Herpesviral replication compartments move and coalesce at nuclear speckles to enhance export of viral late mRNA

Abstract

The role of the intranuclear movement of chromatin in gene expression is not well-understood. Herpes simplex virus forms replication compartments (RCs) in infected cell nuclei as sites of viral DNA replication and late gene transcription. These structures develop from small compartments that grow in size, move, and coalesce. Quantitative analysis of RC trajectories, derived from 4D images, shows that most RCs move by directed motion. Directed movement is impaired in the presence of actin and myosin inhibitors as well as a transcription inhibitor. In addition, RCs coalesce at and reorganize nuclear speckles. Lastly, distinct effects of actin and myosin inhibitors on viral gene expression suggest that RC movement is not required for transcription, but rather, movement results in the bridging of transcriptionally active RCs with nuclear speckles to form structures that enhance export of viral late mRNAs.gene movement | nuclear export M ovement of chromatin and its interactions and location in the nucleus are believed to be important for regulation of gene expression (1). A number of studies have shown an association between the movement of chromosomal domains and changes in transcriptional activity. In yeast, activation of genes is associated with targeting of the gene to the nuclear periphery (2, 3), and this is believed to be because of the linkage of actively transcribed genes to the nuclear pore complex for RNA export (4). Lymphoid cell genes move to positions near centromeric heterochromatin coincident with transcriptional silencing (5). Transcriptional activation of the β-globin locus involves movement of the gene away from centromeric heterochromatin (6). During B-cell development, the IgH and Igκ loci are localized at the nuclear periphery in hematopoietic progenitor cells, but they move internally coincident with transcriptional activation in pro-B cells (7). Similarly, the β-globin gene relocalizes to the nuclear interior as it is activated during murine erythroid differentiation (8). Actively transcribed genes are also thought to be brought into close contact with nuclear speckles (9, 10), sites often regarded as hubs of RNA metabolism. Although these results link gene movement with changes in transcriptional activity, it is unclear whether intranuclear movement of chromatin is the cause or result of changes in transcriptional activity (1). Therefore, it is important to define systems in which the function of chromatin movement can be experimentally assessed.Nuclear forms of actin and myosin have been implicated in mediating long-range directed movement (11-13), but the mechanism by which this putative nucleoskeletal system operates is unknown. In addition to their possible roles in mediating intranuclear movement, there is increasing evidence for roles of nuclear actin and myosin in cellular transcription, chromatin remodeling, and mRNA export (reviewed in ref. 14). How and whether their roles in movement are connected to regulation of gene activity are unclear.Viruses are simple genetic entities and ha...

Persistent nuclear actin filaments inhibit transcription by RNA polymerase II

“…The following constructs were as previously described: EGFP-supervillin fragment (Wulfkuhle et al, 1999), α-actin-GFP and the V163M mutant (Domazetovska et al, 2007a), the EYFP-NLS-β-actin construct and mutations of it (Chang et al, 2011;Posern et al, 2002), GFP-RNAPII (Sugaya et al, 2000) and the MHCIITA promoter luciferase construct . To generate the mCherry-supervillin fragment, the original construct was cut using EcoRI, purified, and inserted into the pmCherry-C1 vector backbone (Clontech).…”

Nuclear α-catenin mediates the DNA damage response via β-catenin and nuclear actin

“…To generate mCherry-Myc-NLS-α-catenin and the corresponding mCherry-Myc-NLS-tagged Nterminal fragment of α-catenin, Myc-NLS-α-catenin was digested using restriction enzymes EcorI/ApaI, purified, and inserted into the pmCherry-C1 vector backbone (Clonetech). The EYFP-NLS-β-actin constructs and mutations were previously described (Chang et al, 2011;Posern et al, 2002). Lifeact-NLS-RFP was created by PCR mutagenesis from Lifeact-RFP, a kind gift from Dr Alexander Bershadsky (Weizmann Institute of Science, Israel).…”

Viral and cellular subnuclear structures in human cytomegalovirus-infected cells

“…It is likely that nuclear speckles are involved in HCMV IE gene transcription, but it is unknown what transcriptional events they might facilitate. In HSV-infected cells, it has been reported that nuclear speckles are involved in export of viral RNA transcripts (Chang et al, 2011). Thus, nuclear speckles may serve in the processing and export of HCMV RNA transcripts early in infection.…”

“…It is likely that, as in HSVinfected cells (Chang et al, 2011), these are nuclear speckles that remain associated with HCMV replication compartments throughout virus replication. The directed movement of nuclear speckles with HSV replication compartments, or vice versa, is associated with efficient viral RNA transcription in HSV-infected cells, in particular promoting the export of late HSV RNA transcripts to the cytoplasm (Chang et al, 2011). The function of nuclear speckles associated with replication compartments in HCMV-infected cells has yet to be described, although it is likely that they are involved in viral RNA transcription.…”

“…The mechanisms driving coalescence of HCMV replication compartments have yet to be defined. Replication compartment coalescence in HSVinfected cells occurs via non-random movement and is dependent upon nuclear actin, myosin and viral transcription (Chang et al, 2011).…”