Cytomegalovirus recruitment of cellular kinases to dissolve the nuclear lamina

Cellular stress and signal transduction responses to human cytomegalovirus infection

Human cytomegalovirus (HCMV) receptor-ligand interactions and viral entry excite cellular responses such as receptor tyrosine kinase and mitogen-activated protein kinase signaling, cytoskeletal rearrangement, and the induction of transcription factors, prostaglandins, and cytokines. Bi-phasic stimulation of these pathways, excepting interferon, facilitates productive viral infection and likely contributes to viral pathogenesis.

Redistribution of cellular and herpes simplex virus proteins from the trans-golgi network to cell junctions without enveloped capsids

Herpes simplex virus (HSV) and other alphaherpesviruses assemble enveloped virions in the trans-Golgi network (TGN) or endosomes. Enveloped particles are formed when capsids bud into TGN/endosomes and virus particles are subsequently ferried to the plasma membrane in TGN-derived vesicles. Little is known about the last stages of virus egress from the TGN/endosomes to cell surfaces except that the HSV directs transport of nascent virions to specific cell surface domains, i.e., epithelial cell junctions. Previously, we showed that HSV glycoprotein gE/gI accumulates extensively in the TGN at early times after infection and also when expressed without other viral proteins. At late times of infection, gE/gI and a cellular membrane protein, TGN46, were redistributed from the TGN to epithelial cell junctions. We show here that gE/gI and a second glycoprotein, gB, TGN46, and another cellular protein, carboxypeptidase D, all moved to cell junctions after infection with an HSV mutant unable to produce cytoplasmic capsids. This redistribution did not involve L particles. In contrast to TGN membrane proteins, several cellular proteins that normally adhere to the cytoplasmic face of TGN, Golgi, and endosomal membranes remained primarily dispersed throughout the cytoplasm. Therefore, cellular and viral membrane TGN proteins move to cell junctions at late times of HSV infection when the production of enveloped particles is blocked. This is consistent with the hypothesis that there are late HSV proteins that reorganize or redistribute TGN/endosomal compartments to promote virus egress and cell-to-cell spread.

Nuclear export of the nonenveloped parvovirus virion is directed by an unordered protein signal exposed on the capsid surface

It is uncertain whether nonenveloped karyophilic virus particles may actively traffic from the nucleus outward. The unordered amino-terminal domain of the VP2 major structural protein (2Nt) of the icosahedral parvovirus minute virus of mice (MVM) is internal in empty capsids, but it is exposed outside of the shell through the fivefold axis of symmetry in virions with an encapsidated single-stranded DNA genome, as well as in empty capsids subjected to a heat-induced structural transition. In productive infections of transformed and normal fibroblasts, mature MVM virions were found to efficiently exit from the nucleus prior to cell lysis, in contrast to the extended nuclear accumulation of empty capsids. Newly formed mutant viruses lacking the three phosphorylated serine residues of 2Nt were hampered in their exit from the human transformed NB324K nucleus, in correspondence with the capacity of 2Nt to drive microinjected phosphorylated heated capsids out of the nucleus. However, in normal mouse A9 fibroblasts, in which the MVM capsid was phosphorylated at similar sites but with a much lower rate, the nuclear exit of virions and microinjected capsids harboring exposed 2Nt required the infection process and was highly sensitive to inhibition of the exportin CRM1 in the absence of a demonstrable interaction. Thus, the MVM virion exits the nucleus by accessing nonconventional export pathways relying on cell physiology that can be intensified by infection but in which the exposure of 2Nt remains essential for transport. The flexible 2Nt nuclear transport signal may illustrate a common structural solution used by nonenveloped spherical viruses to propagate in undamaged host tissues.

Role of protein kinase C delta in reactivation of Kaposi's sarcoma-associated herpesvirus

TPA (12-O-tetradecanoylphorbol-13-acetate), a well-known activator of protein kinase C (PKC), can experimentally induce reactivation of Kaposi's sarcoma-associated herpesvirus (KSHV) in certain latently infected cells. We selectively blocked the activity of PKC isoforms by using GF 109203X or rottlerin and demonstrated that this inhibition largely decreased lytic KSHV reactivation by TPA. Translocation of the PKCdelta isoform was evident shortly after TPA stimulation. Overexpression of the dominant-negative PKCdelta mutant supported an essential role for the PKCdelta isoform in virus reactivation, yet overexpression of PKCdelta alone was not sufficient to induce lytic reactivation of KSHV, suggesting that additional signaling molecules participate in this pathway.

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.

