Fate of the inner nuclear membrane protein lamin B receptor and nuclear lamins in herpes simplex virus type 1 infection

Mechanisms for assembly of the nucleoplasmic reticulum

The nuclear envelope consists of an outer membrane connected to the endoplasmic reticulum, an inner membrane facing the nucleoplasm and a perinuclear space separating the two bilayers. The inner and outer nuclear membranes are physically connected at nuclear pore complexes that mediate selective communication and transfer of materials between the cytoplasm and nucleus. The spherical shape of the nuclear envelope is maintained by counterbalancing internal and external forces applied by cyto- and nucleo-skeletal networks, and the nuclear lamina and chromatin that underly the inner nuclear membrane. Despite its apparent rigidity, the nuclear envelope can invaginate to form an intranuclear membrane network termed the nucleoplasmic reticulum (NR) consisting of Type-I NR contiguous with the inner nuclear membrane and Type-II NR containing both the inner and outer nuclear membranes. The NR extends deep into the nuclear interior potentially facilitating communication and exchanges between the nuclear interior and the cytoplasm. This review details the evidence that NR intrusions that regulate cytoplasmic communication and genome maintenance are the result of a dynamic interplay between membrane biogenesis and remodelling, and physical forces exerted on the nuclear lamina derived from the cyto- and nucleo-skeletal networks.

Comprehensive Analysis of the Tegument Proteins Involved in Capsid Transport and Virion Morphogenesis of Alpha, Beta and Gamma Herpesviruses

Herpesviruses are enveloped and have an amorphous protein layer surrounding the capsid, which is termed the tegument. Tegument proteins perform critical functions throughout the viral life cycle. This review provides a comprehensive and comparative analysis of the roles of specific tegument proteins in capsid transport and virion morphogenesis of selected, well-studied prototypes of each of the three subfamilies of Herpesviridae i.e., human herpesvirus-1/herpes simplex virus-1 (Alphaherpesvirinae), human herpesvirus-5/cytomegalovirus (Betaherpesvirinae) and human herpesvirus -8/Kaposi's sarcomavirus (Gammaherpesvirinae). Most of the current knowledge is based on alpha herpesviruses, in particular HSV-1. While some tegument proteins are released into the cytoplasm after virus entry, several tegument proteins remain associated with the capsid and are responsible for transport to and docking at the nucleus. After replication and capsid formation, the capsid is enveloped at the nuclear membrane, which is referred to as primary envelopment, followed by de-envelopment and release into the cytoplasm. This requires involvement of at least three tegument proteins. Subsequently, multiple interactions between tegument proteins and capsid proteins, other tegument proteins and glycoproteins are required for assembly of the virus particles and envelopment at the Golgi, with certain tegument proteins acting as the central hub for these interactions. Some redundancy in these interactions ensures appropriate morphogenesis.

Three-Dimensional Chromatin Structure of the EBV Genome: A Crucial Factor in Viral Infection

Epstein-Barr Virus (EBV) is a human gamma-herpesvirus that is widespread worldwide. To this day, about 200,000 cancer cases per year are attributed to EBV infection. EBV is capable of infecting both B cells and epithelial cells. Upon entry, viral DNA reaches the nucleus and undergoes a process of circularization and chromatinization and establishes a latent lifelong infection in host cells. There are different types of latency all characterized by different expressions of latent viral genes correlated with a different three-dimensional architecture of the viral genome. There are multiple factors involved in the regulation and maintenance of this three-dimensional organization, such as CTCF, PARP1, MYC and Nuclear Lamina, emphasizing its central role in latency maintenance.

The Role of Lamins in the Nucleoplasmic Reticulum, a Pleiomorphic Organelle That Enhances Nucleo-Cytoplasmic Interplay

Invaginations of the nuclear membrane occur in different shapes, sizes, and compositions. Part of these pleiomorphic invaginations make up the nucleoplasmic reticulum (NR), while others are merely nuclear folds. We define the NR as tubular invaginations consisting of either both the inner and outer nuclear membrane, or only the inner nuclear membrane. Specifically, invaginations of both the inner and outer nuclear membrane are also called type II NR, while those of only the inner nuclear membrane are defined as type I NR. The formation and structure of the NR is determined by proteins associated to the nuclear membrane, which induce a high membrane curvature leading to tubular invaginations. Here we review and discuss the current knowledge of nuclear invaginations and the NR in particular. An increase in tubular invaginations of the nuclear envelope is associated with several pathologies, such as laminopathies, cancer, (reversible) heart failure, and Alzheimer’s disease. Furthermore, viruses can induce both type I and II NR. In laminopathies, the amount of A-type lamins throughout the nucleus is generally decreased or the organization of lamins or lamin-associated proteins is disturbed. Also, lamin overexpression or modulation of lamin farnesylation status impacts NR formation, confirming the importance of lamin processing in NR formation. Virus infections reorganize the nuclear lamina via (de)phosphorylation of lamins, leading to an uneven thickness of the nuclear lamina and in turn lobulation of the nuclear membrane and the formation of invaginations of the inner nuclear membrane. Since most studies on the NR have been performed with cell cultures, we present additional proof for the existence of these structures in vivo, focusing on a variety of differentiated cardiovascular and hematopoietic cells. Furthermore, we substantiate the knowledge of the lamin composition of the NR by super-resolution images of the lamin A/C and B1 organization. Finally, we further highlight the essential role of lamins in NR formation by demonstrating that (over)expression of lamins can induce aberrant NR structures.

Intranuclear HSV-1 DNA ejection induces major mechanical transformations suggesting mechanoprotection of nucleus integrity

Maintaining nuclear integrity is essential to cell survival when exposed to mechanical stress. Herpesviruses, like most DNA and some RNA viruses, put strain on the nuclear envelope as hundreds of viral DNA genomes replicate and viral capsids assemble. It remained unknown, however, how nuclear mechanics is affected at the initial stage of herpesvirus infection-immediately after viral genomes are ejected into the nuclear space-and how nucleus integrity is maintained despite an increased strain on the nuclear envelope. With an atomic force microscopy force volume mapping approach on cell-free reconstituted nuclei with docked herpes simplex type 1 (HSV-1) capsids, we explored the mechanical response of the nuclear lamina and the chromatin to intranuclear HSV-1 DNA ejection into an intact nucleus. We discovered that chromatin stiffness, measured as Young's modulus, is increased by ∼14 times, while nuclear lamina underwent softening. Those transformations could be associated with a mechanism of mechanoprotection of nucleus integrity facilitating HSV-1 viral genome replication. Indeed, stiffening of chromatin, which is tethered to the lamina meshwork, helps to maintain nuclear morphology. At the same time, increased lamina elasticity, reflected by nucleus softening, acts as a "shock absorber," dissipating the internal mechanical stress on the nuclear membrane (located on top of the lamina wall) and preventing its rupture.

Nuclear Cytoskeleton in Virus Infection

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

Herpes Simplex Virus 1 UL34 Mutants That Affect Membrane Budding Regulation and Nuclear Lamina Disruption

Nuclear envelope budding in herpesvirus nuclear egress may be negatively regulated, since the pUL31/pUL34 nuclear egress complex heterodimer can induce membrane budding without capsids when expressed ectopically or on artificial membranes in vitro, but not in the infected cell. We have previously described a pUL34 mutant that contained alanine substitutions at R158 and R161 and that showed impaired growth, impaired pUL31/pUL34 interaction, and unregulated budding. Here, we determine the phenotypic contributions of the individual substitutions to these phenotypes. Neither substitution alone was able to reproduce the impaired growth or nuclear egress complex (NEC) interaction phenotypes. Either substitution, however, could fully reproduce the unregulated budding phenotype, suggesting that misregulated budding may not substantially impair virus replication. In addition, the R158A substitution caused relocalization of the NEC to intranuclear punctate structures and recruited lamin A/C to these structures, suggesting that this residue might be important for recruitment of kinases for dispersal of nuclear lamins. IMPORTANCE Herpesvirus nuclear egress is a complex, regulated process coordinated by two virus proteins that are conserved among the herpesviruses that form a heterodimeric nuclear egress complex (NEC). The NEC drives budding of capsids at the inner nuclear membrane and recruits other viral and host cell proteins for disruption of the nuclear lamina, membrane scission, and fusion. The structural basis of individual activities of the NEC, apart from membrane budding, are not clear, nor is the basis of the regulation of membrane budding. Here, we explore the properties of NEC mutants that have an unregulated budding phenotype, determine the significance of that regulation for virus replication, and also characterize a structural requirement for nuclear lamina disruption.

