and even death. Due to the relatively low number of people affected, these diseases typically fail to attract a great deal of commercial investment and research interest, and the affected patients thus have unmet medical needs. Advances in genomics biology, gene editing, and gene delivery can now offer potentially effective options for treating rare monogenic diseases. Herein, we review the application of gene therapy strategies (traditional gene therapy and gene editing) against various rare monogenic diseases with nuclear or mitochondrial gene mutations, including eye, central nervous system, pulmonary, systemic, and blood cell diseases. We summarize their pathologic features, address the barriers to gene delivery for these diseases, discuss available therapies in the clinic and in clinical trials, and sum up in-development gene delivery systems for various rare monogenic disorders. Finally, we elaborate the possible directions and outlook of gene therapy for rare monogenic disorders.

Highly Basic Clusters in the Herpes Simplex Virus 1 Nuclear Egress Complex Drive Membrane Budding by Inducing Lipid Ordering

During replication of herpesviruses, capsids escape from the nucleus into the cytoplasm by budding at the inner nuclear membrane. This unusual process is mediated by the viral nuclear egress complex (NEC) that deforms the membrane around the capsid by oligomerizing into a hexagonal, membrane-bound scaffold. Here, we found that highly basic membrane-proximal regions (MPRs) of the NEC alter lipid order by inserting into the lipid headgroups and promote negative Gaussian curvature. We also find that the electrostatic interactions between the MPRs and the membranes are essential for membrane deformation. One of the MPRs is phosphorylated by a viral kinase during infection, and the corresponding phosphomimicking mutations block capsid nuclear egress. We show that the same phosphomimicking mutations disrupt the NEC-membrane interactions and inhibit NEC-mediated budding in vitro, providing a biophysical explanation for the in vivo phenomenon. Our data suggest that the NEC generates negative membrane curvature by both lipid ordering and protein scaffolding and that phosphorylation acts as an off switch that inhibits the membrane-budding activity of the NEC to prevent capsid-less budding. IMPORTANCE Herpesviruses are large viruses that infect nearly all vertebrates and some invertebrates and cause lifelong infections in most of the world's population. During replication, herpesviruses export their capsids from the nucleus into the cytoplasm by an unusual mechanism in which the viral nuclear egress complex (NEC) deforms the nuclear membrane around the capsid. However, how membrane deformation is achieved is unclear. Here, we show that the NEC from herpes simplex virus 1, a prototypical herpesvirus, uses clusters of positive charges to bind membranes and order membrane lipids. Reducing the positive charge or introducing negative charges weakens the membrane deforming ability of the NEC. We propose that the virus employs electrostatics to deform nuclear membrane around the capsid and can control this process by changing the NEC charge through phosphorylation. Blocking NEC-membrane interactions could be exploited as a therapeutic strategy.

Nucleolar protein NPM1 is essential for circovirus replication by binding to viral capsid

Entry of circovirus into the host cell nucleus is essential for viral replication during the early stage of infection. However, the mechanisms by which nucleolar shuttle proteins are used during viral replication is still not well understood. Here, we report a previously unidentified nucleolar localization signal in circovirus capsid protein (Cap), and that circovirus hijacks the nucleolar phosphoprotein nucleophosmin-1 (NPM1) to facilitate its replication. Colocalization analysis showed that NPM1 translocates from the nucleolus to the nucleoplasm and cytoplasm during viral infection. Coimmunoprecipitation and glutathione S-transferase pull-down assays showed that Cap interacts directly with NPM1. Binding domain mapping showed that the arginine-rich N-terminal motif ¹MTYPRRRYRRRRHRPRSHLG²⁰ of Cap, and residue serine-48 of the N-terminal oligomerization domain of NPM1, are essential for the interaction. Virus rescue experiments showed that all arginine to alanine substitution in the N-terminal arginine-rich motif of Cap resulted in diminished viral replication. Knockdown of NPM1 and substitution of serine-48 in NPM1 to glutamic acid also decreased viral replication. In addition, binding assays showed that the arginine-rich motif of Cap is a nucleolar localization signal. Taken together, our findings demonstrate that circovirus protein Cap is a nucleolus-located, and regulates viral replication by directly binding to NPM1.

When in Need of an ESCRT: The Nature of Virus Assembly Sites Suggests Mechanistic Parallels between Nuclear Virus Egress and Retroviral Budding

The proper assembly and dissemination of progeny virions is a fundamental step in virus replication. As a whole, viruses have evolved a myriad of strategies to exploit cellular compartments and mechanisms to ensure a successful round of infection. For enveloped viruses such as retroviruses and herpesviruses, acquisition and incorporation of cellular membrane is an essential process during the formation of infectious viral particles. To do this, these viruses have evolved to hijack the host Endosomal Sorting Complexes Required for Transport (ESCRT-I, -II, and -III) to coordinate the sculpting of cellular membrane at virus assembly and dissemination sites, in seemingly different, yet fundamentally similar ways. For instance, at the plasma membrane, ESCRT-I recruitment is essential for HIV-1 assembly and budding, while it is dispensable for the release of HSV-1. Further, HSV-1 was shown to recruit ESCRT-III for nuclear particle assembly and egress, a process not used by retroviruses during replication. Although the cooption of ESCRTs occurs in two separate subcellular compartments and at two distinct steps for these viral lifecycles, the role fulfilled by ESCRTs at these sites appears to be conserved. This review discusses recent findings that shed some light on the potential parallels between retroviral budding and nuclear egress and proposes a model where HSV-1 nuclear egress may occur through an ESCRT-dependent mechanism.

