Role for A-Type Lamins in Herpesviral DNA Targeting and Heterochromatin Modulation

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Abstract
Posttranslational modification of histones is known to regulate chromatin structure and transcriptional activity, and the nuclear lamina is thought to serve as a site for heterochromatin maintenance and transcriptional silencing. In this report, we show that the nuclear lamina can also play a role in the downregulation of heterochromatin and in gene activation. Herpes simplex virus DNA initiates replication in replication compartments near the inner edge of the nucleus, and histones are excluded from these structures. To define the role of nuclear lamins in HSV replication, we examined HSV infection in wild-type and A-type lamin–deficient (Lmna−/−) murine embryonic fibroblasts (MEFs). In Lmna−/− cells, viral replication compartments are reduced in size and fail to target to the nuclear periphery, as observed in WT cells. Chromatin immunoprecipitation and immunofluorescence studies demonstrate that HSV DNA is associated with increased heterochromatin in Lmna−/− MEFs. These results argue for a functional role for A-type lamins as viral gene expression, DNA replication, and growth are reduced in Lmna−/− MEFs, with the greatest effect on viral replication at low multiplicity of infection. Thus, lamin A/C is required for targeting of the viral genome and the reduction of heterochromatin on viral promoters during lytic infection. The nuclear lamina can serve as a molecular scaffold for DNA genomes and the protein complexes that regulate both euchromatin and heterochromatin histone modifications. Transcription of eukaryotic genes is regulated by sequence-specific DNA-binding proteins that bind to the DNA as well as the structure of the chromatin associated with the specific gene. Posttranslational modification of histones plays a major role in the higher order structure of the chromatin and whether it serves as heterochromatin or inactive chromatin versus euchromatin or active chromatin. The nuclear lamina has been shown to promote the maintenance of heterochromatin in mammalian cells, but little is known about where heterochromatin is modulated. In this study, we find that the A-type lamins are required for the targeting of herpes simplex virus genomic DNA to the periphery of the infected cell nucleus and for the prevention or reduction of heterochromatin on the viral genome and transcriptional silencing of the viral genome. This raises the broader function of the nuclear lamina in the regulation of both euchromatin and heterochromatin. We propose that the nuclear lamina is a platform for the organization of chromatin remodeling and histone modification enzymes that regulate both euchromatin and heterochromatin.