Synthesis of infective poliovirus in BSC-1 monkey cells enucleated with cytochalasin B

Nucleocytoplasmic Trafficking Perturbation Induced by Picornaviruses

Picornaviruses are positive-stranded RNA viruses. Even though replication and translation of their genome take place in the cytoplasm, these viruses evolved different strategies to disturb nucleocytoplasmic trafficking of host proteins and RNA. The major targets of picornavirus are the phenylalanine-glycine (FG)-nucleoporins, which form a mesh in the central channel of the nuclear pore complex through which protein cargos and karyopherins are actively transported in both directions. Interestingly, while enteroviruses use the proteolytic activity of their 2A protein to degrade FG-nucleoporins, cardioviruses act by triggering phosphorylation of these proteins by cellular kinases. By targeting the nuclear pore complex, picornaviruses recruit nuclear proteins to the cytoplasm, where they increase viral genome translation and replication; they affect nuclear translocation of cytoplasmic proteins such as transcription factors that induce innate immune responses and retain host mRNA in the nucleus thereby preventing cell emergency responses and likely making the ribosomal machinery available for translation of viral RNAs.

Exploitation of nuclear functions by human rhinovirus, a cytoplasmic RNA virus

Protein production, genomic RNA replication, and virion assembly during infection by picornaviruses like human rhinovirus and poliovirus take place in the cytoplasm of infected human cells, making them the quintessential cytoplasmic pathogens. However, a growing body of evidence suggests that picornavirus replication is promoted by a number of host proteins localized normally within the host cell nucleus. To systematically identify such nuclear proteins, we focused on those that appear to re-equilibrate from the nucleus to the cytoplasm during infection of HeLa cells with human rhinovirus via quantitative protein mass spectrometry. Our analysis revealed a highly selective re-equilibration of proteins with known mRNA splicing and transport-related functions over nuclear proteins of all other functional classes. The multifunctional splicing factor proline and glutamine rich (SFPQ) was identified as one such protein. We found that SFPQ is targeted for proteolysis within the nucleus by viral proteinase 3CD/3C, and a fragment of SFPQ was shown to migrate to the cytoplasm at mid-to-late times of infection. Cells knocked down for SFPQ expression showed significantly reduced rhinovirus titers, viral protein production, and viral RNA accumulation, consistent with SFPQ being a pro-viral factor. The SFPQ fragment that moved into the cytoplasm was able to bind rhinovirus RNA either directly or indirectly. We propose that the truncated form of SFPQ promotes viral RNA stability or replication, or virion morphogenesis. More broadly, our findings reveal dramatic changes in protein compartmentalization during human rhinovirus infection, allowing the virus to systematically hijack the functions of proteins not normally found at its cytoplasmic site of replication.

We explored the dynamics of host cell protein relocalization from the nucleus to the cytoplasm during an infection by human rhinovirus using quantitative mass spectrometry, confocal imaging, and Western blot analysis. We discovered a highly selective re-equilibration of proteins with known mRNA splicing and transport-related functions, including splicing factor proline and glutamine rich (SFPQ). Using RNAi experiments and viral replication assays, we demonstrated that SFPQ is a pro-viral factor required for rhinovirus growth. Our studies provide new insights into how this cytoplasmic RNA virus is able to alter and hijack the functions of host proteins that normally reside in the nucleus.

Picornaviruses and nuclear functions: targeting a cellular compartment distinct from the replication site of a positive-strand RNA virus