Mechanism of Nuclear Lamina Disruption and the Role of pUS3 in HSV-1 Nuclear Egress

Herpes simplex virus capsid envelopment at the nuclear membrane is coordinated by nuclear egress complex (NEC) proteins, pUL34 and pUL31, and is accompanied by alteration in the nuclear architecture and local disruption of nuclear lamina. Here, we examined the role of capsid envelopment in the changes of the nuclear architecture by characterizing HSV-1 recombinants that do not form capsids. Typical changes in nuclear architecture and disruption of the lamina were observed in the absence of capsids, suggesting that disruption of the nuclear lamina occurs prior to capsid envelopment. Surprisingly, in the absence of capsid envelopment, lamin A/C becomes concentrated at the nuclear envelope in a pUL34-independent and cell type-specific manner, suggesting that ongoing nuclear egress may be required for the dispersal of lamins observed in wild-type infection. Mutation of virus-encoded protein kinase, pUS3, on a wild-type virus background has been shown to cause accumulation of perinuclear enveloped capsids, formation of NEC aggregates, and exacerbated lamina disruption. We observed that mutation of US3 in the absence of capsids results in identical NEC aggregation and lamina disruption phenotypes, suggesting that they do not result from accumulation of perinuclear virions. TEM analysis revealed that, in the absence of capsids, NEC aggregates correspond to multi-folded nuclear membrane structures, suggesting that pUS3 may control NEC self-association and membrane deformation. To determine the significance of the pUS3 nuclear egress function for virus growth, the replication of single and double UL34 and US3 mutants was measured, showing that the significance of pUS3 nuclear egress function is cell-type specific.ImportanceThe nuclear lamina is an important player in infection by viruses that replicate in the nucleus. Herpesviruses alter the structure of the nuclear lamina to facilitate transport of capsids from the nucleus to the cytoplasm and use both viral and cellular effectors to disrupt the protein-protein interactions that maintain the lamina. Here we explore the role of capsid envelopment and the virus-encoded protein kinase, pUS3, in the disruption of lamina structure. We show that capsid envelopment is not necessary for the lamina disruption, or for US3 mutant phenotypes, including exaggerated lamina disruption, that accompany nuclear egress. These results clarify the mechanisms behind alteration of nuclear lamina structure and support a function for pUS3 in regulating the aggregation state of the nuclear egress machinery.

Host and Viral Factors Involved in Nuclear Egress of Herpes Simplex Virus 1

Herpes simplex virus 1 (HSV-1) replicates its genome and packages it into capsids within the nucleus. HSV-1 has evolved a complex mechanism of nuclear egress whereby nascent capsids bud on the inner nuclear membrane to form perinuclear virions that subsequently fuse with the outer nuclear membrane, releasing capsids into the cytosol. The viral-encoded nuclear egress complex (NEC) plays a crucial role in this vesicle-mediated nucleocytoplasmic transport. Nevertheless, similar system mediates the movement of other cellular macromolecular complexes in normal cells. Therefore, HSV-1 may utilize viral proteins to hijack the cellular machinery in order to facilitate capsid transport. However, little is known about the molecular mechanisms underlying this phenomenon. This review summarizes our current understanding of the cellular and viral factors involved in the nuclear egress of HSV-1 capsids.

Characterization of lamin B receptor of Sf9 cells and its fate during Autographa californica nucleopolyhedrovirus infection

Baculovirus nucleocapsids egress from the nuclear membrane during infection. However, details of alternation of nuclear membrane structure during baculovirus egress are unknown. In this study, we examined the changes of lamin B receptor (LBR), a main inner nuclear membrane component, during Autographa californica nucleopolyhedrovirus (AcMNPV) infection. Firstly, the open reading frame (Orf) of Sf9 lbr was cloned by reverse transcription PCR, and the distribution of LBR in Sf9 cells were observed by fusing LBR with the red fluorescence protein mcherry. Besides, the amount of endogenous LBR during AcMNPV infection was detected by western blotting. Moreover, the distribution of LBR after AcMNPV infection was observed under the confocal fluorescence microscopy. Furthermore, the effects of protein kinase C (PKC) inhibitor on stability of LBR and release of budded virus (BVs) were determined. The results showed that Sf9 lbr contains an Orf of 2040 nucleotides (NTs), which encodes a predicted protein of 679 amino acids (AAs). Fluorescence microscopy showed that LBR is localized to the nuclear membrane. Western blotting result showed that the amount of endogenous LBR is significantly reduced after AcMNPV infection. Transfection and infection assay demonstrated that the fluorescence of LBR nearly completely disappeared after viral infection. PKC inhibitor can suppress the degradation of LBR induced by AcMNPV, resulting in the reduction of viral titer of progeny viruses. The electron microscopy analysis demonstrated that PKC inhibitor did not influence virion entry, uncoating, and assembly, but may partially protect the nuclear membrane from disruption by AcMNPV. Taken together, AcMNPV infection can distort the expression of LBR, which may promote the egress of nucleocapsids.

Roles of the Interhexamer Contact Site for Hexagonal Lattice Formation of the Herpes Simplex Virus 1 Nuclear Egress Complex in Viral Primary Envelopment and Replication

During the nuclear export of nascent nucleocapsids of herpes simplex virus 1 (HSV-1), the nucleocapsids acquire a primary envelope by budding through the inner nuclear membrane into the perinuclear space between the inner and outer nuclear membranes. This unique budding process, termed primary envelopment, is initiated by the nuclear egress complex (NEC), composed of the HSV-1 UL31 and UL34 proteins. Earlier biochemical approaches have shown that the NEC has an intrinsic ability to vesiculate membranes through the formation of a hexagonal lattice structure. The significance of intrahexamer interactions of the NEC in the primary envelopment of HSV-1-infected cells has been reported. In contrast, the contribution of lattice formation of the NEC hexamer to primary envelopment in HSV-1-infected cells remains to be elucidated. Therefore, we constructed and characterized a recombinant HSV-1 strain carrying an amino acid substitution in a UL31 residue that is an interhexamer contact site for the lattice formation of the NEC hexamer. This mutation was reported to destabilize the interhexamer interactions of the HSV-1 NEC. Here, we demonstrate that the mutation causes the aberrant accumulation of nucleocapsids in the nucleus and reduces viral replication in Vero and HeLa cells. Thus, the ability of HSV-1 to form the hexagonal lattice structure of the NEC was linked to an increase in primary envelopment and viral replication. Our results suggest that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment.IMPORTANCE The scaffolding proteins of several envelope viruses required for virion assembly form high-order lattice structures. However, information on the significance of their lattice formation in infected cells is limited. Herpesviruses acquire envelopes twice during their viral replication. The first envelop acquisition (primary envelopment) is one of the steps in the vesicle-mediated nucleocytoplasmic transport of nascent nucleocapsids, which is unique in biology. HSV-1 NEC, thought to be conserved in all members of the Herpesviridae family, is critical for primary envelopment and was shown to form a hexagonal lattice structure. Here, we investigated the significance of the interhexamer contact site for hexagonal lattice formation of the NEC in HSV-1-infected cells and present evidence suggesting that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment. Our results provide insights into the mechanisms of the envelopment of herpesviruses and other envelope viruses.

