Molecular and functional significance of cellular modifications induced by herpes simplex virus infection

Five families of diverse DNA viruses comprehensively restructure the nucleus

Many viruses have evolved ways to restructure their host cell's nucleus profoundly and unexpectedly upon infection. In particular, DNA viruses that need to commandeer their host's cellular synthetic functions to produce their progeny can induce the condensation and margination of host chromatin during productive infection, a phenomenon known as virus-induced reorganization of cellular chromatin (ROCC). These ROCC-inducing DNA viruses belong to 5 families (herpesviruses, baculoviruses, adenoviruses, parvoviruses, and geminiviruses) that infect a wide range of hosts and are important for human and ecosystem health, as well as for biotechnology. Although the study of virus-induced ROCC is in its infancy, investigations are already raising important questions, such as why only some DNA viruses that replicate their genomes in the nucleus elicit ROCC. Studying the shared and distinct properties of ROCC-inducing viruses will provide valuable insights into viral reorganization of host chromatin that could have implications for future therapies that target the viral life cycle.

Three-dimensional visualization of gammaherpesvirus life cycle in host cells by electron tomography

Gammaherpesviruses are etiologically associated with human tumors. A three-dimensional (3D) examination of their life cycle in the host is lacking, significantly limiting our understanding of the structural and molecular basis of virus-host interactions. Here, we report the first 3D visualization of key stages of the murine gammaherpesvirus 68 life cycle in NIH 3T3 cells, including viral attachment, entry, assembly, and egress, by dual-axis electron tomography. In particular, we revealed the transient processes of incoming capsids injecting viral DNA through nuclear pore complexes and nascent DNA being packaged into progeny capsids in vivo as a spool coaxial with the putative portal vertex. We discovered that intranuclear invagination of both nuclear membranes is involved in nuclear egress of herpesvirus capsids. Taken together, our results provide the structural basis for a detailed mechanistic description of gammaherpesvirus life cycle and also demonstrate the advantage of electron tomography in dissecting complex cellular processes of viral infection.

Annexation of the interchromosomal space during viral infection

The nucleus is known to be compartmentalized into units of function, but the processes leading to the spatial organization of chromosomes and nuclear compartments are not yet well defined. Here we report direct quantitative analysis of the global structural perturbations of interphase chromosome and interchromosome domain distribution caused by infection with herpes simplex virus-1 (HSV-1). Our results show that the peripheral displacement of host chromosomes that correlates with expansion of the viral replication compartment (VRC) is coupled to a twofold increase in nuclear volume. Live cell dynamic measurements suggest that viral compartment formation is driven by the functional activity of viral components and underscore the significance of spatial regulation of nuclear activities.

In situ hybridization and immuno-electron microscope analyses of the Us11 gene of herpes simplex virus type 1 during transient expression

The distribution of Us11 RNA and of its encoded protein have been investigated at the ultrastructural level in HeLa cells transiently expressing the Us11 gene of herpes simplex virus type 1. In these transfected cells, Us11 protein accumulates at sites identical to those of lytically infected cells, i.e., in nucleoli and in regions of the cytoplasm that contain ribosomes. Us11 RNA and polyadenylated RNA are scattered over the ribosome-rich areas of the cytoplasm. They also accumulate in the nucleoplasm on clustered ribonucleoprotein (RNP) fibrils but also in clusters of interchromatin granules, some of them contiguous to nucleoli. However they are never found in nucleoli. These data reveal the involvement of interchromatin granules in some steps of Us11 mRNA maturation and/or transport.

Induction of complete segregation of cellular DNA and non-encapsidated viral genomes in herpes simplex virus type 1 infected HeLa cells as revealed by in situ hybridization

The intranuclear distribution of human Alu elements and herpes simplex virus type 1 (HSV-1) genomes was examined in HeLa cells by post-embedding in situ hybridization using in parallel appropriate biotinylated DNA probes. The bound probes were detected by direct immunogold labeling. In non-infected cells, human Alu elements detected by BLUR 8 were randomly scattered over the masses and strands of chromatin throughout the nucleus. The marked asynchrony of the HSV-1 cycle in individual HeLa cells of 17 h infected cultures allowed us to study the respective distributions of cell and viral DNA during the course of the infectious cycle. Labeling of human Alu elements revealed that cellular DNA had become confined to the border of infected nuclei without extension of cellular DNA fibers into the newly formed electron-translucent regions that occupied the centers of the infected nuclei. Labeling of HSV-1 DNA detected by a viral DNA probe revealed that non-encapsidated viral genomes were present exclusively within this centrally located viral region whereas encapsidated HSV-1 genomes, which were more widely distributed in the infected cell, were seen within the marginated host chromatin as well as the central viral region. Therefore, HSV-1 infection induces a regrouping of human Alu elements, that is, of host chromatin at the nuclear border. Non-encapsidated HSV-1 genomes and cellular DNA do not co-localize. Instead, they always constitute two adjacent compartments without spatial interrelationships.

Compartmentalization of cellular and viral DNAs in adenovirus type 5 infection as revealed by ultrastructural in situ hybridization

We describe the respective distributions of cellular DNA and viral genomes in adenovirus type 5 (Ad5)-infected HeLa cells by means of electron microscope in situ hybridization using biotinylated Alu DNA and Ad5 DNA probes in a post-embedding technique. When hybridization was performed on sections of formaldehyde-fixed material, Alu elements were restricted to the cellular chromatin, irrespective of the stage of the infectious cycle, without any interpenetration within the virus-induced regions. Viral DNA was confined entirely to the latter. Similar topological distributions of cellular and viral DNA were obtained when in situ hybridizations were performed at the optical level on methanol-acetone fixed cells. Such separation between cellular and viral DNA persisted under experimental conditions which partially loosened the nucleoproteins. In addition, the viral DNA of the different viral structures spread without interpenetration. On the other hand, the procedure did not dissociate the clusters of viruses which remained, as usual, closely surrounded by innumerable viral genomes. It also preserved the shapes, content (viral ssDNA + viral 72 kD protein) and the topological localization of the pleomorphic viral ssDNA accumulation sites. These data, which suggest that all viral DNAs are spatially linked in the infected nuclei, suggest a prominent role for the internal nuclear matrix in virus replication, assembly and maturation.

Putative site for the acquisition of human herpesvirus 6 virion tegument

The virion of human herpesvirus 6 (HHV-6) contains a very distinct tegument layer, occupying the space between the nucleocapsid and the virion envelope. Ultrastructural analyses of thymocytes infected with HHV-6 revealed the presence of intranuclear spherical compartments, approximately 1.5 microns in diameter, in which tegumentation seems to take place. These compartments, termed tegusomes, were bounded by two membranes and contained ribosomes, consistent with their derivation by cytoplasmic invagination into the nucleus. Capsids located within the nucleus outside the tegusomes were all naked, while those located in the cytoplasm were uniformly tegumented. In contrast, capsids present inside the tegusomes contains teguments of variable thicknesses. In addition, nucleocapsids were documented in the process of budding into the tegusomes. We thus suggest that the tegusomes represent a cellular site in which HHV-6 virions acquire their tegument.