The compartmentalization of DNA replication and gene transcription in the nucleus and protein production in the cytoplasm is a defining feature of eukaryotic cells. The nucleus functions to maintain the integrity of the nuclear genome of the cell and to control gene expression based on intracellular and environmental signals received through the cytoplasm. The spatial separation of the major processes that lead to the expression of protein-coding genes establishes the necessity of a transport network to allow biomolecules to translocate between these two regions of the cell. The nucleocytoplasmic transport network is therefore essential for regulating normal cellular functioning. The Picornaviridae virus family is one of many viral families that disrupt the nucleocytoplasmic trafficking of cells to promote viral replication. Picornaviruses contain positive-sense, single-stranded RNA genomes and replicate in the cytoplasm of infected cells. As a result of the limited coding capacity of these viruses, cellular proteins are required by these intracellular parasites for both translation and genomic RNA replication. Being of messenger RNA polarity, a picornavirus genome can immediately be translated upon entering the cell cytoplasm. However, the replication of viral RNA requires the activity of RNA-binding proteins, many of which function in host gene expression, and are consequently localized to the nucleus. As a result, picornaviruses disrupt nucleocytoplasmic trafficking to exploit protein functions normally localized to a different cellular compartment from which they translate their genome to facilitate efficient replication. Furthermore, picornavirus proteins are also known to enter the nucleus of infected cells to limit host-cell transcription and down-regulate innate antiviral responses. The interactions of picornavirus proteins and host-cell nuclei are extensive, required for a productive infection, and are the focus of this review.

Delayed kinetics of poliovirus RNA synthesis in a human cell line with reduced levels of hnRNP C proteins

The hnRNP C heterotetramer [(C1₃)C2] binds RNA polymerase II transcripts in the nucleus, along with other proteins of the core hnRNP complex, and plays an important role in mRNA biogenesis and transport. Infection of HeLa cells with poliovirus causes hnRNP C to re-localize from the nucleus, where it is normally retained during interphase, to the cytoplasm. We have proposed that in the cytoplasm, the protein isoforms of hnRNP C participate in the recognition of viral specific RNAs by the poliovirus replication proteins and/or in the assembly of membrane-bound RNA replication complexes. In SK-OV-3 cells, which express reduced levels of hnRNP C compared to HeLa cells or 293 cells, the kinetics of poliovirus replication are delayed. hnRNP C is also re-localized from the nucleus to the cytoplasm in SK-OV-3 cells infected with poliovirus. Increased expression of hnRNP C in SK-OV-3 cells by transient transfection increases the rate of virus production and overall yield over that seen in mock-transfected cells. We propose that hnRNP C interacts with poliovirus RNA and replication proteins to increase the efficiency of viral genomic RNA synthesis.

Nucleocytoplasmic traffic disorder induced by cardioviruses

Some picornaviruses, for example, poliovirus, increase bidirectional permeability of the nuclear envelope and suppress active nucleocytoplasmic transport. These activities require the viral protease 2A(pro). Here, we studied nucleocytoplasmic traffic in cells infected with encephalomyocarditis virus (EMCV; a cardiovirus), which lacks the poliovirus 2A(pro)-related protein. EMCV similarly enhanced bidirectional nucleocytoplasmic traffic. By using the fluorescent "Timer" protein, which contains a nuclear localization signal, we showed that the cytoplasmic accumulation of nuclear proteins in infected cells was largely due to the nuclear efflux of "old" proteins rather than impaired active nuclear import of newly synthesized molecules. The nuclear envelope of digitonin-treated EMCV-infected cells permitted rapid efflux of a nuclear marker protein. Inhibitors of poliovirus 2A(pro) did not prevent the EMCV-induced efflux. Extracts from EMCV-infected cells and products of in vitro translation of viral RNAs contained an activity increasing permeability of the nuclear envelope of uninfected cells. This activity depended on the expression of the viral leader protein. Mutations disrupting the zinc finger motif of this protein abolished its efflux-inducing ability. Inactivation of the L protein phosphorylation site (Thr47-->Ala) resulted in a delayed efflux, while a phosphorylation-mimicking (Thr47-->Asp) replacement did not significantly impair the efflux-inducing ability. Such activity of extracts from EMCV-infected cells was suppressed by the protein kinase inhibitor staurosporine. As evidenced by electron microscopy, cardiovirus infection resulted in alteration of the nuclear pores, but it did not trigger degradation of the nucleoporins known to be degraded in the poliovirus-infected cells. Thus, two groups of picornaviruses, enteroviruses and cardioviruses, similarly alter the nucleocytoplasmic traffic but achieve this by strikingly different mechanisms.

