A viral histone-like protein exploits antagonism between linker histones and HMGB proteins to obstruct the cell cycle

Viral imitation is the sincerest form of epigenetic flattery

Viruses use multiple strategies to successfully generate progeny and overcome host defenses. In recent years, it has become increasingly evident that epigenetic mechanisms of host gene regulation are vulnerable to viral manipulation. In the form of histone mimicry, viral invasion of host chromatin is a striking example of how viruses have evolved to invade every aspect of cellular function for viral benefit. In this perspective, we will review how three viruses-influenza A, SARS-CoV-2, and Cotesia plutellae bracovirus-use histone mimicry to promote viral success through immune evasion. These examples highlight the importance of this burgeoning field and point toward the wealth of knowledge we have yet to uncover.

Controlling Much? Viral Control of Host Chromatin Dynamics

Viruses are exemplary molecular biologists and have been integral to scientific discovery for generations. It is therefore no surprise that nuclear replicating viruses have evolved to systematically take over host cell function through astoundingly specific nuclear and chromatin hijacking. In this review, we focus on nuclear replicating DNA viruses-herpesviruses and adenoviruses-as key examples of viral invasion in the nucleus. We concentrate on critical features of nuclear architecture, such as chromatin and the nucleolus, to illustrate the complexity of the virus-host battle for resources in the nucleus. We conclude with a discussion of the technological advances that have enabled the discoveries we describe and upcoming steps in this burgeoning field.

Human cytomegalovirus induces neuronal gene expression for viral maturation

Viral invasion of the host cell causes some of the most dramatic changes in biology. Human cytomegalovirus (HCMV) extensively remodels host cells, altering nuclear shape and generating a cytoplasmic viral-induced assembly compartment (vIAC). How these striking morphology changes take place in the context of host gene regulation is still emerging. Here, we discovered that histone variant macroH2A1 is essential for producing infectious progeny. Because virion maturation and cellular remodeling are closely linked processes, we investigated structural changes in the host cell upon HCMV infection. We discovered that macroH2A1 is necessary for HCMV-induced reorganization of the host nucleus, cytoskeleton, and endoplasmic reticulum. Furthermore, using RNA-seq we found that while all viral genes were highly expressed in the absence of macroH2A1, many HCMV-induced host genes were not. Remarkably, hundreds of these HCMV-induced macroH2A1-dependent host genes are associated with neuronal synapse formation and vesicle trafficking. Knock-down of these HCMV-induced neuronal genes during infection resulted in malformed vIACs and smaller plaques, establishing their importance to HCMV infection. Together, our findings demonstrate that HCMV manipulates host gene expression by hijacking a dormant neuronal secretory pathway for efficient virion maturation.

The nucleic acid binding protein SFPQ represses EBV lytic reactivation by promoting histone H1 expression

Epstein-Barr virus (EBV) uses a biphasic lifecycle of latency and lytic reactivation to infect >95% of adults worldwide. Despite its central role in EBV persistence and oncogenesis, much remains unknown about how EBV latency is maintained. We used a human genome-wide CRISPR/Cas9 screen to identify that the nuclear protein SFPQ was critical for latency. SFPQ supported expression of linker histone H1, which stabilizes nucleosomes and regulates nuclear architecture, but has not been previously implicated in EBV gene regulation. H1 occupied latent EBV genomes, including the immediate early gene BZLF1 promoter. Upon reactivation, SFPQ was sequestered into sub-nuclear puncta, and EBV genomic H1 occupancy diminished. Enforced H1 expression blocked EBV reactivation upon SFPQ knockout, confirming it as necessary downstream of SFPQ. SFPQ knockout triggered reactivation of EBV in B and epithelial cells, as well as of Kaposi's sarcoma-associated herpesvirus in B cells, suggesting a conserved gamma-herpesvirus role. These findings highlight SFPQ as a major regulator of H1 expression and EBV latency.

SET protein as an epigenetics target

The SET gene has four transcripts reported in NCBI, coding two isoforms of SET proteins. The most known function of SET protein is inhibiting protein phosphatase 2A, a tumor suppressor, which has been associated with different biological processes. In this review, our focus was on exploring the other SET functions related to epigenetic mechanisms, which impact cellular migration, cell cycle and apoptosis.