Structural Perspectives of Beak and Feather Disease Virus and Porcine Circovirus Proteins

Circoviruses represent a rapidly expanding group of viruses that infect both vertebrate and invertebrate hosts. Members are responsible for diseases of veterinary and economic importance, including postweaning multisystemic wasting syndrome in pigs, and beak and feather disease (BFD) in birds. These viruses are associated with lymphoid depletion and immunosuppressive conditions in infected animals leading to systemic illness. Circoviruses are small nonenveloped DNA viruses containing a single-stranded circular genome, encoding two major proteins: the capsid-associated protein (Cap), comprising the entirety of the viral capsid, and the replication-associated protein (Rep). Cap is the only protein component of the virion and plays crucial roles throughout the virus replication cycle, including viral attachment, cell entry, genome uncoating, and packaging of newly formed viral particles. Rep mediates recognition of replication origin motifs in the viral genome sequence and is responsible for endonuclease activity enabling nicking of the circular DNA and initiation of rolling-circle replication (RCR). Porcine circovirus 2 (PCV2) was the first circovirus capsid structure to be solved at atomic resolution using X-ray crystallography. The structure revealed an assembly comprising 60 monomeric subunits to form virus-like particles. Each Cap monomer harbors a canonical viral jelly roll domain composed of two, four-stranded antiparallel β-sheets. Crystal structures of two distinct macromolecular assemblies from BFD virus Cap were also resolved at high resolution. In these structures, the exposure of the N-terminal arginine-rich motif, responsible for DNA binding and nuclear localization is reversed. Additional structural investigations have also elucidated a PCV2 type-specific neutralizing epitope, and interaction between the PCV2 capsid and polymers such as heparin. In this review, we provide a snapshot of the structural and functional aspects of circovirus proteins.

Chimeric Virus Made from crTMV RNA and the Coat Protein of Potato Leafroll Virus is Targeted to the Nucleolus and Can Infect Nicotiana benthamiana Mechanically

A genetically engineered chimeric virus crTMV-CP-PLRV composed of the crucifer-infecting tobacco mosaic virus (crTMV) RNA and the potato leafroll virus (PLRV) coat protein (CP) was obtained by agroinfiltration of Nicotiana benthamiana with the binary vector pCambia-crTMV-CP_PLRV. The significant levels of the chimeric virus enabled direct visualization of crTMV-CP-PLRV in the cell and to investigate the mechanism of the pathogenesis. Localization of the crTMV-CP-PLRV in plant cells was examined by immunoblot techniques, as well as light, and transmission electron microscopy. The chimera can transfer between vascular and nonvascular tissues. The chimeric virus inoculum is capable to infect N. benthamiana mechanically. The distinguishing feature of the chimeric virus, the RNA virus with the positive genome, was found to localize in the nucleolus. We also investigated the role of the N-terminal sequence of the PLRV P3 coat protein in the cellular localization of the virus. We believe that the gene of the PLRV CP can be substituted with genes from other challenging-to-study plant pathogens to produce other useful recombinant viruses.

EHV-1: A Constant Threat to the Horse Industry

Equine herpesvirus-1 (EHV-1) is one of the most important and prevalent viral pathogens of horses and a major threat to the equine industry throughout most of the world. EHV-1 primarily causes respiratory disease but viral spread to distant organs enables the development of more severe sequelae; abortion and neurologic disease. The virus can also undergo latency during which viral genes are minimally expressed, and reactivate to produce lytic infection at any time. Recently, there has been a trend of increasing numbers of outbreaks of a devastating form of EHV-1, equine herpesviral myeloencephalopathy. This review presents detailed information on EHV-1, from the discovery of the virus to latest developments on treatment and control of the diseases it causes. We also provide updates on recent EHV-1 research with particular emphasis on viral biology which enables pathogenesis in the natural host. The information presented herein will be useful in understanding EHV-1 and formulating policies that would help limit the spread of EHV-1 within horse populations.

Cellular p32 Is a Critical Regulator of Porcine Circovirus Type 2 Nuclear Egress

Circoviruses are the smallest DNA viruses known to infect mammalian and avian species. Although circoviruses are known to be associated with a range of clinical diseases, the details of circovirus DNA release still remain unknown. Here, we identified p32 as a key regulator for porcine circoviral nuclear egress. Upon porcine circovirus type 2 (PCV2) infection, p32 was recruited into the nucleus by the viral capsid (Cap) protein; simultaneously, protein kinase C isoform δ (PKC-δ) was phosphorylated at threonine 505 by phospholipase C (PLC)-mediated signaling at the early stage of infection, which was further amplified by Jun N-terminal protein kinase (JNK) and extracellular signal-regulated kinase (ERK) signaling at the late infection phase. p32 functioned as an adaptor to recruit phosphorylated PKC-δ and Cap to the nuclear membrane to phosphorylate lamin A/C, resulting in a rearrangement of nuclear lamina and thus facilitating viral nuclear egress. Consistent with these findings, knockout (KO) of p32 in PCV2-infected cells markedly reduced the phosphorylation of PKC-δ and impeded the recruitment of p-PKC-δ and Cap to the nuclear membrane, hence abolishing the phosphorylation of lamin A/C and the rearrangement of nuclear lamina. As a result, p32 depletion profoundly impaired the production of cell-free viruses during PCV2 infection. We further identified the N-terminal 24RRR26 of Cap to be crucial for binding to p32, and mutation of these three arginine residues significantly weakened the replication and pathogenesis of PCV2 in vivo In summary, our findings highlight a critical role of p32 in the activation and recruitment of PKC-δ to phosphorylate lamin A/C and facilitate porcine circoviral nuclear egress, and they certainly help understanding of the mechanism of PCV2 replication.IMPORTANCE Circovirus infections are highly prevalent in mammalian and avian species. Circoviral capsid protein is the only structural protein of the virion that plays an essential role in viral assembly. However, the machinery of circovirus nuclear egress is currently unknown. In this work, we identified p32 as a key regulator of porcine circovirus type 2 (PCV2) nuclear egress that forms a complex with the viral capsid (Cap) protein to enhance protein kinase C isoform δ (PKC-δ) activity; this resulted in a recruitment of phosphorylated PKC-δ to the nuclear membrane, which further phosphorylates lamin A/C to promote the rearrangement of nuclear lamina and facilitate viral nuclear egress. Notably, we found that the N-terminal 24RRR26 of Cap, a highly conserved motif among circovirus species, was required for interacting with p32, and that mutation of this motif markedly impeded PCV2 nuclear egress. These data indicate that p32 is a critical regulator of PCV2 nuclear egress and reveal the importance of this finding in circovirus replication.