Bidirectional increase in permeability of nuclear envelope upon poliovirus infection and accompanying alterations of nuclear pores

Poliovirus and some other picornaviruses trigger relocation of certain nuclear proteins into the cytoplasm. Here, by using a protein changing its fluorescence color with time and containing a nuclear localization signal (NLS), we demonstrate that the poliovirus-triggered relocation is largely due to the exit of presynthesized nuclear protein into the cytoplasm. The leakiness of the nuclear envelope was also documented by the inability of nuclei from digitonin-permeabilized, virus-infected (but not mock-infected) cells to retain an NLS-containing derivative of green fluorescent protein (GFP). The cytoplasm-to-nucleus traffic was also facilitated during infection, as evidenced by experiments with GAPDH (glyceraldehyde-3-phosphate dehydrogenase), cyclin B1, and an NLS-lacking derivative of GFP, which are predominantly cytoplasmic in uninfected cells. Electron microscopy demonstrated that a bar-like barrier structure in the channel of the nuclear pores, seen in uninfected cells, was missing in the infected cells, giving the impression of fully open pores. Transient expression of poliovirus 2A protease also resulted in relocation of the nuclear proteins. Lysates from poliovirus-infected or 2A-expressing cells induced efflux of 3xEGFP-NLS from the nuclei of permeabilized uninfected cells. This activity was inhibited by the elastase inhibitors elastatinal and N-(methoxysuccinyl)-L-alanyl-L-alanyl-L-prolyl-L-valine chloromethylketone (drugs known also to be inhibitors of poliovirus protease 2A), a caspase inhibitor zVAD(OMe), fmk, and some other protease inhibitors. These data suggest that 2A elicited nuclear efflux, possibly in cooperation with a zVAD(OMe).fmk-sensitive protease. However, poliovirus infection facilitated nuclear protein efflux also in cells deficient in caspase-3 and caspase-9, suggesting that the efflux may occur without the involvement of these enzymes. The biological relevance of nucleocytoplasmic traffic alterations in infected cells is discussed.

Early alteration of nucleocytoplasmic traffic induced by some RNA viruses

A HeLa cell line expressing the green fluorescent protein fused to the SV40 T-antigen nuclear localization signal (EGFP-NLS) was established. Fluorescence in these cells was confined to the nuclei. After poliovirus infection, cytoplasmic fluorescence in a proportion of cells could be detected by 1 h postinfection (p.i.) and in virtually all of the fluorescent cells by 2 h p.i. The relocation could be prevented by cycloheximide but not by inhibition of poliovirus replication by guanidine. HCl. Nuclear exit of a protein composed of three copies of GFP fused to the NLS also occurred upon poliovirus infection. A similar redistribution of EGFP-NLS took place upon infection with coxsakievirus B3 and, to a lesser extent, with vesicular stomatitis virus. The EGFP-NLS efflux was not due to the loss of NLS. Thus, some positive-strand and negative-strand RNA viruses trigger a rapid nonspecific relocation of nuclear proteins.

Intracellular membrane proliferation in E. coli induced by foot-and-mouth disease virus 3A gene products

During picornavirus infection replication of genomic RNA occurs in membrane-associated ribonucleoprotein complexes. These replication complexes contain different nonstructural viral proteins with mostly unknown function. To examine the function of nonstructural picornaviral proteins in more detail, cDNA of foot-and-mouth-disease virus (FMDV) strain O1 Lausanne was cloned into lambda ZAP II, and different parts of the P3-coding sequence were expressed in E. coli by the T7 polymerase system. Expression products constituted (a) fusion proteins composed of N-terminal leader peptide of bacteriophage T7 phi 10 protein fused to FMDV P3-sequences of different lengths, (b) translation products of authentic P3-region genes, and (c) carboxy-terminally truncated 3A proteins. Expression products were characterized by NaDodSO4-polyacrylamide gel electrophoresis, immunoblotting, as well as electron and immunoelectron microscopy. We show here that in the T7 polymerase system a high level of expression of 3A-containing peptides is achieved in E. coli. Remarkably, the expression of 3A-derived proteins induced a dramatic intracellular membrane proliferation in E. coli cells, similar to the vesicle induction observed in FMDV-infected cells. By immunoelectron microscopy, 3A-reactive material was found associated with these membranes. We hypothesize that the FMDV 3A protein is instrumental in eliciting intracellular membrane proliferation in infected cells as a prerequisite for viral RNA replication.