Comprehensive mutational analysis of a herpesvirus gene in the viral genome context reveals a region essential for virus replication

Essential viral proteins perform vital functions during morphogenesis via a complex interaction with other viral and cellular gene products. Here, we present a novel approach to comprehensive mutagenesis of essential cytomegalovirus genes and biological analysis in the 230-kbp-genome context. A random Tn7-based mutagenesis procedure at the single-gene level was combined with site-specific recombination via the FLP/FLP recognition target site system for viral genome reconstitution. We show the function of more than 100 mutants from a larger library of M50/p35, a protein involved in capsid egress from the nucleus. This protein recruits other viral proteins and cellular enzymes to the inner nuclear membrane. Our approach enabled us to rapidly discriminate between essential and nonessential regions within the coding sequence. Based on the prediction of the screen, we were able to map a site essential for viral protein-protein interaction at the amino acid level.

Mutations in the human cytomegalovirus UL27 gene that confer resistance to maribavir

Previous drug selection experiments resulted in the isolation of a human cytomegalovirus (CMV) UL97 phosphotransferase mutant resistant to the benzimidazole compound maribavir (1263W94), reflecting the anti-UL97 effect of this drug. Three other CMV strains were plaque purified during these experiments. These strains showed lower-grade resistance to maribavir than the UL97 mutant. Extensive DNA sequence analyses showed no changes from the baseline strain AD169 in UL97, the genes involved in DNA replication, and most structural proteins. However, changes were identified in UL27 where each strain contained a different mutation (R233S, W362R, or a combination of A406V and a stop at codon 415). The mutation at codon 415 is predicted to truncate the expressed UL27 protein by 193 codons (32% of UL27) with a loss of nuclear localization. The expression of full-length UL27 as a green fluorescent fusion protein in uninfected fibroblasts resulted in nuclear and nucleolar fluorescence, whereas cytoplasmic localization was observed when codons 1 to 415 were similarly expressed. Viable UL27 deletion mutants were created by recombination and showed slight growth attenuation and maribavir resistance in cell culture. Marker transfer experiments confirmed that UL27 mutations conferred maribavir resistance. The UL27 sequence was well conserved in a sample of 16 diverse clinical isolates. Mutation in UL27, a betaherpesvirus-specific early gene of unknown biological function, may adapt the virus for growth in the absence of UL97 activity.

Conformational changes in the nuclear lamina induced by herpes simplex virus type 1 require genes U(L)31 and U(L)34

The herpes simplex virus type 1 (HSV-1) U(L)31 and U(L)34 proteins are dependent on each other for proper targeting to the nuclear membrane and are required for efficient envelopment of nucleocapsids at the inner nuclear membrane. In this work, we show that whereas the solubility of lamins A and C (lamin A/C) was not markedly increased, HSV induced conformational changes in the nuclear lamina of infected cells, as viewed after staining with three different lamin A/C-specific antibodies. In one case, reactivity with a monoclonal antibody that recognizes an epitope in the lamin tail domain was greatly reduced in HSV-infected cells. This apparent HSV-induced epitope masking required both U(L)31 and U(L)34, but these proteins were not sufficient to mask the epitope in uninfected cells, indicating that other HSV proteins are also required. In the second case, staining with a rabbit polyclonal antibody that primarily recognizes epitopes in the lamin A/C rod domain revealed that U(L)34 is required for HSV-induced decreased availability of epitopes for reaction with the antibody, whereas U(L)31 protein was dispensable for this effect. Still another polyclonal antibody indicated virtually no difference in lamin A/C staining in infected versus uninfected cells, indicating that the HSV-induced changes are more conformational than the result of lamin depletion at the nuclear rim. Further evidence supporting an interaction between the nuclear lamina and the U(L)31/U(L)34 protein complex includes the observations that (i) overexpression of the U(L)31 protein in uninfected cells was sufficient to relocalize lamin A/C from the nuclear rim into nucleoplasmic aggregates, (ii) overexpression of U(L)34 was sufficient to relocalize some lamin A/C into the cytoplasm, and (iii) both U(L)31 and U(L)34 could directly bind lamin A/C in vitro. These studies suggest that the U(L)31 and U(L)34 proteins modify the conformation of the nuclear lamina in infected cells, possibly by direct interaction with lamin A/C, and that other proteins are also likely involved. Given that the nuclear lamina potentially excludes nucleocapsids from envelopment sites at the inner nuclear membrane, the lamina alteration may reflect a role of the U(L)31/U(L)34 protein complex in perturbing the lamina to promote nucleocapsid egress from the nucleus. Alternatively, the data are compatible with a role of the lamina in targeting the U(L)31/U(L)34 protein complex to the nuclear membrane.

