Herpes simplex virus capsids are assembled and packaged in the nucleus and move by diffusion through the nucleoplasm to the nuclear envelope for egress. Analyzing their motion provides conclusions not only on capsid transport but also on the properties of the nuclear environment during infection. We utilized live-cell imaging and single-particle tracking to characterize capsid motion relative to the host chromatin. The data indicate that as the chromatin was marginalized toward the nuclear envelope it presented a restrictive barrier to the capsids. However, later in infection this barrier became more permissive and the probability of capsids to enter the chromatin increased. Thus, although chromatin marginalization initially restricted capsid transport to the nuclear envelope, a structural reorganization of the chromatin counteracted that to promote capsid transport later. Analyses of capsid motion revealed that it was subdiffusive, and that the diffusion coefficients were lower in the chromatin than in regions lacking chromatin. In addition, the diffusion coefficient in both regions increased during infection. Throughout the infection, the capsids were never enriched at the nuclear envelope, which suggests that instead of nuclear export the transport through the chromatin is the rate-limiting step for the nuclear egress of capsids. This provides motivation for further studies by validating the importance of intranuclear transport to the life cycle of HSV-1.

Parvoviruses are small single-stranded (ss) DNA viruses, which replicate in the nucleoplasm and affect both the structure and function of the nucleus. The nuclear stage of the parvovirus life cycle starts at the nuclear entry of incoming capsids and culminates in the successful passage of progeny capsids out of the nucleus. In this review, we will present past, current, and future microscopy and biochemical techniques and demonstrate their potential in revealing the dynamics and molecular interactions in the intranuclear processes of parvovirus infection. In particular, a number of advanced techniques will be presented for the detection of infection-induced changes, such as DNA modification and damage, as well as protein–chromatin interactions.

Herpes simplex virus type 1 (HSV-1) infection remodels the host nucleus, marginalizing chromatin and forming viral replication compartments (VRCs). Nuclear speckles, nuclear bodies enriched in RNA-processing factors, reposition around VRCs and undergo structural changes. While viral mRNAs are transcribed in VRCs and host transcription is largely suppressed, the nuclear routes used by viral and upregulated host transcripts and their relationship with nuclear bodies remain unclear. We show that immediate-early (IE) viral transcripts uniquely accumulate in nuclear speckles prior to export, unlike early or late transcripts, revealing a selective nuclear speckle-dependent pathway. Similarly, host mRNAs upregulated during infection traffic into nuclear speckles after transcription. Moreover, nuclear speckles are structurally remodeled, marked by the long non-coding RNA (lncRNA) MALAT1 removal and increased dynamics of the nuclear speckle core protein SRRM2. Finally, we found that blocking mRNA export causes IE transcripts to accumulate in nuclear speckles and that nuclear speckle disassembly severely impairs IE mRNA export, preventing downstream viral gene expression. These findings establish nuclear speckles as dynamic regulatory hubs that selectively facilitate the processing and export of IE viral mRNAs during HSV-1 infection.