Development of a novel DsRed-NLS vector with a monopartite classical nuclear localization signal

The nuclear localization signal (NLS) marks proteins for transport to the nucleus and is used in various applications in many fields. NLSs are used to achieve efficient and stable transport of biomolecules. Previously, commercial vectors used in NLS studies contained three iterations of the NLS sequence, but these sequences can affect experimental results and alter protein function. Here, we investigated a new vector using a single classical NLS sequence with a mutation in pDsRed2-C1-wt to reduce experimental artifacts. In the newly constructed pDsRed2-C1-1NLS vector, the NLS sequence is placed near the multiple cloning sites of pDsRed2-C1-wt, and the multiple cloning site region was designed to facilitate insertion of the desired gene by site-directed mutagenesis. Fluorescent protein expression in the nucleus can be visually confirmed. The results show that the fluorescent protein was bound to the transport protein. The constructed vector had a cell survival rate of 89-95% and a transfection efficiency of 39-56% when introduced into animal cells, which are similar to those of other NLS vectors. Additionally, the constructed NLS vector can be used to demonstrate complementary binding between target proteins, and that the target protein is transported by the NLS transport system. Especially, we show that the vector can be useful for experiments involving the S100A10 gene. In addition, the constructed vector is useful for studies of genes and proteins that show potential for gene therapy or drug delivery applications.

Nucleocytoplasmic Shuttling of Geminivirus C4 Protein Mediated by Phosphorylation and Myristoylation Is Critical for Viral Pathogenicity

Many geminivirus C4 proteins induce severe developmental abnormalities in plants. We previously demonstrated that Tomato leaf curl Yunnan virus (TLCYnV) C4 induces plant developmental abnormalities at least partically by decreasing the accumulation of NbSKη, an ortholog of Arabidopsis BIN2 kinase involved in the brassinosteroid signaling pathway, in the nucleus through directing it to the plasma membrane. However, the molecular mechanism by which the membrane-associated C4 modifies the localization of NbSKη in the host cell remains unclear. Here, we show that TLCYnV C4 is a nucleocytoplasmic shuttle protein, and that C4 shuttling is accompanied by nuclear export of NbSKη. TLCYnV C4 is phosphorylated by NbSKη in the nucleus, which promotes myristoylation of the viral protein. Myristoylation of phosphorylated C4 favors its interaction with exportin-α (XPO I), which in turn facilitates nuclear export of the C4/NbSKη complex. Supporting this model, chemical inhibition of N-myristoyltransferases or exportin-α enhanced nuclear retention of C4, and mutations of the putative phosphorylation or myristoylation sites in C4 resulted in increased nuclear retention of C4 and thus decreased severity of C4-induced developmental abnormalities. The impact of C4 on development is also lessened when a nuclear localization signal or a nuclear export signal is added to its C-terminus, restricting it to a specific cellular niche and therefore impairing nucleocytoplasmic shuttling. Taken together, our results suggest that nucleocytoplasmic shuttling of TLCYnV C4, enabled by phosphorylation by NbSKη, myristoylation, and interaction with exportin-α, is critical for its function as a pathogenicity factor.

Influenza Virus: A Master Tactician in Innate Immune Evasion and Novel Therapeutic Interventions

Influenza is a contagion that has plagued mankind for many decades, and continues to pose concerns every year, with millions of infections globally. The frequent mutations and recombination of the influenza A virus (IAV) cast a looming threat that antigenically novel strains/subtypes will rise with unpredictable pathogenicity and fear of it evolving into a pandemic strain. There have been four major influenza pandemics, since the beginning of twentieth century, with the great 1918 pandemic being the most severe, killing more than 50 million people worldwide. The mechanisms of IAV infection, host immune responses, and how viruses evade from such defensive responses at the molecular and structural levels have been greatly investigated in the past 30 years. While this has advanced our understanding of virus–host interactions and human immunology, and has led to the development of several antiviral drugs, they have minimal impact on the clinical outcomes of infection. The heavy use of these drugs has also imposed selective pressure on IAV to evolve and develop resistance. Vaccination remains the cornerstone of public health efforts to protect against influenza; however, rapid mass-production of sufficient vaccines is unlikely to occur immediately after the beginning of a pandemic. This, therefore, requires novel therapeutic strategies against this continually emerging infectious virus with higher specificity and cross-reactivity against multiple strains/subtypes of IAVs. This review discusses essential virulence factors of IAVs that determine sustainable human-to-human transmission, the mechanisms of viral hijacking of host cells and subversion of host innate immune responses, and novel therapeutic interventions that demonstrate promising antiviral properties against IAV. This hopefully will promote discussions and investigations on novel avenues of prevention and treatment strategies of influenza, that are effective and cross-protective against multiple strains/subtypes of IAV, in preparation for the advent of future IAVs and pandemics.

Upregulation of Coxsackie Adenovirus Receptor Sensitizes Cisplatin-Resistant Lung Cancer Cells to CRAd-Induced Inhibition

Objective. Conditionally replicating adenoviruses (CRAds) have been proven potent oncolytic viruses in previous studies. They selectively replicate in the tumor cells because of incorporated survivin promoter and ultimately lead to their killing with minimal side effects on normal tissue. Chemotherapy with cisplatin is commonly employed for treating tumors, but its cytotoxic effects and development of resistance remained major concerns to be dealt with. The aim of this study was to explore the anticancer potential of survivin regulated CRAd alone or in combination with cisplatin in the A549 lung cancer cell line and cisplatin-resistant lung cancer cell line, A549-DDPR.