Novel nuclear herniations induced by nuclear localization of a viral protein

A common consequence of viral infection is perturbation of host cell nuclear functions. For cytoplasmically replicating viruses, this process may require regulated transport of specific viral proteins into the nucleus. Here, we describe a novel form of virus-induced perturbation of host cell nuclear structures. Active signal-mediated nuclear import of the reovirus sigma1s protein results in redistribution of nuclear pore complexes and nuclear lamins and formation of nuclear herniations. These herniations represent a previously undescribed mechanism by which cytoplasmic viral infection can perturb nuclear architecture and induce cytopathic effects, which ultimately lead to disease pathogenesis in the infected host.

Herpes simplex virus 1 U(L)31 and U(L)34 gene products promote the late maturation of viral replication compartments to the nuclear periphery

Herpes simplex virus 1 (HSV-1) forms replication compartments (RCs), domains in which viral DNA replication, late-gene transcription, and encapsidation take place, in the host cell nucleus. The formation of these domains leads to compression and marginalization of host cell chromatin, which forms a dense layer surrounding the viral RCs and constitutes a potential barrier to viral nuclear egress or primary envelopment at the inner nuclear membrane. Surrounding the chromatin layer is the nuclear lamina, a further host cell barrier to egress. In this study, we describe an additional phase in RC maturation that involves disruption of the host chromatin and nuclear lamina so that the RC can approach the nuclear envelope. During this phase, the structure of the chromatin layer is altered so that it no longer forms a continuous layer around the RCs but instead is fragmented, forming islands between which RCs extend to reach the nuclear periphery. Coincident with these changes, the nuclear lamina components lamin A/C and lamin-associated protein 2 appear to be redistributed via a mechanism involving the U(L)31 and U(L)34 gene products. Viruses in which the U(L)31 or U(L)34 gene has been deleted are unable to undergo this phase of chromatin reorganization and lamina alterations and instead form RCs which are bounded by an intact host cell chromatin layer and nuclear lamina. We postulate that these defects in chromatin restructuring and lamina reorganization explain the previously documented growth defects of these mutant viruses.

The Epstein-Barr virus BFRF1 and BFLF2 proteins interact and coexpression alters their cellular localization

The BFRF1 protein of Epstein-Barr virus (EBV) is a recently identified membrane protein that is the homolog of the alphaherpesvirus UL34 gene product. We report here that a yeast two-hybrid screen identified the BFLF2 gene product, a homolog of alphaherpesvirus UL31, as a protein that interacts with BFRF1. Expression of BFLF2 in mammalian cells revealed a protein of approximately 28 kDa that associated with BFRF1 in a noncovalently linked complex. When expressed alone, the BFRF1 protein was found in the cytoplasm and perinuclear region. BFLF2 was found diffusely in the nucleus in the absence of BFRF1, but coexpression of BFRF1 and BFLF2 resulted in colocalization of the two proteins at the nuclear rim. These data recapitulate the behavior of the alphaherpesvirus homologs of BFRF1 and BFLF2 and suggest that functional as well as structural and positional homology may be conserved.

Simultaneous deletion of pseudorabies virus tegument protein UL11 and glycoprotein M severely impairs secondary envelopment

The pseudorabies virus (PrV) proteins UL11, glycoprotein E (gE), and gM are involved in secondary envelopment of tegumented nucleocapsids in the cytoplasm. To assess the relative contributions of these proteins to the envelopment process, virus mutants with deletions of either UL11, gM, or gE as well as two newly constructed mutant viruses with simultaneous deletions of UL11 and gE or of UL11 and gM were analyzed in cell culture for their growth phenotype. We show here that simultaneous deletion of UL11 and gE reduced plaque size in an additive manner over the reduction observed by deletion of only UL11 or gE. However, one-step growth was not further impaired beyond the level of the UL11 deletion mutant. Moreover, in electron microscopic analyses PrV-DeltaUL11/gE exhibited a phenotype similar to that of the UL11 mutant virus. In contrast, plaque formation was virtually abolished by the simultaneous absence of UL11 and gM, and one-step growth was significantly reduced. Electron microscopy showed the presence of huge intracytoplasmic inclusions in PrV-DeltaUL11/gM-infected cells, with a size reaching 3 micro m and containing nucleocapsids embedded in tegument. We hypothesize that UL11 and gM are involved in different steps during secondary envelopment and that simultaneous deletion of both interrupts both processes, resulting in the observed drastic impairment of secondary envelopment.