A Nondimensional Model Reveals Alterations in Nuclear Mechanics upon Hepatitis C Virus Replication

Morphology of the nucleus is an important regulator of gene expression. Nuclear morphology is in turn a function of the forces acting on it and the mechanical properties of the nuclear envelope. Here, we present a two-parameter, nondimensional mechanical model of the nucleus that reveals a relationship among nuclear shape parameters, such as projected area, surface area, and volume. Our model fits the morphology of individual nuclei and predicts the ratio between forces and modulus in each nucleus. We analyzed the changes in nuclear morphology of liver cells due to hepatitis C virus (HCV) infection using this model. The model predicted a decrease in the elastic modulus of the nuclear envelope and an increase in the pre-tension in cortical actin as the causes for the change in nuclear morphology. These predictions were validated biomechanically by showing that liver cells expressing HCV proteins possessed enhanced cellular stiffness and reduced nuclear stiffness. Concomitantly, cells expressing HCV proteins showed downregulation of lamin-A,C and upregulation of β-actin, corroborating the predictions of the model. Our modeling assumptions are broadly applicable to adherent, monolayer cell cultures, making the model amenable to investigate changes in nuclear mechanics due to other stimuli by merely measuring nuclear morphology. Toward this, we present two techniques, graphical and numerical, to use our model for predicting physical changes in the nucleus.

Nuclear envelope impairment is facilitated by the herpes simplex virus 1 Us3 kinase

Background: Capsids of herpes simplex virus 1 (HSV-1) are assembled in the nucleus, translocated either to the perinuclear space by budding at the inner nuclear membrane acquiring tegument and envelope, or released to the cytosol in a "naked" state via impaired nuclear pores that finally results in impairment of the nuclear envelope. The Us3 gene encodes a protein acting as a kinase, which is responsible for phosphorylation of numerous viral and cellular substrates. The Us3 kinase plays a crucial role in nucleus to cytoplasm capsid translocation. We thus investigate the nuclear surface in order to evaluate the significance of Us3 in maintenance of the nuclear envelope during HSV-1 infection. Methods: To address alterations of the nuclear envelope and capsid nucleus to cytoplasm translocation related to the function of the Us3 kinase we investigated cells infected with wild type HSV-1 or the Us3 deletion mutant R7041(∆Us3) by transmission electron microscopy, focused ion-beam electron scanning microscopy, cryo-field emission scanning electron microscopy, confocal super resolution light microscopy, and polyacrylamide gel electrophoresis. Results: Confocal super resolution microscopy and cryo-field emission scanning electron microscopy revealed decrement in pore numbers in infected cells. Number and degree of pore impairment was significantly reduced after infection with R7041(∆Us3) compared to infection with wild type HSV-1. The nuclear surface was significantly enlarged in cells infected with any of the viruses. Morphometric analysis revealed that additional nuclear membranes were produced forming multiple folds and caveolae, in which virions accumulated as documented by three-dimensional reconstruction after ion-beam scanning electron microscopy. Finally, significantly more R7041(∆Us3) capsids were retained in the nucleus than wild-type capsids whereas the number of R7041(∆Us3) capsids in the cytosol was significantly lower. Conclusions: The data indicate that Us3 kinase is involved in facilitation of nuclear pore impairment and, concomitantly, in capsid release through impaired nuclear envelope.

Epstein-Barr Virus-Induced Nodules on Viral Replication Compartments Contain RNA Processing Proteins and a Viral Long Noncoding RNA

Profound alterations in host cell nuclear architecture accompany the lytic phase of Epstein-Barr virus (EBV) infection. Viral replication compartments assemble, host chromatin marginalizes to the nuclear periphery, cytoplasmic poly(A)-binding protein translocates to the nucleus, and polyadenylated mRNAs are sequestered within the nucleus. Virus-induced changes to nuclear architecture that contribute to viral host shutoff (VHS) must accommodate selective processing and export of viral mRNAs. Here we describe additional previously unrecognized nuclear alterations during EBV lytic infection in which viral and cellular factors that function in pre-mRNA processing and mRNA export are redistributed. Early during lytic infection, before formation of viral replication compartments, two cellular pre-mRNA splicing factors, SC35 and SON, were dispersed from interchromatin granule clusters, and three mRNA export factors, Y14, ALY, and NXF1, were depleted from the nucleus. During late lytic infection, virus-induced nodular structures (VINORCs) formed at the periphery of viral replication compartments. VINORCs were composed of viral (BMLF1 and BGLF5) and cellular (SC35, SON, SRp20, and NXF1) proteins that mediate pre-mRNA processing and mRNA export. BHLF1 long noncoding RNA was invariably found in VINORCs. VINORCs did not contain other nodular nuclear cellular proteins (PML or coilin), nor did they contain viral proteins (BRLF1 or BMRF1) found exclusively within replication compartments. VINORCs are novel EBV-induced nuclear structures. We propose that EBV-induced dispersal and depletion of pre-mRNA processing and mRNA export factors during early lytic infection contribute to VHS; subsequent relocalization of these pre-mRNA processing and mRNA export proteins to VINORCs and viral replication compartments facilitates selective processing and export of viral mRNAs.IMPORTANCE In order to make protein, mRNA transcribed from DNA in the nucleus must enter the cytoplasm. Nuclear export of mRNA requires correct processing of mRNAs by enzymes that function in splicing and nuclear export. During the Epstein-Barr virus (EBV) lytic cycle, nuclear export of cellular mRNAs is blocked, yet export of viral mRNAs is facilitated. Here we report the dispersal and dramatic reorganization of cellular (SC35, SON, SRp20, Y14, ALY, and NXF1) and viral (BMLF1 and BGLF5) proteins that play key roles in pre-mRNA processing and export of mRNA. These virus-induced nuclear changes culminate in formation of VINORCs, novel nodular structures composed of viral and cellular RNA splicing and export factors. VINORCs localize to the periphery of viral replication compartments, where viral mRNAs reside. These EBV-induced changes in nuclear organization may contribute to blockade of nuclear export of host mRNA, while enabling selective processing and export of viral mRNA.

Remodeling of host membranes during herpesvirus assembly and egress

Many viruses, enveloped or non-enveloped, remodel host membrane structures for their replication, assembly and escape from host cells. Herpesviruses are important human pathogens and cause many diseases. As large enveloped DNA viruses, herpesviruses undergo several complex steps to complete their life cycles and produce infectious progenies. Firstly, herpesvirus assembly initiates in the nucleus, producing nucleocapsids that are too large to cross through the nuclear pores. Nascent nucleocapsids instead bud at the inner nuclear membrane to form primary enveloped virions in the perinuclear space followed by fusion of the primary envelopes with the outer nuclear membrane, to translocate the nucleocapsids into the cytoplasm. Secondly, nucleocapsids obtain a series of tegument proteins in the cytoplasm and bud into vesicles derived from host organelles to acquire viral envelopes. The vesicles are then transported to and fuse with the plasma membrane to release the mature virions to the extracellular space. Therefore, at least two budding and fusion events take place at cellular membrane structures during herpesviruses assembly and egress, which induce membrane deformations. In this review, we describe and discuss how herpesviruses exploit and remodel host membrane structures to assemble and escape from the host cell.