Autoregulation of tubulin synthesis in enucleated cells

The effects on tubulin messenger RNA levels and tubulin protein synthesis of treating cells with microtubule-depolymerizing drugs or directly microinjecting cells with tubulin have suggested that non-polymerized tubulin depresses its own synthesis. The precise level of this control is unclear. It has been shown that enucleated cells, termed cytoplasts, retain many properties of the original cell, including maintenance of cell shape, pinocytic activity and locomotion as well as biosynthetic activities such as protein synthesis and replication of cytoplasmic viruses. Furthermore, cytoplasts retain most of the components of the cytoskeleton including the centrioles. If cytoplasmic activities alone are responsible for regulating tubulin biosynthesis, cytoplasts should contain the necessary components. To distinguish between regulation which would occur in the nucleus, that is, alterations in mRNA synthesis or modifications of the mRNA, from alterations in mRNA stability and/or translatability which would take place in the cytoplasm, we examined the autoregulation of tubulin synthesis in enucleated cells. Here, we report that enucleated mouse fibroblasts retain the ability to turn off tubulin protein synthesis in response to microtubule depolymerization, the reduction in tubulin synthesis being accompanied by a corresponding decrease in tubulin mRNA levels. Thus, transcription, processing and transport of tubulin mRNA from the nucleus are not likely to be the loci of regulation. Instead, tubulin must reduce, either directly or indirectly, the translatability of its own mRNA.

Intranuclear crystal formation in picornavirus-infected cells

Cells infected with echovirus or poliovirus were incubated at a suboptimal temperature (28 degrees C). When the cells were examined by electron microscopy, not only were crystalline arrays of mature virus particles observed in the cytoplasm but crystals composed of immature virus-like particles were also evident in the less electron-dense central region of the nucleus. Immunofluorescent and autoradiographic studies revealed the presence of viral proteins and RNA in the nucleus. These findings strongly suggest that the intranuclear crystals are composed of incomplete virus particles and that crystal formation at such a low temperature may be a remarkable feature common to picornaviruses. Possible mechanisms for the intranuclear synthesis of viral constituents are discussed.

The regeneration and division of mouse L-cell karyoplasts

A technique for efficient cytochalasin-induced enucleation was used to prepare "karyoplasts"--nuclei surrounded by a thin shell of cytoplasm and an outer cell membrane. Methods for estimating the quantity of cytoplasm remaining in karyoplasts indicated that they contained less than 10% of the amount found in whole cells. Procedures for separating karyoplasts from contaminating cytoplasmic fragments and whole cells are also described. Freshly prepared L-cell karyoplasts were unable to adhere to and spread upon a surface. However, after incubation for several days, about 30% of the karyoplasts regained these abilities to some degree. A portion of the regenerating karyoplasts were then observed to divide. These events were confirmed and recorded by time-lapse cinematography. In addition, by culturing karyoplasts under appropriate conditions, clones were isolated.

Rhabdovirus replication in enucleated host cells

Infection of enucleated TC-7 monkey cells with rabies virus resulted in the synthesis of virus-directed RNA and the production of rabies antigens but not of infectious virus. The yield of infectious vesicular stomatitis virus from enucleated TC-7 cells, on the other hand, was almost as high as that from intact cells. Inhibition of the mitochondrial functions of enucleated cells by treatment with ethidium bromide did not influence the development of rabies antigens or the production of infectious vesicular stomatitis virus.