The pseudorabies virus US3 protein is a component of primary and of mature virions

Herpesviruses acquire a primary envelope by budding of capsids at the inner leaflet of the nuclear membrane. They then traverse into the cytoplasm after fusion of the primary envelope with the outer leaflet of the nuclear membrane. In the alphaherpesvirus pseudorabies virus (PrV), the latter process is impaired when the US3 protein is absent. Acquisition of final tegument and envelope occurs in the cytoplasm. Besides the capsid components, only the UL31 and UL34 gene products of PrV have unequivocally been shown to be part of primary enveloped virions, whereas they lack several tegument proteins present in mature virions (reviewed by T. C. Mettenleiter, J. Virol. 76:1537-1547, 2002). Using immunoelectron microscopy, we show that the US3 protein is present in primary enveloped as well as in mature virions. It is also detectable in intracytoplasmic inclusions produced in the absence of other viral tegument components or envelope-associated glycoproteins. In particular, inclusions formed in the absence of the inner tegument protein UL37 contained the US3 protein. Thus, the US3 protein is a tegument component of both forms of enveloped alphaherpes virions. We hypothesize that US3 protein in primary virions modulates deenvelopment at the outer leaflet of the nuclear membrane and is either lost from primary virions during nuclear egress and subsequently reacquired early during tegumentation or is retained during transit of the nucleocapsid through the nuclear membrane.

Herpes simplex virus type 1 primary envelopment: UL34 protein modification and the US3-UL34 catalytic relationship

The herpes simplex virus type 1 (HSV-1) US3 kinase is likely important for primary envelopment of progeny nucleocapsids since it localizes to the nuclear envelope of infected cells and largely determines the phosphorylation state and localization of the necessary primary envelopment factor, the UL34 protein. In HEp-2 cells, the production of infectious US3 null progeny is delayed and decreased relative to that of the parental strain, HSV-1(F). Furthermore, the US3 kinase affects the morphology of primary envelopment such that in its absence, UL34 protein-containing enveloped virions accumulate within membrane-bound vesicles. These vesicles are most often found along the interior periphery of the nucleus and may be derived from the inner nuclear membrane. Since the US3 and UL34 proteins comprise a kinase-substrate pair, a reasonable hypothesis is that the US3 kinase influences these replication parameters by direct phosphorylation of the UL34 protein. For this report, recombinant viruses were constructed to determine the significance of UL34 protein phosphorylation and US3 catalytic activity on UL34 protein localization, single-step growth, and envelopment morphology in both HEp-2 and Vero cells. The data presented suggest that the significance of UL34 phosphorylation is cell type dependent and that efficient viral morphogenesis requires US3-mediated phosphorylation of an infected cell protein other than UL34.

Protein kinases conserved in herpesviruses potentially share a function mimicking the cellular protein kinase cdc2

Herpesviruses encode protein kinases. A subset of these proteins, represented by HSV-1 UL13, are conserved throughout all members of the Herpesviridae, and here, are designated CHPKs (conserved herpesvirus protein kinases). In addition to conserved gene products like CHPKs, herpesviruses encode genes specific to respective herpesviruses. When acting upon conserved viral gene products or cellular factors, CHPKs may play conserved roles in the life cycles of herpesviruses. CHPKs may also express unique functions within the infectious process of individual herpesviruses when specific viral gene products are targeted. CHPKs demonstrate specific activity in multiple herpesvirus infections, functioning in the regulation of viral gene expression in HSV-1, tissue tropism in VZV, and viral DNA synthesis, encapsidation and egress from the nucleus in HCMV. The HCMV CHPK, however, can partially substitute for the HSV-1 CHPK. Representative CHPKs from all Herpesviridae subfamilies can also facilitate the hyperphosphorylation of the cellular translation factor, EF-1delta. This indicates that CHPKs have conserved functions. Recent data have shown that both CHPKs and a cellular protein kinase, cdc2, phosphorylate the same amino acid residues of target proteins. Thus, CHPKs may mimic cdc2 function in infected cells.