Three-dimensional imaging reveals endo(sarco)plasmic reticulum-containing invaginations within the nucleoplasm of muscle

The mammalian nucleus has invaginations from the cytoplasm, termed nucleoplasmic reticulum (NR). With increased resolution of cellular imaging, progress has been made in understanding the formation and function of NR. In fact, nucleoplasmic Ca²⁺ homeostasis has been implicated in the regulation of gene expression, DNA repair, and cell death. However, the majority of studies focus on cross-sectional or single-plane analyses of NR invaginations, providing an incomplete assessment of its distribution and content. Here, we provided advanced imaging and three-dimensional reconstructive analyses characterizing the molecular constituents of nuclear invaginations in the nucleoplasm in HEK293 cells, murine C₂C₁₂ muscle cells, and cardiac myocytes. We demonstrated the presence of critical Ca²⁺ regulatory channels, including sarco(endo)plasmic reticulum Ca²⁺-ATPase 2a (SERCA2a), stromal interaction molecule 1 (STIM1), and Ca²⁺ release-activated Ca²⁺ channel protein 1 (ORAI1), in the nucleoplasm in isolated primary mouse cardiomyocytes. We have shown for the first time the presence of STIM1 and ORAI1 in the nucleoplasm, suggesting the presence of store-operated calcium entry (SOCE) mechanism in nucleoplasmic Ca²⁺ regulation. These results show that nucleoplasmic invaginations contain continuous endoplasmic reticulum components, mitochondria, and intact nuclear membranes, highlighting the extremely detailed and complex nature of this organellar structure.

Baculovirus infection induces disruption of the nuclear lamina

Baculovirus nucleocapsids egress from the nucleus primarily via budding at the nuclear membrane. The nuclear lamina underlying the nuclear membrane represents a substantial barrier to nuclear egress. Whether the nuclear lamina undergoes disruption during baculovirus infection remains unknown. In this report, we generated a clonal cell line, Sf9-L, that stably expresses GFP-tagged Drosophila lamin B. GFP autofluorescence colocalized with immunofluorescent anti-lamin B at the nuclear rim of Sf9-L cells, indicating GFP-lamin B was incorporated into the nuclear lamina. Meanwhile, virus was able to replicate normally in Sf9-L cells. Next, we investigated alterations to the nuclear lamina during baculovirus infection in Sf9-L cells. A portion of GFP-lamin B localized diffusely at the nuclear rim, and some GFP-lamin B was redistributed within the nucleus during the late phase of infection, suggesting the nuclear lamina was partially disrupted. Immunoelectron microscopy revealed associations between GFP-lamin B and the edges of the electron-dense stromal mattes of the virogenic stroma, intranuclear microvesicles, and ODV envelopes and nucleocapsids within the nucleus, indicating the release of some GFP-lamin B from the nuclear lamina. Additionally, GFP-lamin B phosphorylation increased upon infection. Based on these data, baculovirus infection induced lamin B phosphorylation and disruption of the nuclear lamina.

The human cytomegalovirus nuclear egress complex unites multiple functions: Recruitment of effectors, nuclear envelope rearrangement, and docking to nuclear capsids

BACKGROUND

Nuclear replication represents a common hallmark of herpesviruses achieved by a number of sequentially unrolled regulatory processes. A rate-limiting step is provided by nucleo-cytoplasmic capsid export, for which a defined multiregulatory protein complex, namely, the nuclear egress complex (NEC), is assembled comprising both viral and cellular components. The NEC regulates at least 3 aspects of herpesviral nuclear replication: (1) multimeric recruitment of NEC-associated effector proteins, (2) reorganization of the nuclear lamina and membranes, and (3) the docking to nuclear capsids. Here, we review published data and own experimental work that characterizes the NEC of HCMV and other herpesviruses.

METHODS

A systematic review of information on nuclear egress of HCMV compared to selected alpha-, beta-, and gamma-herpesviruses: proteomics-based approaches, high-resolution imaging techniques, and functional investigations.

RESULTS

A large number of reports on herpesviral NECs have been published during the last two decades, focusing on protein-protein interactions, nuclear localization, regulatory phosphorylation, and functional validation. The emerging picture provides an illustrated example of well-balanced and sophisticated protein networking in virus-host interaction.

CONCLUSIONS

Current evidence refined the view about herpesviral NECs. Datasets published for HCMV, murine CMV, herpes simplex virus, and Epstein-Barr virus illustrate the marked functional consistency in the way herpesviruses achieve nuclear egress. However, this compares with only limited sequence conservation of core NEC proteins and a structural conservation restricted to individual domains. The translational use of this information might help to define a novel antiviral strategy on the basis of NEC-directed small molecules.

Transcriptome-wide analysis of alternative RNA splicing events in Epstein-Barr virus-associated gastric carcinomas

Multiple human diseases including cancer have been associated with a dysregulation in RNA splicing patterns. In the current study, modifications to the global RNA splicing landscape of cellular genes were investigated in the context of Epstein-Barr virus-associated gastric cancer. Global alterations to the RNA splicing landscape of cellular genes was examined in a large-scale screen from 295 primary gastric adenocarcinomas using high-throughput RNA sequencing data. RT-PCR analysis, mass spectrometry, and co-immunoprecipitation studies were also used to experimentally validate and investigate the differential alternative splicing (AS) events that were observed through RNA-seq studies. Our study identifies alterations in the AS patterns of approximately 900 genes such as tumor suppressor genes, transcription factors, splicing factors, and kinases. These findings allowed the identification of unique gene signatures for which AS is misregulated in both Epstein-Barr virus-associated gastric cancer and EBV-negative gastric cancer. Moreover, we show that the expression of Epstein-Barr nuclear antigen 1 (EBNA1) leads to modifications in the AS profile of cellular genes and that the EBNA1 protein interacts with cellular splicing factors. These findings provide insights into the molecular differences between various types of gastric cancer and suggest a role for the EBNA1 protein in the dysregulation of cellular AS.

Dynamics and Structure-Function Relationships of the Lamin B Receptor (LBR)

The lamin B receptor (LBR) is a multi-spanning membrane protein of the inner nuclear membrane that is often employed as a "reporter" of nuclear envelope dynamics. We show here that the diffusional mobility of full-length LBR exhibits significant regional variation along the nuclear envelope, consistent with the existence of discrete LBR microdomains and the occurrence of multiple, asymmetrically-spaced anastomoses along the nuclear envelope-endoplasmic reticulum interface. Interestingly, a commonly used fusion protein that contains the amino-terminal region and the first transmembrane domain of LBR exhibits reduced mobility at the nuclear envelope, but behaves similarly to full-length LBR in the endoplasmic reticulum. On the other hand, carboxy-terminally truncated mutants that retain the first four transmembrane domains and a part or the whole of the amino-terminal region of LBR are generally hyper-mobile. These results suggest that LBR dynamics is structure and compartment specific. They also indicate that native LBR is probably "configured" by long-range interactions that involve the loops between adjacent transmembrane domains and parts of the amino-terminal region.

Extragenic Suppression of a Mutation in Herpes Simplex Virus 1 UL34 That Affects Lamina Disruption and Nuclear Egress

Nuclear egress of herpesviruses is accompanied by changes in the architecture of the nuclear membrane and nuclear lamina that are thought to facilitate capsid access to the inner nuclear membrane (INM) and curvature of patches of the INM around the capsid during budding. Here we report the properties of a point mutant of pUL34 (Q163A) that fails to induce gross changes in nuclear architecture or redistribution of lamin A/C. The UL34(Q163A) mutant shows a roughly 100-fold defect in single-step growth, and it forms small plaques. This mutant has a defect in nuclear egress, and furthermore, it fails to disrupt nuclear shape or cause observable displacement of lamin A/C despite retaining the ability to recruit the pUS3 and PKC protein kinases and to mediate phosphorylation of emerin. Extragenic suppressors of the UL34(Q163A) phenotype were isolated, and all of them carry a single mutation of arginine 229 to leucine in UL31. Surprisingly, although this UL31 mutation largely restores virus replication, it does not correct the lamina disruption defect, suggesting that, in Vero cells, changes in nuclear shape and gross displacements of lamin A/C may facilitate but are unnecessary for nuclear egress.