The surface morphology and fine structure of CHO (Chinese hamster ovary) cells following enucleation

Chinese hamster ovary cells grown in monolayer culture and exposed to cytochalasin B were enucleated by centrifugation. Thereafter, the karyoplasts (the nucleated parts obtained from the bottoms of the centrifuge tubes) and the cytoplasts (the enucleated cytoplasmic parts attached to the coverslips) were allowed to recover and subsequently were examined by scanning and transmission electron microscopy. Microscopy of thin sections revealed that the karyoplasts, limited by an intact plasma membrane, contain an intact nucleus surrounded by a layer of cytoplasm that includes ribosomes, mitochondria, and fragments of the endoplasmic reticulum, but no centrioles or microtubules. The cytoplasts, similarly examined, appear to contain all cytoplasmic organelles and systems, including centrioles and microtubules. The karyoplasts, when replated in fresh medium adhere to the substrate but remain essentially spherical and are incapable of motility. They disintegrate in about 72 hr. The cytoplasts, under identical conditions, recover a shape similar to that of the whole Chinese hamster ovary cell and display some motility. They generally survive not more than 48 hr. It appears that this enucleation procedure consistently separates the nucleus and limited cytoplasm from the centrosphere and microtubule-containing cytoplasts and, furthermore, that the formdetermining and motility mechanisms reside in the cytoplast and function without nuclear participation for the short period of viability.

Reconstruction of mammalian cells from nuclear and cytoplasmic components separated by treatment with cytochalasin B

Mouse L929 cells were separated into enucleated cytoplasmic components (cytoplasts) and nucleated subcellular fractions (karyoplasts) in the presence of cytochalasin B. Karyoplasts from cells containing tritiated nuclei were fused, using inactivated Sendai virus, to cytoplasts from cells containing large (1.0-mum diameter) latex spheres in the cytoplasm. Mononucleated cells containing radioactive nuclei and large latex spheres in the cytoplasm were observed among the products of the fusion reaction. Some of these cells were in mitotic configurations. The results indicate that cells capable of undergoing mitosis can be reconstructed from the products of cellular enucleation in the presence of cytochalasin B.

Establishment and maintenance of the interferon-induced antiviral state: studies in enucleated cells

Requirements for the physical presence of the cell's nucleus for the establishment and maintenance of the interferon-induced antiviral state were investigated. Enucleated chicken embryo fibroblasts were obtained by cytochalasin B treatment during centrifugation. The inhibition of vaccinia virus cytoplasmic DNA synthesis, monitored by autoradiography, was used to measure the antiviral activity resulting from interferon treatment. The antiviral state is not established in cells treated with interferon after removal of their nuclei. On the other hand, cells first treated with interferon for 6 or 12 h and then enucleated express the antiviral state. Furthermore, the antiviral state is maintained in enucleated cells for 16 h after enucleation. The antiviral state appears to be more stable in enucleates than in the residual nucleated cells found in the same cultures. Single cells of antiviral populations are found to be either fully permissive or fully restrictive to vaccinia DNA synthesis. The effect of an increasing intracellular multiplicity of infectious virus is to overcome the antiviral cell's block against viral DNA synthesis.

Virus replication in enucleate cells: vesicular stomatitis virus and influenza virus

The requirement of the presence of a nucleus for the replication of vesicular stomatitis virus and influenza virus has been examined by following the growth and development of these viruses in enucleate BS-C-1 cells. Vesicular stomatitis virus replicates normally in enucleate cells with the rate of production of infectious virus, the amount of virus-specific protein synthesis, and the type of proteins produced being essentially the same in nucleate and enucleate cells. Influenza virus does not replicate in enucleate cells, no virus gene products can be detected, and there is no inhibition of cellular protein synthesis.

Vaccinia virus replication in enucleate BSC-1 cells: particle production and synthesis of viral DNA and proteins

The growth of vaccinia virus in monolayers of BSC-1 cells enucleated by centrifugation in the presence of cytochalasin B has been studied. No evidence for the production of infectious virus in these cells was obtained, and the production of virus particles was reduced to 8.3% compared with the yield from cytochalasin-treated, uncentrifuged cells. Virus DNA and early and late polypeptides were synthesized with normal timing in enucleate cells, but in reduced amounts; cleavage of structural polypeptide precursors P4a and Px also occurred in enucleate cells. Factories containing immature virus particles were demonstrated in enucleate cells by electron microscopy; these factories were reduced in number and size compared with those found in cytochalasin-treated, uncentrifuged cells.