Effects of charged cluster mutations on the function of herpes simplex virus type 1 UL34 protein

Herpes simplex virus type 1 (HSV-1) is a DNA virus that acquires an envelope by budding into the inner nuclear membrane of an infected cell. Recombinant HSV-1 lacking the U(L)34 gene cannot undergo this event. U(L)34 and U(L)31, another viral protein, colocalize in an infected cell and are necessary and sufficient to target both proteins to the inner nuclear envelope. In order to define and characterize sequences of U(L)34 that are necessary for primary envelopment to occur, a library of 19 U(L)34 charged cluster mutants and a truncation mutant lacking the putative transmembrane domain (DeltaTM) were generated. Mutants in this library were analyzed in a complementation assay for their ability to function in the production of infectious virus. Seven of the mutants failed to complement a U(L)34-null virus. The remainder of the mutants complemented at or near wild-type U(L)34 levels. Failure of a mutant protein to function might be the result of incorrect subcellular localization. To address this possibility, confocal microscopy was used to determine the localization of the U(L)34 protein in charged cluster mutants and DeltaTM. In transfection-infection experiments, all of the functional U(L)34 mutants and four of the six noncomplementing mutants localized to the inner nuclear envelope in a manner indistinguishable from that of wild-type U(L)34. All of the noncomplementing U(L)34 mutants mediated proper localization of U(L)31. Charged clusters critical for U(L)34 function are dispersed throughout the protein sequence and do not correlate well with highly conserved regions of the protein. These data suggest that U(L)34 has at least one function in addition to mediating proper localization of U(L)31 in infected cells and provide further support for the role of U(L)34 in mediating proper localization of U(L)31 in infected cells.

Herpes simplex virus replication compartments can form by coalescence of smaller compartments

Herpes simplex virus (HSV) uses intranuclear compartmentalization to concentrate the viral and cellular factors required for the progression of the viral life cycle. Processes as varied as viral DNA replication, late gene expression, and capsid assembly take place within discrete structures within the nucleus called replication compartments. Replication compartments are hypothesized to mature from a few distinct structures, called prereplicative sites, that form adjacent to cellular nuclear matrix-associated ND10 sites. During productive infection, the HSV single-stranded DNA-binding protein ICP8 localizes to replication compartments. To further the understanding of replication compartment maturation, we have constructed and characterized a recombinant HSV-1 strain that expresses an ICP8 molecule with green fluorescent protein (GFP) fused to its C terminus. In transfected Vero cells that were infected with HSV, the ICP8-GFP protein localized to prereplicative sites in the presence of the viral DNA synthesis inhibitor phosphonoacetic acid (PAA) or to replication compartments in the absence of PAA. A recombinant HSV-1 strain expressing the ICP8-GFP virus replicated in Vero cells, but the yield was increased by 150-fold in an ICP8-complementing cell line. Using the ICP8-GFP protein as a marker for replication compartments, we show here that these structures start as punctate structures early in infection and grow into large, globular structures that eventually fill the nucleus. Large replication compartments were formed by small structures that either moved through the nucleus to merge with adjacent compartments or remained relatively stationary within the nucleus and grew by accretion and fused with neighboring structures.

Nuclear lipid signalling

During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide-phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.

Nuclear envelope dynamics in oocytes: from germinal vesicle breakdown to mitosis

We have recently gained new insight into the mechanisms involved in nuclear envelope breakdown, the irreversible step that commits a cell to the M phase. Results from mammalian cell and starfish oocyte studies suggest that mechanical forces of the cytoskeleton, as well as biochemical disassembly of nuclear envelope protein complexes, play important roles in this process.

The human cytomegalovirus UL97 protein kinase, an antiviral drug target, is required at the stage of nuclear egress

Human cytomegalovirus encodes an unusual protein kinase, UL97, that activates the established antiviral drug ganciclovir and is specifically inhibited by a new antiviral drug, maribavir. We used maribavir and a UL97 null mutant, which is severely deficient in viral replication, to determine what stage of virus infection critically requires UL97. Compared with wild-type virus, there was little or no decrease in immediate-early gene expression, viral DNA synthesis, late gene expression, or packaging of viral DNA into nuclease-resistant structures in mutant-infected or maribavir-treated cells under conditions where the virus yield was severely impaired. Electron microscopy studies revealed similar proportions of various capsid forms, including DNA-containing capsids, in the nuclei of wild-type- and mutant-infected cells. However, capsids were rare in the cytoplasm of mutant-infected or maribavir-treated cells; the magnitudes of these decreases in cytoplasmic capsids were similar to those for virus yield. Thus, genetic and pharmacological evidence indicates that UL97 is required at the stage of infection when nucleocapsids exit from the nucleus (nuclear egress), and this poorly understood stage of virus infection can be targeted by antiviral drugs. Understanding UL97 function and maribavir action should help elucidate this interesting biological process and help identify new antiviral drug targets for an important pathogen in immunocompromised patients.