Methods. CRAd was genetically engineered in our laboratory by removing its E1B region and adding survivin promoter to control its replication. A549, H292, and H661 lung cancer cell lines were procured from the CAS-China. The anti-tumor effectiveness of combined treatment (cisplatin plus CRAd) was evaluated in vitro through MTS assays and in vivo through mouse model experimentation. RT- PCR was used to assess MDR gene and mRNA expression of coxsackie adenoviral receptor (CAR).

Results. Results of in vitro studies established that A549 lung cancer cells were highly sensitive to cisplatin showing dose-dependent inhibition. The resistant cells of A549-DDPR exhibited very less sensitivity to cisplatin but were infected with CRAd more efficiently as compared to A549. A549-DDPR cells exhibited higher expression of MDR gene and CAR in the RT-PCR analysis. The nearly similar rise in the CAR expression was seen when lung cancer cell lines received cisplatin in combined treatment (cisplatin plus CRAd). Combined anti-cancer therapy (cisplatin plus oncolytic virus) proved more efficient than monotherapy in the killing of cancer cells. Results of in vivo experiments recapitulated nearly similar tumor inhibition activities.

Conclusion. This study highlighted the significant role of survivin in gene therapy as it has the potential to render CRAd more tumor specific. It also establishes that higher CAR expression plays a vital role in the success of adenovirus-based therapies. Furthermore, a careful combination of chemotherapy drugs and oncolytic viruses can culminate in significant therapeutic achievements against cancer.

Parvovirus Capsid Structures Required for Infection: Mutations Controlling Receptor Recognition and Protease Cleavages

Parvovirus capsids are small but complex molecular machines responsible for undertaking many of the steps of cell infection, genome packing, and cell-to-cell as well as host-to-host transfer. The details of parvovirus infection of cells are still not fully understood, but the processes must involve small changes in the capsid structure that allow the endocytosed virus to escape from the endosome, pass through the cell cytoplasm, and deliver the single-stranded DNA (ssDNA) genome to the nucleus, where viral replication occurs. Here, we examine capsid substitutions that eliminate canine parvovirus (CPV) infectivity and identify how those mutations changed the capsid structure or altered interactions with the infectious pathway. Amino acid substitutions on the exterior surface of the capsid (Gly299Lys/Ala300Lys) altered the binding of the capsid to transferrin receptor type 1 (TfR), particularly during virus dissociation from the receptor, but still allowed efficient entry into both feline and canine cells without successful infection. These substitutions likely control specific capsid structural changes resulting from TfR binding required for infection. A second set of changes on the interior surface of the capsid reduced viral infectivity by >100-fold and included two cysteine residues and neighboring residues. One of these substitutions, Cys270Ser, modulates a VP2 cleavage event found in ∼10% of the capsid proteins that also was shown to alter capsid stability. A neighboring substitution, Pro272Lys, significantly reduced capsid assembly, while a Cys273Ser change appeared to alter capsid transport from the nucleus. These mutants reveal additional structural details that explain cell infection processes of parvovirus capsids.

IMPORTANCE

Parvoviruses are commonly found in both vertebrate and invertebrate animals and cause widespread disease. They are also being developed as oncolytic therapeutics and as gene therapy vectors. Most functions involved in infection or transduction are mediated by the viral capsid, but the structure-function correlates of the capsids and their constituent proteins are still incompletely understood, especially in relation to identifying capsid processes responsible for infection and release from the cell. Here, we characterize the functional effects of capsid protein mutations that result in the loss of virus infectivity, giving a better understanding of the portions of the capsid that mediate essential steps in successful infection pathways and how they contribute to viral infectivity.

Synthetic hydrogel mimics of the nuclear pore complex display selectivity dependent on FG-repeat concentration and electrostatics

Synthetic hydrogels were utilized to explore influence of both charge and phenylalanine-glycine (FG) repeat concentration on translocation of select proteins. Hydrogels studied represent a biomimetic platform of the nuclear pore complex (NPC) found in eukaryotic cells. Polyacrylamide/phenylalanine-serine-phenylalanine-glycine (FSFG) peptide copolymers have previously demonstrated similar selectivity to native NPCs. Entry of a nuclear transport receptor (Impβ) into hydrogels was monitored with fluorescence microscopy and observed to be greater within gels that contained larger concentrations of FG peptide. Low-resolution structural studies of gels demonstrated changes in morphology and porous network dimensions as FG-repeat concentration was varied. Copolymerization of charged acrylates within the polyacrylamide/FSFG matrix was performed to produce charged hydrogels. Enhanced entry of Impβ, which is negatively charged, was observed in positively charged hydrogels, whereas entry was greatly diminished in negatively charged gels. Synthetic NPC mimics provide a useful testbed for further investigation of nucleocytoplasmic transport and may illuminate new routes for biomimetic separations.

Epitope Mapping of Avian Influenza M2e Protein: Different Species Recognise Various Epitopes

A common approach for developing diagnostic tests for influenza virus detection is the use of mouse or rabbit monoclonal and/or polyclonal antibodies against a target antigen of the virus. However, comparative mapping of the target antigen using antibodies from different animal sources has not been evaluated before. This is important because identification of antigenic determinants of the target antigen in different species plays a central role to ensure the efficiency of a diagnostic test, such as competitive ELISA or immunohistochemistry-based tests. Interest in the matrix 2 ectodomain (M2e) protein of avian influenza virus (AIV) as a candidate for a universal vaccine and also as a marker for detection of virus infection in vaccinated animals (DIVA) is the rationale for the selection of this protein for comparative mapping evaluation. This study aimed to map the epitopes of the M2e protein of avian influenza virus H5N1 using chicken, mouse and rabbit monoclonal or monospecific antibodies. Our findings revealed that rabbit antibodies (rAbs) recognized epitope ⁶EVETPTRN¹³ of the M2e, located at the N-terminal of the protein, while mouse (mAb) and chicken antibodies (cAbs) recognized epitope ¹⁰PTRNEWECK¹⁸, located at the centre region of the protein. The findings highlighted the difference between the M2e antigenic determinants recognized by different species that emphasized the importance of comparative mapping of antibody reactivity from different animals to the same antigen, especially in the case of multi-host infectious agents such as influenza. The findings are of importance for antigenic mapping, as well as diagnostic test and vaccine development.