IMPORTANCE

Herpesvirus nuclear egress is an essential and conserved process that requires close association of the viral capsid with the inner nuclear membrane and budding of the capsid into that membrane. Access to the nuclear membrane and tight curvature of that membrane are thought to require disruption of the nuclear lamina that underlies the inner nuclear membrane, and consistent with this idea, herpesvirus infection induces biochemical and architectural changes at the nuclear membrane. The significance of the nuclear membrane architectural changes is poorly characterized. The results presented here address that deficiency in our understanding and show that a combination of mutations in two of the viral nuclear egress factors results in a failure to accomplish at least two components of lamina disruption while still allowing relatively efficient viral replication, suggesting that changes in nuclear shape and displacement of lamins are not necessary for herpes simplex virus 1 (HSV-1) nuclear egress.

Visualization of DNA G-quadruplexes in herpes simplex virus 1-infected cells

We have previously shown that clusters of guanine quadruplex (G4) structures can form in the human herpes simplex-1 (HSV-1) genome. Here we used immunofluorescence and immune-electron microscopy with a G4-specific monoclonal antibody to visualize G4 structures in HSV-1 infected cells. We found that G4 formation and localization within the cells was virus cycle dependent: viral G4s peaked at the time of viral DNA replication in the cell nucleus, moved to the nuclear membrane at the time of virus nuclear egress and were later found in HSV-1 immature virions released from the cell nucleus. Colocalization of G4s with ICP8, a viral DNA processing protein, was observed in viral replication compartments. G4s were lost upon treatment with DNAse and inhibitors of HSV-1 DNA replication. The notable increase in G4s upon HSV-1 infection suggests a key role of these structures in the HSV-1 biology and indicates new targets to control both the lytic and latent infection.

Global Profiling of the Cellular Alternative RNA Splicing Landscape during Virus-Host Interactions

Alternative splicing (AS) is a central mechanism of genetic regulation which modifies the sequence of RNA transcripts in higher eukaryotes. AS has been shown to increase both the variability and diversity of the cellular proteome by changing the composition of resulting proteins through differential choice of exons to be included in mature mRNAs. In the present study, alterations to the global RNA splicing landscape of cellular genes upon viral infection were investigated using mammalian reovirus as a model. Our study provides the first comprehensive portrait of global changes in the RNA splicing signatures that occur in eukaryotic cells following infection with a human virus. We identify 240 modified alternative splicing events upon infection which belong to transcripts frequently involved in the regulation of gene expression and RNA metabolism. Using mass spectrometry, we also confirm modifications to transcript-specific peptides resulting from AS in virus-infected cells. These findings provide additional insights into the complexity of virus-host interactions as these splice variants expand proteome diversity and function during viral infection.

Observing Mitotic Division and Dynamics in a Live Zebrafish Embryo

Mitosis is critical for organismal growth and differentiation. The process is highly dynamic and requires ordered events to accomplish proper chromatin condensation, microtubule-kinetochore attachment, chromosome segregation, and cytokinesis in a small time frame. Errors in the delicate process can result in human disease, including birth defects and cancer. Traditional approaches investigating human mitotic disease states often rely on cell culture systems, which lack the natural physiology and developmental/tissue-specific context advantageous when studying human disease. This protocol overcomes many obstacles by providing a way to visualize, with high resolution, chromosome dynamics in a vertebrate system, the zebrafish. This protocol will detail an approach that can be used to obtain dynamic images of dividing cells, which include: in vitro transcription, zebrafish breeding/collecting, embryo embedding, and time-lapse imaging. Optimization and modifications of this protocol are also explored. Using H2A.F/Z-EGFP (labels chromatin) and mCherry-CAAX (labels cell membrane) mRNA-injected embryos, mitosis in AB wild-type, auroraB(hi1045) (,) and esco2(hi2865) mutant zebrafish is visualized. High resolution live imaging in zebrafish allows one to observe multiple mitoses to statistically quantify mitotic defects and timing of mitotic progression. In addition, observation of qualitative aspects that define improper mitotic processes (i.e., congression defects, missegregation of chromosomes, etc.) and improper chromosomal outcomes (i.e., aneuploidy, polyploidy, micronuclei, etc.) are observed. This assay can be applied to the observation of tissue differentiation/development and is amenable to the use of mutant zebrafish and pharmacological agents. Visualization of how defects in mitosis lead to cancer and developmental disorders will greatly enhance understanding of the pathogenesis of disease.

Viruses and the nuclear envelope

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

p32 is a novel target for viral protein ICP34.5 of herpes simplex virus type 1 and facilitates viral nuclear egress

As a large double-stranded DNA virus, herpes simplex virus type 1 (HSV-1) assembles capsids in the nucleus where the viral particles exit by budding through the inner nuclear membrane. Although a number of viral and host proteins are involved, the machinery of viral egress is not well understood. In a search for host interacting proteins of ICP34.5, which is a virulence factor of HSV-1, we identified a cellular protein, p32 (gC1qR/HABP1), by mass spectrophotometer analysis. When expressed, ICP34.5 associated with p32 in mammalian cells. Upon HSV-1 infection, p32 was recruited to the inner nuclear membrane by ICP34.5, which paralleled the phosphorylation and rearrangement of nuclear lamina. Knockdown of p32 in HSV-1-infected cells significantly reduced the production of cell-free viruses, suggesting that p32 is a mediator of HSV-1 nuclear egress. These observations suggest that the interaction between HSV-1 ICP34.5 and p32 leads to the disintegration of nuclear lamina and facilitates the nuclear egress of HSV-1 particles.

Nuclear envelope breakdown induced by herpes simplex virus type 1 involves the activity of viral fusion proteins

Herpesvirus infection reorganizes components of the nuclear lamina usually without loss of integrity of the nuclear membranes. We report that wild-type HSV infection can cause dissolution of the nuclear envelope in transformed mouse embryonic fibroblasts that do not express torsinA. Nuclear envelope breakdown is accompanied by an eight-fold inhibition of virus replication. Breakdown of the membrane is much more limited during infection with viruses that lack the gB and gH genes, suggesting that breakdown involves factors that promote fusion at the nuclear membrane. Nuclear envelope breakdown is also inhibited during infection with virus that does not express UL34, but is enhanced when the US3 gene is deleted, suggesting that envelope breakdown may be enhanced by nuclear lamina disruption. Nuclear envelope breakdown cannot compensate for deletion of the UL34 gene suggesting that mixing of nuclear and cytoplasmic contents is insufficient to bypass loss of the normal nuclear egress pathway.

O-Linked β-N-acetylglucosamine (O-GlcNAc) regulates emerin binding to barrier to autointegration factor (BAF) in a chromatin- and lamin B-enriched "niche"

Emerin, a membrane component of nuclear "lamina" networks with lamins and barrier to autointegration factor (BAF), is highly O-GlcNAc-modified ("O-GlcNAcylated") in mammalian cells. Mass spectrometry analysis revealed eight sites of O-GlcNAcylation, including Ser-53, Ser-54, Ser-87, Ser-171, and Ser-173. Emerin O-GlcNAcylation was reduced ~50% by S53A or S54A mutation in vitro and in vivo. O-GlcNAcylation was reduced ~66% by the triple S52A/S53A/S54A mutant, and S173A reduced O-GlcNAcylation of the S52A/S53A/S54A mutant by ~30%, in vivo. We separated two populations of emerin, A-type lamins and BAF; one population solubilized easily, and the other required sonication and included histones and B-type lamins. Emerin and BAF associated only in histone- and lamin-B-containing fractions. The S173D mutation specifically and selectively reduced GFP-emerin association with BAF by 58% and also increased GFP-emerin hyper-phosphorylation. We conclude that β-N-acetylglucosaminyltransferase, an essential enzyme, controls two regions in emerin. The first region, defined by residues Ser-53 and Ser-54, flanks the LEM domain. O-GlcNAc modification at Ser-173, in the second region, is proposed to promote emerin association with BAF in the chromatin/lamin B "niche." These results reveal direct control of a conserved LEM domain nuclear lamina component by β-N-acetylglucosaminyltransferase, a nutrient sensor that regulates cell stress responses, mitosis, and epigenetics.