HSV-1 exploits host heterochromatin for nuclear egress

Herpes simplex virus (HSV-1) progeny form in the nucleus and exit to successfully infect other cells. Newly formed capsids navigate complex chromatin architecture to reach the inner nuclear membrane (INM) and egress. Here, we demonstrate by transmission electron microscopy (TEM) that HSV-1 capsids traverse heterochromatin associated with trimethylation on histone H3 lysine 27 (H3K27me3) and the histone variant macroH2A1. Through chromatin profiling during infection, we revealed global redistribution of these marks whereby massive host genomic regions bound by macroH2A1 and H3K27me3 correlate with decreased host transcription in active compartments. We found that the loss of these markers resulted in significantly lower viral titers but did not impact viral genome or protein accumulation. Strikingly, we discovered that loss of macroH2A1 or H3K27me3 resulted in nuclear trapping of capsids. Finally, by live-capsid tracking, we quantified this decreased capsid movement. Thus, our work demonstrates that HSV-1 takes advantage of the dynamic nature of host heterochromatin formation during infection for efficient nuclear egress.

Adenovirus protein VII binds the A-box of HMGB1 to repress interferon responses

Viruses hijack host proteins to promote infection and dampen host defenses. Adenovirus encodes the multifunctional protein VII that serves both to compact viral genomes inside the virion and disrupt host chromatin. Protein VII binds the abundant nuclear protein high mobility group box 1 (HMGB1) and sequesters HMGB1 in chromatin. HMGB1 is an abundant host nuclear protein that can also be released from infected cells as an alarmin to amplify inflammatory responses. By sequestering HMGB1, protein VII prevents its release, thus inhibiting downstream inflammatory signaling. However, the consequences of this chromatin sequestration on host transcription are unknown. Here, we employ bacterial two-hybrid interaction assays and human cell biological systems to interrogate the mechanism of the protein VII-HMGB1 interaction. HMGB1 contains two DNA binding domains, the A- and B-boxes, that bend DNA to promote transcription factor binding while the C-terminal tail regulates this interaction. We demonstrate that protein VII interacts directly with the A-box of HMGB1, an interaction that is inhibited by the HMGB1 C-terminal tail. By cellular fractionation, we show that protein VII renders A-box containing constructs insoluble, thereby acting to prevent their release from cells. This sequestration is not dependent on HMGB1's ability to bind DNA but does require post-translational modifications on protein VII. Importantly, we demonstrate that protein VII inhibits expression of interferon β, in an HMGB1- dependent manner, but does not affect transcription of downstream interferon- stimulated genes. Together, our results demonstrate that protein VII specifically harnesses HMGB1 through its A-box domain to depress the innate immune response and promote infection.

Human Adenovirus Gene Expression and Replication Is Regulated through Dynamic Changes in Nucleoprotein Structure throughout Infection

Human adenovirus (HAdV) is extremely common and can rapidly spread in confined populations such as daycare centers, hospitals, and retirement homes. Although HAdV usually causes only minor illness in otherwise healthy patients, HAdV can cause significant morbidity and mortality in certain populations, such as the very young, very old, or immunocompromised individuals. During infection, the viral DNA undergoes dramatic changes in nucleoprotein structure that promote the rapid expression of viral genes, replication of the DNA, and generation of thousands of new infectious virions-each process requiring a distinct complement of virus and host-encoded proteins. In this review, we summarize our current understanding of the nucleoprotein structure of HAdV DNA during the various phases of infection, the cellular proteins implicated in mediating these changes, and the role of epigenetics in HAdV gene expression and replication.

Doxorubicin impacts chromatin binding of HMGB1, Histone H1 and retinoic acid receptor

Doxorubicin (Dox), a widely used anticancer DNA-binding drug, affects chromatin in multiple ways, and these effects contribute to both its efficacy and its dose-limiting side effects, especially cardiotoxicity. Here, we studied the effects of Dox on the chromatin binding of the architectural proteins high mobility group B1 (HMGB1) and the linker histone H1, and the transcription factor retinoic acid receptor (RARα) by fluorescence recovery after photobleaching (FRAP) and fluorescence correlation spectroscopy (FCS) in live cells. At lower doses, Dox increased the binding of HMGB1 to DNA while decreasing the binding of the linker histone H1. At higher doses that correspond to the peak plasma concentrations achieved during chemotherapy, Dox reduced the binding of HMGB1 as well. This biphasic effect is interpreted in terms of a hierarchy of competition between the ligands involved and Dox-induced local conformational changes of nucleosome-free DNA. Combined, FRAP and FCS mobility data suggest that Dox decreases the overall binding of RARα to DNA, an effect that was only partially overcome by agonist binding. The intertwined interactions described are likely to contribute to both the effects and side effects of Dox.