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.

Methods to examine the impact of nonsynonymous SNPs on protein degradation and function of human ABC transporter

Clinical studies have strongly suggested that genetic polymorphisms and/or mutations of certain ATP-binding cassette (ABC) transporter genes might be regarded as significant factors affecting patients' responses to medication and/or the risk of diseases. In the case of ABCG2, certain single nucleotide polymorphisms (SNPs) in the encoding gene alter the substrate specificity and/or enhance endoplasmic reticulum-associated degradation (ERAD) of the de novo synthesized ABCG2 protein via the ubiquitin-mediated proteasomal proteolysis pathway. Hitherto accumulated clinical data imply that several nonsynonymous SNPs affect the ABCG2-mediated clearance of drugs or cellular metabolites, although some controversies still exist. Therefore, we recently developed high-speed functional screening and ERAD of ABC transporters so as to evaluate the effect of genetic polymorphisms on their function and protein expression levels in vitro. In this chapter we present in vitro experimental methods to elucidate the impact of nonsynonymous SNPs on protein degradation of ABCG2 as well as on its transport function.

Nucleocytoplasmic shuttling of viral proteins in borna disease virus infection

Nuclear import and export of viral RNA and proteins are critical to the replication cycle of viruses that replicate in the nucleus. Borna disease virus (BDV) is a nonsegmented, negative-strand RNA virus that belongs to the order Mononegavirales. BDV has several distinguishing features, one of the most striking being the site of its replication. BDV RNA is transcribed and replicated in the nucleus, while most other negative-strand RNA viruses replicate in the cytoplasm. Therefore, the nucleocytoplasmic trafficking of BDV macromolecules plays a key role in virus replication. Growing evidence indicates that several BDV proteins, including the nucleoprotein, phosphoprotein, protein X and large protein, contribute to the nucleocytoplasmic trafficking of BDV ribonucleoprotein (RNP). The directional control of BDV RNP trafficking is likely determined by the ratios of and interactions between the nuclear localization signals and nuclear export signals in the RNP. In this review, we present a comprehensive view of several unique mechanisms that BDV has developed to control its RNP trafficking and discuss the significance of BDV RNP trafficking in the replication cycle of BDV.

HUH site-specific recombinases for targeted modification of the human genome

Site-specific recombinases (SSRs) have been crucial in the development of mammalian transgenesis. For gene therapy purposes, this approach remains challenging, because, for example, SSR delivery is largely unresolved and SSR DNA substrates must pre-exist in target cells. In this review, we discuss the potential of His-hydrophobic-His (HUH) recombinases to overcome some of the limitations of conventional SSRs. Members of the HUH protein family cleave single-stranded (ss)DNA, but can mediate site-specific integration with the aid of the host replication machinery. Adeno-associated virus (AAV) Rep remains the only known example to support site-specific integration in human cells, and AAV is an excellent gene delivery vector that can be targeted to specific cells and organelles. Bacterial protein TrwC catalyzes integration into human sequences and can be delivered to human cells covalently linked to DNA, offering attractive new features for targeted genome modification.

Helper virus-independent transcription and multimerization of a satellite RNA associated with cucumber mosaic virus

Satellite RNAs are the smallest infectious agents whose replication is thought to be completely dependent on their helper virus (HV). Here we report that, when expressed autonomously in the absence of HV, a variant of satellite RNA (satRNA) associated with Cucumber mosaic virus strain Q (Q-satRNA) has a propensity to localize in the nucleus and be transcribed, generating genomic and antigenomic multimeric forms. The involvement of the nuclear phase of Q-satRNA was further confirmed by confocal microscopy employing in vivo RNA-tagging and double-stranded-RNA-labeling assays. Sequence analyses revealed that the Q-satRNA multimers formed in the absence of HV, compared to when HV is present, are distinguished by the addition of a template-independent heptanucleotide motif at the monomer junctions within the multimers. Collectively, the involvement of a nuclear phase in the replication cycle of Q-satRNA not only provides a valid explanation for its persistent survival in the absence of HV but also suggests a possible evolutionary relationship to viroids that replicate in the nucleus.

Baculovirus capsid display potentiates OVA cytotoxic and innate immune responses

Baculoviruses (BV) are DNA viruses that are pathogenic for insects. Although BV infect a range of mammalian cell types, they do not replicate in these cells. Indeed, the potential effects of these insect viruses on the immune responses of mammals are only just beginning to be studied. We show in this paper that a recombinant Autographa californica multiple nuclear polyhedrosis virus carrying a fragment of ovalbumin (OVA) on the VP39 capsid protein (BV-OVA) has the capacity to act as an adjuvant and vector of antigens in mice, thereby promoting specific CD4 and cytotoxic T cell responses against OVA. BV also induced in vivo maturation of dendritic cells and the production of inflammatory cytokines, thus promoting innate and adaptive immune responses. The OVA-specific response induced by BV-OVA was strong enough to reject a challenge with OVA-expressing melanoma cells (MO5 cells) and effectively prolonged survival of MO5 bearing mice. All these findings, together with the absence of pre-existing immunity to BV in humans and the lack of viral gene expression in mammalian cells, make BV a candidate for vaccination.

Detecting RNA viruses in living mammalian cells by fluorescence microscopy

Traditional methods that rely on viral isolation and culture techniques continue to be the gold standards used for detection of infectious viral particles. However, new techniques that rely on visualization of live cells can shed light on understanding virus-host interaction for early stage detection and potential drug discovery. Live-cell imaging techniques that incorporate fluorescent probes into viral components provide opportunities for understanding mRNA expression, interaction, and virus movement and localization. Other viral replication events inside a host cell can be exploited for non-invasive detection, such as single-virus tracking, which does not inhibit viral infectivity or cellular function. This review highlights some of the recent advances made using these novel approaches for visualization of viral entry and replication in live cells.