HIV protease inhibitors do not cause the accumulation of prelamin A in PBMCs from patients receiving first line therapy: the ANRS EP45 "aging" study

BACKGROUND

The ANRS EP45 "Aging" study investigates the cellular mechanisms involved in the accelerated aging of HIV-1 infected and treated patients. The present report focuses on lamin A processing, a pathway known to be altered in systemic genetic progeroid syndromes.

METHODS

35 HIV-1 infected patients being treated with first line antiretroviral therapy (ART, mean duration at inclusion: 2.7±1.3 years) containing boosted protease inhibitors (PI/r) (comprising lopinavir/ritonavir in 65% of patients) were recruited together with 49 seronegative age- and sex-matched control subjects (http://clinicaltrials.gov/, NCT01038999). In more than 88% of patients, the viral load was <40 copies/ml and the CD4+ cell count was >500/mm³. Prelamin A processing in peripheral blood mononuclear cells (PBMCs) from patients and controls was analysed by western blotting at inclusion. PBMCs from patients were also investigated at 12 and 24 months after enrolment in the study. PBMCs from healthy controls were also incubated with boosted lopinavir in culture medium containing various concentrations of proteins (4 to 80 g/L).

RESULTS

Lamin A precursor was not observed in cohort patient PBMC regardless of the PI/r used, the dose and the plasma concentration. Prelamin A was detected in PBMC incubated in culture medium containing a low protein concentration (4 g/L) but not in plasma (60-80 g/L) or in medium supplemented with BSA (40 g/L), both of which contain a high protein concentration.

CONCLUSIONS

Prelamin A processing abnormalities were not observed in PBMCs from patients under the PI/r first line regimen. Therefore, PI/r do not appear to contribute to lamin A-related aging in PBMCs. In cultured PBMCs from healthy donors, prelamin A processing abnormalities were only observed when the protein concentration in the culture medium was low, thus increasing the amount of PI available to enter cells. ClinicalTrials.gov NCT01038999 http://clinicaltrials.gov/ct2/show/NCT01038999.

Mouse cytomegalovirus egress protein pM50 interacts with cellular endophilin-A2

The herpesvirus replication cycle comprises maturation processes in the nucleus and cytoplasm of the infected cells. After their nuclear assembly viral capsids translocate via primary envelopment towards the cytoplasm. This event is mediated by the nuclear envelopment complex, which is composed by two conserved viral proteins belonging to the UL34 and UL31 protein families. Here, we generated recombinant viruses, which express affinity-tagged pM50 and/or pM53, the pUL34 and pUL31 homologues of the murine cytomegalovirus. We extracted pM50- and pM53-associated protein complexes from infected cells and analysed their composition after affinity purification by mass spectrometry. We observed reported interaction partners and identified new putative protein-protein interactions for both proteins. Endophilin-A2 was observed as the most prominent cellular partner of pM50. We found that endophilin-A2 binds to pM50 directly, and this interaction seems to be conserved in the pUL34 family.

Alternative nuclear transport for cellular protein quality control

Herpesvirus capsids traverse the nuclear envelope (NE) by utilizing an unusual export pathway termed nuclear egress. In this process, the viral capsid is delivered into the perinuclear space (PNS), producing a vesicular intermediate after fission. After fusion with the outer nuclear membrane (ONM), the naked capsid is released into the cytosol. A recent study now suggests that this pathway might be an endogenous cellular pathway, co-opted by viruses, that serves to transport cellular cargo exceeding the size limit imposed by the nuclear pore complex (NPC). We propose that one function of this pathway is to transport nuclear protein aggregates to the cytosolic autophagy machinery. Our model has implications for our understanding of laminopathies and related diseases affecting proteins residing at the inner nuclear membrane (INM) and nuclear lamina.

Nuclear actin and lamins in viral infections

Lamins are the best characterized cytoskeletal components of the cell nucleus that help to maintain the nuclear shape and participate in diverse nuclear processes including replication or transcription. Nuclear actin is now widely accepted to be another cytoskeletal protein present in the nucleus that fulfills important functions in the gene expression. Some viruses replicating in the nucleus evolved the ability to interact with and probably utilize nuclear actin for their replication, e.g., for the assembly and transport of capsids or mRNA export. On the other hand, lamins play a role in the propagation of other viruses since nuclear lamina may represent a barrier for virions entering or escaping the nucleus. This review will summarize the current knowledge about the roles of nuclear actin and lamins in viral infections.

Intragenic and extragenic suppression of a mutation in herpes simplex virus 1 UL34 that affects both nuclear envelope targeting and membrane budding

Late in infection herpesviruses move DNA-filled capsids from the nucleus to the cytoplasm by enveloping DNA-containing capsids at the inner nuclear membrane (INM) and deenveloping them at the outer nuclear membrane. This process requires two conserved herpesvirus proteins, pUL31 and pUL34. Interaction between pUL34 and pUL31 is essential for targeting both proteins to the nuclear envelope (NE), and sequences that mediate the targeting interaction have been mapped in both proteins. Here, we show that a mutation in the INM-targeting domain of pUL34 fails to support production of infectious virus or plaque formation. The mutation results in multiple defects, including impaired interaction between pUL34 and pUL31, poor NE targeting of pUL34, and misregulated, capsid-independent budding of the NE. The mutant defects in virus production, plaque formation, and pUL31 interaction can be suppressed by other mutations in the INM-targeting domain of pUL31 and by additional mutations in the pUL34 coding sequence.

A functional role for TorsinA in herpes simplex virus 1 nuclear egress

Herpes simplex virus 1 (HSV-1) capsids leave the nucleus by a process of envelopment and de-envelopment at the nuclear envelope (NE) that is accompanied by structural alterations of the NE. As capsids translocate across the NE, transient primary enveloped virions form in the perinuclear space. Here, we provide evidence that torsinA (TA), a ubiquitously expressed ATPase, has a role in HSV-1 nuclear egress. TA resides within the lumen of the endoplasmic reticulum (ER)/NE and functions in maintaining normal NE architecture. We show that perturbation of TA normal function by overexpressing torsinA wild type (TAwt) inhibits HSV-1 production. Ultrastructural analysis of infected cells overexpressing TAwt revealed reduced levels of surface virions in addition to accumulation of novel, double-membrane structures called virus-like vesicles (VLVs). Although mainly found in the cytoplasm, VLVs resemble primary virions in their size, by the appearance of the inner membrane, and by the presence of pUL34, a structural component of primary virions. Collectively, our data suggest a model in which interference of TA normal function by overexpression impairs de-envelopment of the primary virions leading to their accumulation in a cytoplasmic membrane compartment. This implies novel functions for TA at the NE.

Herpesviruses and intermediate filaments: close encounters with the third type

Intermediate filaments (IF) are essential to maintain cellular and nuclear integrity and shape, to manage organelle distribution and motility, to control the trafficking and pH of intracellular vesicles, to prevent stress-induced cell death, and to support the correct distribution of specific proteins. Because of this, IF are likely to be targeted by a variety of pathogens, and may act in favor or against infection progress. As many IF functions remain to be identified, however, little is currently known about these interactions. Herpesviruses can infect a wide variety of cell types, and are thus bound to encounter the different types of IF expressed in each tissue. The analysis of these interrelationships can yield precious insights into how IF proteins work, and into how viruses have evolved to exploit these functions. These interactions, either known or potential, will be the focus of this review.

