Viral and host mRNA expression during HSV- 1 infection
Herpes simplex virus type 1 (HSV-1) infection extensively remodels the host nucleus, marginalizing chromatin to the nuclear periphery and forming prominent viral replication compartments (VRCs). Nuclear speckles, nuclear bodies enriched with RNA-processing factors, are repositioned adjacent to VRCs and undergo structural changes, including enlargement and rounding up. While viral mRNAs are transcribed within VRCs and host transcription is largely suppressed, the nuclear pathways that viral and upregulated host transcripts take en route to the cytoplasm and their relationship with nuclear bodies, remain poorly understood. Furthermore, it is unclear why the infected nucleus continues to uphold these nuclear bodies as the nucleus restructures. Here, we demonstrate that immediate-early (IE) viral transcripts uniquely accumulate within nuclear speckles prior to export, a phenomenon not observed for early or late viral transcripts, highlighting a selective nuclear speckle-dependent processing pathway. Host mRNAs transcriptionally upregulated during infection similarly traffic into nuclear speckles after transcription. Structural remodeling of nuclear speckles includes removal of the long non-coding RNA MALAT1 from nuclear speckles, and increased dynamics of the core nuclear speckle protein SRRM2. Additionally, inhibiting mRNA export results in the accumulation of IE viral mRNAs in nuclear speckles, and disassembling nuclear speckles significantly impairs IE viral mRNA export, effectively blocking downstream viral gene expression, confirming nuclear speckles as critical intermediate hubs for viral transcript processing and nuclear export. Altogether, these findings establish nuclear speckles as indispensable and dynamic regulatory centers that selectively facilitate the processing and export of immediate-early viral mRNAs during HSV-1 infection. Overall design: HeLa cells were infected with HSV1 and samples for GRO-Seq were collected 4 or 8 hours post infection.