Subcellular localization of V2 protein of Tomato leaf curl Java virus by using green fluorescent protein and yeast hybrid system

Tomato leaf curl Java virus-A (ToLCJV-A[ID]) from Southeast Asia is a new member of the emerging group of monopartite begomoviruses that require a betasatellite component for symptom induction. Previously, we have elucidated the role of V1 ORF encoded by ToLCJV-A[ID] in cell-to-cell movement. In this study, the role of V2 (PreCP) in localization was determined. Subcellular localization of ToLCJV-A[ID] V2 in plant tissues showed that this protein is co-localized to the cell cytoplasm, perinuclear and associated with the endoplasmic reticulum network. The results obtained from deletion analysis indicate that fusion of N-terminal part of the V2, containing the nuclear export signals (NES), directed the accumulation of fluorescence towards the cell cytoplasm. Furthermore, functionality of the NES ((20)LAVKYLQLV(29)) in the N-terminal part of the V2 protein was confirmed by one-hybrid yeast system. Taken together, these results suggest that V2 enhances the coat protein-mediated nuclear export of ToLCJV-A[ID] and is consistent with the model in which V2 mediates viral DNA export from the nucleus to the plasmodesmata.

Role of the CM2 protein in the influenza C virus replication cycle

CM2 is the second membrane protein of influenza C virus. Although its biochemical characteristics, coding strategy, and properties as an ion channel have been extensively studied, the role(s) of CM2 in the virus replication cycle remains to be clarified. In order to elucidate this role, in the present study we generated CM2-deficient influenza C virus-like particles (VLPs) and examined the VLP-producing 293T cells, VLPs, and VLP-infected HMV-II cells. Quantification of viral RNA (vRNA) in the VLPs by real-time PCR revealed that the CM2-deficient VLPs contain approximately one-third of the vRNA found in wild-type VLPs although no significant differences were detected in the expression levels of viral components in VLP-producing cells or in the number and morphology of the generated VLPs. This finding suggests that CM2 is involved in the genome packaging process into VLPs. Furthermore, HMV-II cells infected with CM2-deficient VLPs exhibited significantly reduced reporter gene expression. Although CM2-deficient VLPs could be internalized into HMV-II cells as efficiently as wild-type VLPs, a smaller amount of vRNA was detected in the nuclear fraction of CM2-deficient VLP-infected cells than in that of wild-type VLP-infected cells, suggesting that the uncoating process of the CM2-deficient VLPs in the infected cells did not proceed in an appropriate manner. Taken together, the data obtained in the present study indicate that CM2 has a potential role in the genome packaging and uncoating processes of the virus replication cycle.

Nuclear localization of Beet black scorch virus capsid protein and its interaction with importin α

Beet black scorch virus (BBSV) is a positive-sense, single-stranded RNA virus belonging to Necrovirus genus. In order to better understand the life cycle of BBSV, we have investigated the subcellular localization of BBSV capsid protein (CP) by its fusion with green fluorescent protein (GFP) agroinfiltrated into Nicotiana benthamiana leaves and by particle bombardment into onion (Allium cepa) epidermal cells. Confocal laser scanning microscopy (CLSM) showed that BBSV CP fused to GFP displayed enhanced fluorescence in nuclei and nuclear import of the CP was confirmed in BBSV-infected N. benthamiana leaves. Mutational analysis revealed that the N-terminal basic amino acid cluster (4)KRNKGGKKSR(13) of the CP is essential for nuclear localization. Bimolecular fluorescence complementation (BiFC) assays indicated that the CP could interact with the nuclear import factor importin α, suggesting that the CP is possibly imported into the nucleus via an importin α-dependent pathway. This is the first report of the nuclear localization of the CP encoded by a necrovirus.

Effect of nuclear localization and hydrodynamic delivery-induced cell division on phiC31 integrase activity

Phage φC31 integrase is a recombinase that can be expressed in mammalian cells to integrate plasmids carrying an attB sequence into the genome at specific pseudo attP locations. We demonstrate by immunofluoresence that wild-type φC31 integrase is cytoplasmic and that addition of a SV40 nuclear localization signal (NLS) localizes φC31 integrase to the nucleus. Unexpectedly, the NLS depressed integration efficiency in HeLa cells and provided no benefit when used to integrate the human Factor IX (hFIX) gene into mouse liver. Since breakdown of the nuclear membrane during mitosis could allow cytoplasmic integrase access to the chromosomes, we analyzed whether cell division was required for integration into liver cells in vivo. Hepatocytes were labeled with iododeoxyuridine to mark cells that underwent DNA replication during the week following hydrodynamic injection. Hydrodynamic delivery led to DNA replication in one-third of hepatocytes. Approximately 3 out of 4 cells having φC31 integrase-mediated stable hFIX expression did not undergo replication, indicating that cell division was not required for integrase function in liver. Therefore, although the bulk of φC31 integrase protein appears to be cytoplasmic in mammalian cells, integration can still occur in the nucleus, even without cell division.

Virus entry paradigms

Viruses, despite being relatively simple in structure and composition, have evolved to exploit complex cellular processes for their replication in the host cell. After binding to their specific receptor on the cell surface, viruses (or viral genomes) have to enter cells to initiate a productive infection. Though the entry processes of many enveloped viruses is well understood, that of most non-enveloped viruses still remains unresolved. Recent studies have shown that compared to direct fusion at the plasma membrane, endocytosis is more often the preferred means of entry into the target cell. Receptor-mediated endocytic pathways such as the dynamin-dependent clathrin and caveolar pathways are well characterized as viral entry portals. However, many viruses are able to utilize multiple uptake pathways. Fluid phase uptake, though relatively non-specific in terms of its cargo, potentially aids viral infection by its ability to intersect with the endocytic pathway. In fact, many viruses despite using specialized pathways for entry are still able to generate productive infection via fluid phase uptake. Macropinocytosis, a major fluid uptake pathway found in epithelial cells and fibroblasts, is stimulated by growth factor receptors. Many viruses can induce these signaling cascades in cells leading to macropinocytosis. Though endocytic trafficking is utilized by both enveloped and non-enveloped viruses, key differences lie in the way membranes are traversed to deposit the viral genome at its site of replication. This review will discuss recent developments in the rapidly evolving field of viral entry.