Breach of the nuclear lamina during assembly of herpes simplex viruses

Beneath the inner nuclear membrane lies the dense meshwork of the nuclear lamina, which provides structural support for the nuclear envelope and serves as an important organizing center for a number of nuclear and cytoplasmic constituents and processes. Herpesviruses have a significant and wide-ranging impact on human health, and their capacity to replicate and cause disease includes events that occur in the host cell nucleus. Herpesviruses begin assembly of progeny virus in the nuclei of infected cells and their capsids must escape the confines of the nucleus by budding through the inner nuclear membrane (INM) to proceed with later stages of virion assembly and egress. Access of viral capsids to the INM thus necessitates disruption of the dense nuclear lamina layer. We review herpesvirus effects on the nuclear lamina and in particular the roles of the herpes simplex virus-encoded nuclear envelope complex and viral kinases on lamin phosphorylation, dissociation, and nucleocapsid envelopment at the INM.

Herpes simplex virus 1 pUL34 plays a critical role in cell-to-cell spread of virus in addition to its role in virus replication

Herpes simplex virus (HSV) pUL34 plays a critical role in virus replication by mediating egress of nucleocapsids from the infected cell nucleus. We have identified a mutation in pUL34 (Y68A) that produces a major defect in virus replication and impaired nuclear egress but also profoundly inhibits cell-to-cell spread and trafficking of gE. Virion release to the extracellular medium is not affected by the Y68A mutation, indicating that the mutation specifically inhibits cell-to-cell spread. We isolated extragenic suppressors of the Y68A plaque formation defect and mapped them by a combination of high-throughput Illumina sequencing and PCR-based screening. We found that suppression is highly correlated with a nonsense mutation in the US9 gene, which plays a critical role in cell-to-cell spread of HSV-1 in neurons. The US9 mutation alone is not sufficient to suppress the Y68A spread phenotype, indicating a likely role for multiple viral factors.

The nucleoplasmic reticulum: form and function

The nuclear envelope (NE) physically separates nucleoplasm and cytoplasm, contributes to nuclear structural integrity, controls selective bidirectional transport of ions and macromolecular cargo, regulates gene expression, and acts as a mechanotransducer and a platform for signalling. It is noteworthy however that the NE is not simply a smooth-surfaced outer boundary but is interrupted by invaginations that reach deep within the nucleoplasm and could even traverse the nucleus completely. The existence of such a complex branched network of invaginations forming a nucleoplasmic reticulum (NR) provides sites that are capable of carrying out the 'conventional' NE functions deep within the nucleus in regions that would otherwise be remote from the nuclear periphery. In this review, we describe the structural features of NR in normal and pathological states and discuss the current understanding of their functional and possible pathological roles.

Involvement of the UL24 protein in herpes simplex virus 1-induced dispersal of B23 and in nuclear egress

UL24 of herpes simplex virus 1 (HSV-1) is widely conserved within the Herpesviridae family. Herein, we tested the hypothesis that UL24, which we have previously shown to induce the redistribution of nucleolin, also affects the localization of the nucleolar protein B23. We found that HSV-1-induced dispersal of B23 was dependent on UL24. The conserved N-terminal portion of UL24 was sufficient to induce the redistribution of B23 in transient transfection assays. Mutational analysis revealed that the endonuclease motif of UL24 was important for B23 dispersal in both transfected and infected cells. Nucleolar protein relocalization during HSV-1 infection was also observed in non-immortalized cells. Analysis of infected cells by electron microscopy revealed a decrease in the ratio of cytoplasmic versus nuclear viral particles in cells infected with a UL24-deficient strain compared to KOS-infected cells. Our results suggest that UL24 promotes nuclear egress of nucleocapsids during HSV-1 infection, possibly though effects on nucleoli.

Role of tegument proteins in herpesvirus assembly and egress

Morphogenesis and maturation of viral particles is an essential step of viral replication. An infectious herpesviral particle has a multilayered architecture, and contains a large DNA genome, a capsid shell, a tegument and an envelope spiked with glycoproteins. Unique to herpesviruses, tegument is a structure that occupies the space between the nucleocapsid and the envelope and contains many virus encoded proteins called tegument proteins. Historically the tegument has been described as an amorphous structure, but increasing evidence supports the notion that there is an ordered addition of tegument during virion assembly, which is consistent with the important roles of tegument proteins in the assembly and egress of herpesviral particles. In this review we first give an overview of the herpesvirus assembly and egress process. We then discuss the roles of selected tegument proteins in each step of the process, i.e., primary envelopment, de-envelopment, secondary envelopment and transport of viral particles. We also suggest key issues that should be addressed in the near future.

The alphaherpesvirus US3/ORF66 protein kinases direct phosphorylation of the nuclear matrix protein matrin 3

The protein kinase found in the short region of alphaherpesviruses, termed US3 in herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PRV) and ORF66 in varicella-zoster virus (VZV), affects several viral and host cell processes, and its specific targets remain an area of active investigation. Reports suggesting that HSV-1 US3 substrates overlap with those of cellular protein kinase A (PKA) prompted the use of an antibody specific for phosphorylated PKA substrates to identify US3/ORF66 targets. HSV-1, VZV, and PRV induced very different substrate profiles that were US3/ORF66 kinase dependent. The predominant VZV-phosphorylated 125-kDa species was identified as matrin 3, one of the major nuclear matrix proteins. Matrin 3 was also phosphorylated by HSV-1 and PRV in a US3 kinase-dependent manner and by VZV ORF66 kinase at a novel residue (KRRRT150EE). Since VZV-directed T150 phosphorylation was not blocked by PKA inhibitors and was not induced by PKA activation, and since PKA predominantly targeted matrin 3 S188, it was concluded that phosphorylation by VZV was PKA independent. However, purified VZV ORF66 kinase did not phosphorylate matrin 3 in vitro, suggesting that additional cellular factors were required. In VZV-infected cells in the absence of the ORF66 kinase, matrin 3 displayed intranuclear changes, while matrin 3 showed a pronounced cytoplasmic distribution in late-stage cells infected with US3-negative HSV-1 or PRV. This work identifies phosphorylation of the nuclear matrix protein matrin 3 as a new conserved target of this kinase group.

Escape of herpesviruses from the nucleus

The nuclear envelope of eukaryotic cells is composed of double lipid-bilayer membranes, the membrane-connected nuclear pore complexes and an underlying nuclear lamina network. The nuclear pore complexes serve as gates for regulating the transport of macromolecules between cytoplasm and nucleus. The nuclear lamina not only provides an intact meshwork for maintaining the nuclear stiffness but also presents a natural barrier against most DNA viruses. Herpesviruses are large DNA viruses associated with multiple human and animal diseases. The complex herpesviral virion contains more than 30 viral proteins. After viral DNA replication, the newly synthesised genome is packaged into the pre-assembled intranuclear capsid. The nucleocapsid must then transverse through the nuclear envelope to the cytoplasm for the subsequent maturation process. Information regarding how nucleocapsid breaches the rigid nuclear lamina barrier and accesses the inner nuclear membrane for primary envelopment has emerged recently. From the point of view of both viral components and nuclear structure, this review summarises recent advances in the complicated protein-protein interactions and the phosphorylation regulations involved in the nuclear egress of herpesviral nucleocapsids.