Technical pitfalls and improvements for high-speed screening and QSAR analysis to predict inhibitors of the human bile salt export pump (ABCB11/BSEP)

Drug-induced hepatotoxicity is one of the major problems encountered in drug discovery and development. Selection of a candidate compound for pre-clinical studies in the drug discovery process is a critical step that can determine the speed and expenditure of clinical development. Because inhibition of human adenosine triphosphate-binding cassette transporter ABCB11 (SPGP/bile salt export pump) has severe consequences, which include intrahepatic cholestasis and hepatotoxicity, resulting from exposure to toxic xenobiotics or drug interactions, in vitro screening methods are necessary for quantifying and characterizing the inhibition of ABCB11. In line with such initiatives, we developed methods for in vitro high-speed screening and quantitative structure-activity relationship (QSAR) analysis to investigate the interaction of ABCB11 with a variety of compounds. We identified one set of chemical fragmentation codes closely linked with inhibition of ABCB11. Furthermore, the high-speed screening method enables us to analyze the kinetics of ABCB11-inhibition by test compounds and to distinguish competitive and non-competitive inhibitors. Troglitazone and novobiocin were found to be competitive inhibitors to taurocholate, whereas porphyrins were non-competitive inhibitors. Kinetics-based classification of inhibitors is considered important to improve the accuracy of our QSAR analysis. The present mini-review addresses technical pitfalls and improvements for high-speed screening and QSAR analysis in the ABCB11 inhibition study.

Tissue-specific and transcription factor-mediated nuclear entry of DNA

Low levels of gene transfer and a lack of tissue-specific targeting of vectors have limited the therapeutic potential of non-viral gene therapy. This is due to the numerous cellular barriers that hinder nuclear delivery of vectors and the paucity of methods that restrict expression to specific cells types. In non-dividing cells, the nuclear envelope is an especially problematic hurdle to gene transfer. Given that the majority of target tissues are non-dividing in vivo, the nuclear membrane is a major obstacle to therapeutic gene transfer. In this review, the various barriers to gene transfer are discussed. In particular, the role of the nuclear pore complex (NPC) in regulating passage of plasmid vectors during interphase is reviewed. Several methods of modifying plasmid (pDNA) vectors to enhance nuclear import through the NPC are also discussed, including the use of tissue-specific transcription factors to mediate nuclear entry of pDNA in a cell-specific manner.

Bcl-2 expression and p38MAPK activity in cells infected with influenza A virus: impact on virally induced apoptosis and viral replication

Previous reports have shown that various steps in the influenza A virus life cycle are impaired in cells expressing the antiapoptotic protein Bcl-2 (Bcl-2(+) cells). We demonstrated a direct link between Bcl-2 and the reduced nuclear export of viral ribonucleoprotein (vRNP) complexes in these cells. However, despite its negative impact on viral replication, Bcl-2 did not prevent host cells from undergoing virally triggered apoptosis. The protein's reduced antiapoptotic capacity was related to phosphorylation of its threonine 56 and serine 87 residues by virally activated p38MAPK. In infected Bcl-2(+) cells, activated p38MAPK was found predominantly in the cytoplasm, colocalized with Bcl-2, and both Bcl-2 phosphorylation and virally induced apoptosis were diminished by specific inhibition of p38MAPK activity. In contrast, in Bcl-2-negative (Bcl-2(-)) cells, which are fully permissive to viral infection, p38MAPK activity was predominantly nuclear, and its inhibition decreased vRNP traffic, phosphorylation of viral nucleoprotein, and virus titers in cell supernatants, suggesting that this kinase also contributes to the regulation of vRNP export and viral replication. This could explain why in Bcl-2(+) cells, where p38MAPK is active in the cytoplasm, phosphorylating Bcl-2, influenza viral replication is substantially reduced, whereas apoptosis proceeds at rates similar to those observed in Bcl-2(-) cells. Our findings suggest that the impact of p38MAPK on the influenza virus life cycle and the apoptotic response of host cells to infection depends on whether or not the cells express Bcl-2, highlighting the possibility that the pathological effects of the virus are partly determined by the cell type it targets.

Characterization of signals that dictate nuclear/nucleolar and cytoplasmic shuttling of the capsid protein of Tomato leaf curl Java virus associated with DNA beta satellite

Transport of the viral genome into the nucleus is an obligatory step in the replication cycle of geminiviruses. Capsid proteins (CPs) of geminiviruses are multifunctional proteins thought to be involved in this process. The CP of monopartite geminiviruses is absolutely essential for virus movement. To more precisely examine the role of CP, we have constructed a series of single and double deletions into the coding sequence of Tomato leaf curl Java virus (ToLCJAV) CP and examined sub-cellular localization using transient expression of GFP fusion proteins. In this report, the domains of the CP encoded by ToLCJAV localized in the nucleus/nucleolus and cytoplasm in transfected cells were mapped. Deletion analysis revealed that the Arg-rich cluster from amino acids (aa) (16)KVRRR(20) in the N-terminal region of CP functioned as nuclear/nucleolar localization signals (NLSs). The region from aa (52)RKPR(55) contained basic amino acid cluster was capable to redirect the CP to the nucleus. Further, both transient expression and yeast hybrid assays demonstrated that CP was capable of shuttling between the nucleus and cytoplasm of the cell. Deletion mutant analysis revealed that this property was attributed to a nuclear export signal (NES) sequence consisted of aa ((245)LKIRIY(250)) reside at C-terminal part of CP. This hydrophobic region caused transport of GFP to the cytoplasm. However, ToLCJAV CP NLSs and NES show peculiarities in the number and position of basic residues. Taken together, these results demonstrated that ToLCJAV CP shuttles between the nucleus and cytoplasm, such an activity homolog to bipartite geminivirus BV1 ORF.

Genetic evidence for a connection between Rous sarcoma virus gag nuclear trafficking and genomic RNA packaging

The packaging of retroviral genomic RNA (gRNA) requires cis-acting elements within the RNA and trans-acting elements within the Gag polyprotein. The packaging signal psi, at the 5' end of the viral gRNA, binds to Gag through interactions with basic residues and Cys-His box RNA-binding motifs in the nucleocapsid. Although specific interactions between Gag and gRNA have been demonstrated previously, where and when they occur is not well understood. We discovered that the Rous sarcoma virus (RSV) Gag protein transiently localizes to the nucleus, although the roles of Gag nuclear trafficking in virus replication have not been fully elucidated. A mutant of RSV (Myr1E) with enhanced plasma membrane targeting of Gag fails to undergo nuclear trafficking and also incorporates reduced levels of gRNA into virus particles compared to those in wild-type particles. Based on these results, we hypothesized that Gag nuclear entry might facilitate gRNA packaging. To test this idea by using a gain-of-function genetic approach, a bipartite nuclear localization signal (NLS) derived from the nucleoplasmin protein was inserted into the Myr1E Gag sequence (generating mutant Myr1E.NLS) in an attempt to restore nuclear trafficking. Here, we report that the inserted NLS enhanced the nuclear localization of Myr1E.NLS Gag compared to that of Myr1E Gag. Also, the NLS sequence restored gRNA packaging to nearly wild-type levels in viruses containing Myr1E.NLS Gag, providing genetic evidence linking nuclear trafficking of the retroviral Gag protein with gRNA incorporation.

Expression or subcellular targeting of virus capsid proteins with cloning genome of a canine parvovirus from China

A strain of canine parvovirus (CPV), designated B2004, was isolated from the stool of a sick dog in Beijing. The partial genome (4623bp) was cloned, sequenced with sequence showing B2004 to be a member of the widely distributed CPV-2a subclade. A completed VP2 or 11-residue N-terminal peptide (MAPPAKRARRG) of VP1 from B2004 was also tested for its ability to mediate nuclear transport of a heterologous protein, in this case enhanced green fluorescence protein (EGFP). EGFP was detected in the nucleus when it fused with the VP1 peptide; it was distributed primarily in the nucleus and also in the cytoplasm either when it fused with VP2, or in the cytoplasm when expressed on its own. In common with other parvoviruses the CPV VP1 N-terminal peptide contributes to the nuclear localization of the gene product.

Kaposi's sarcoma-associated herpesvirus utilizes an actin polymerization-dependent macropinocytic pathway to enter human dermal microvascular endothelial and human umbilical vein endothelial cells

Kaposi's sarcoma-associated herpesvirus (KSHV) utilizes clathrin-mediated endocytosis for its infectious entry into human foreskin fibroblast (HFF) cells (S. M. Akula, P. P. Naranatt, N.-S. Walia, F.-Z. Wang, B. Fegley, and B. Chandran, J. Virol. 77:7978-7990, 2003). Here, we characterized KSHV entry into primary human microvascular dermal endothelial (HMVEC-d) and human umbilical vein endothelial (HUVEC) cells. Similar to the results for HMVEC-d cells, KSHV infection of HUVEC cells also resulted in an initial high level and subsequent decline in the expression of the lytic switch gene, ORF50, while latent gene expression persisted. Internalized virus particles enclosed in irregular vesicles were observed by electron microscopy of infected HMVEC-d cells. At an early time of infection, colocalization of KSHV capsid with envelope was observed by immunofluorescence analysis, thus demonstrating endocytosis of intact enveloped virus particles. Chlorpromazine, an inhibitor of clathrin-mediated endocytosis, and filipin (C(35)H(58)O(11)), a caveolar endocytosis inhibitor, did not have any effect on KSHV binding, entry (DNA internalization), or gene expression in HMVEC-d and HUVEC cells. In contrast to the results for HFF cells, virus entry and gene expression in both types of endothelial cells were significantly blocked by macropinocytosis inhibitors (EIPA [5-N-ethyl-N-isoproamiloride] and rottlerin [C(30)H(28)O(8)]) and by cytochalasin D, which affects actin polymerization. Inhibition of lipid raft blocked viral gene expression in HMVEC-d cells but not in HUVEC or HFF cells. In HMVEC-d and HUVEC cells, KSHV induced the actin polymerization and formation of lamellipodial extensions that are essential for macropinocytosis. Inhibition of macropinocytosis resulted in the distribution of viral capsids at the HMVEC-d cell periphery, and capsids did not associate with microtubules involved in the nuclear delivery of viral DNA. Internalized KSHV in HMVEC-d and HUVEC cells colocalized with the macropinocytosis marker dextran and not with the clathrin pathway marker transferrin or with caveolin. Dynasore, an inhibitor of dynamin, did not block viral entry into endothelial cells but did inhibit entry into HFF cells. KSHV was not associated with the early endosome marker EEA-1 in HMVEC-d cells, but rather with the late endosome marker LAMP1, as well as with Rab34 GTPase that is known to regulate macropinocytosis. Silencing Rab34 with small interfering RNA dramatically inhibited KSHV gene expression. Bafilomycin-mediated disruption of endosomal acidification inhibited viral gene expression. Taken together, these findings suggest that KSHV utilizes the actin polymerization-dependent, dynamin-independent macropinocytic pathway that involves a Rab34 GTPase-dependent late endosome and low-pH environment for its infectious entry into HMVEC-d and HUVEC cells. These studies also demonstrate that KSHV utilizes different modes of endocytic entry in fibroblast and endothelial cells.