Significance of host cell kinases in herpes simplex virus type 1 egress and lamin-associated protein disassembly from the nuclear lamina

The nuclear lamina is thought to be a steric barrier to the herpesvirus capsid. Disruption of the lamina accompanied by phosphorylation of lamina proteins is a conserved feature of herpesvirus infection. In HSV-1-infected cells, protein kinase C (PKC) alpha and delta isoforms are recruited to the nuclear membrane and PKC delta has been implicated in phosphorylation of emerin and lamin B. We tested two critical hypotheses about the mechanism and significance of lamina disruption. First, we show that chemical inhibition of all PKC isoforms reduced viral growth five-fold and inhibited capsid egress from the nucleus. However, specific inhibition of either conventional PKCs or PKC delta does not inhibit viral growth. Second, we show hyperphosphorylation of emerin by viral and cellular kinases is required for its disassociation from the lamina. These data support hypothesis that phosphorylation of lamina components mediates lamina disruption during HSV nuclear egress.

Nucleolin is required for efficient nuclear egress of herpes simplex virus type 1 nucleocapsids

Herpesvirus nucleocapsids assemble in the nucleus and must cross the nuclear membrane for final assembly and maturation to form infectious progeny virions in the cytoplasm. It has been proposed that nucleocapsids enter the perinuclear space by budding through the inner nuclear membrane, and these enveloped nucleocapsids then fuse with the outer nuclear membrane to enter the cytoplasm. Little is known about the mechanism(s) for nuclear egress of herpesvirus nucleocapsids and, in particular, which, if any, cellular proteins are involved in the nuclear egress pathway. UL12 is an alkaline nuclease encoded by herpes simplex virus type 1 (HSV-1) and has been suggested to be involved in viral DNA maturation and nuclear egress of nucleocapsids. Using a live-cell imaging system to study cells infected by a recombinant HSV-1 expressing UL12 fused to a fluorescent protein, we observed the previously unreported nucleolar localization of UL12 in live infected cells and, using coimmunoprecipitation analyses, showed that UL12 formed a complex with nucleolin, a nucleolus marker, in infected cells. Knockdown of nucleolin in HSV-1-infected cells reduced capsid accumulation, as well as the amount of viral DNA resistant to staphylococcal nuclease in the cytoplasm, which represented encapsidated viral DNA, but had little effect on these viral components in the nucleus. These results indicated that nucleolin is a cellular factor required for efficient nuclear egress of HSV-1 nucleocapsids in infected cells.

Analysis of a charge cluster mutation of herpes simplex virus type 1 UL34 and its extragenic suppressor suggests a novel interaction between pUL34 and pUL31 that is necessary for membrane curvature around capsids

Interaction between pUL34 and pUL31 is essential for targeting both proteins to the inner nuclear membrane (INM). Sequences mediating the targeting interaction have been mapped by others with both proteins. We have previously reported identification of charge cluster mutants of herpes simplex virus type 1 UL34 that localize properly to the inner nuclear membrane, indicating interaction with UL31, but fail to complement a UL34 deletion. We have characterized one mutation (CL04) that alters a charge cluster near the N terminus of pUL34 and observed the following. (i) The CL04 mutant has a dominant-negative effect on pUL34 function, indicating disruption of some critical interaction. (ii) In infections with CL04 pUL34, capsids accumulate in close association with the INM, but no perinuclear enveloped viruses, cytoplasmic capsids, or virions or cell surface virions were observed, suggesting that CL04 UL34 does not support INM curvature around the capsid. (iii) Passage of UL34-null virus on a stable cell line that expresses CL04 resulted in selection of extragenic suppressor mutants that grew efficiently using the mutant pUL34. (iv) All extragenic suppressors contained an R229-->L mutation in pUL31 that was sufficient to suppress the CL04 phenotype. (v) Immunolocalization and coimmunoprecipitation experiments with truncated forms of pUL34 and pUL31 confirm that N-terminal sequences of pUL34 and a C-terminal domain of pUL31 mediate interaction but not nuclear membrane targeting. pUL34 and pUL31 may make two essential interactions-one for the targeting of the complex to the nuclear envelope and another for nuclear membrane curvature around capsids.

Conserved residues in the UL24 protein of herpes simplex virus 1 are important for dispersal of the nucleolar protein nucleolin

The UL24 family of proteins is widely conserved among herpesviruses. We demonstrated previously that UL24 of herpes simplex virus 1 (HSV-1) is important for the dispersal of nucleolin from nucleolar foci throughout the nuclei of infected cells. Furthermore, the N-terminal portion of UL24 localizes to nuclei and can disperse nucleolin in the absence of any other viral proteins. In this study, we tested the hypothesis that highly conserved residues in UL24 are important for the ability of the protein to modify the nuclear distribution of nucleolin. We constructed a panel of substitution mutations in UL24 and tested their effects on nucleolin staining patterns. We found that modified UL24 proteins exhibited a range of subcellular distributions. Mutations associated with a wild-type localization pattern for UL24 correlated with high levels of nucleolin dispersal. Interestingly, mutations targeting two regions, namely, within the first homology domain and overlapping or near the previously identified PD-(D/E)XK endonuclease motif, caused the most altered UL24 localization pattern and the most drastic reduction in its ability to disperse nucleolin. Viral mutants corresponding to the substitutions G121A and E99A/K101A both exhibited a syncytial plaque phenotype at 39 degrees C. vUL24-E99A/K101A replicated to lower titers than did vUL24-G121A or KOS. Furthermore, the E99A/K101A mutation caused the greatest impairment of HSV-1-induced dispersal of nucleolin. Our results identified residues in UL24 that are critical for the ability of UL24 to alter nucleoli and further support the notion that the endonuclease motif is important for the function of UL24 during infection.

Herpes simplex virus 2 UL13 protein kinase disrupts nuclear lamins

Herpesviruses must cross the inner nuclear membrane and underlying lamina to exit the nucleus. HSV-1 US3 and PKC can phosphorylate lamins and induce their dispersion but do not elicit all of the phosphorylated lamin species produced during infection. UL13 is a serine threonine protein kinase conserved among many herpesviruses. HSV-1 UL13 phosphorylates US3 and thereby controls UL31 and UL34 nuclear rim localization, indicating a role in nuclear egress. Here, we report that HSV-2 UL13 alone induced conformational changes in lamins A and C and redistributed lamin B1 from the nuclear rim to intranuclear granular structures. HSV-2 UL13 directly phosphorylated lamins A, C, and B1 in vitro, and the lamin A1 tail domain. HSV-2 infection recapitulated the lamin alterations seen upon expression of UL13 alone, and other alterations were also observed, indicating that additional viral and/or cellular proteins cooperate with UL13 to alter lamins during HSV-2 infection to allow nuclear egress.

Herpesvirus assembly: an update

The order Herpesvirales contains viruses infecting animals from molluscs to men with a common virion morphology which have been classified into the families Herpesviridae, Alloherpesviridae and Malacoherpesviridae. Herpes virions are among the most complex virus particles containing a multitude of viral and cellular proteins which assemble into nucleocapsid, envelope and tegument. After autocatalytic assembly of the capsid and packaging of the newly replicated viral genome, a process which occurs in the nucleus and resembles head formation and genome packaging in the tailed double-stranded DNA bacteriophages, the nucleocapsid is translocated to the cytoplasm by budding at the inner nuclear membrane followed by fusion of the primary envelope with the outer nuclear membrane. Viral and cellular proteins are involved in mediating this 'nuclear egress' which entails substantial remodeling of the nuclear architecture. For final maturation within the cytoplasm tegument components associate with the translocated nucleocapsid, with themselves, and with the future envelope containing viral membrane proteins in a complex network of interactions resulting in the formation of an infectious herpes virion. The diverse interactions between the involved proteins exhibit a striking redundancy which is still insufficiently understood. In this review, recent advances in our understanding of the molecular processes resulting in herpes virion maturation will be presented and discussed as an update of a previous contribution [Mettenleiter, T.C., 2004. Budding events in herpesvirus morphogenesis. Virus Res. 106, 167-180].