By Florian Full and Michaela U. Gack
Most people acquire herpesviruses as early as infancy. After primary infection, these viruses establish persistent infection in the form of latency and remain in the infected host for life. There are eight known human herpesviruses including important pathogens such as herpes simplex virus type 1 and 2, varicella zoster virus and human cytomegalovirus. In addition, two members of the human herpesvirus family, Epstein-Barr virus and the Kaposi's sarcoma-associated herpesvirus (KSHV), are tumor viruses. Although in most individuals latent herpesvirus infection does not significantly affect health, immunocompromised individuals, such as ones that have AIDS or received organ transplants, are unable to control infection by certain herpesviruses and thus may exhibit severe disease.
Research in our laboratory is focused on the innate immune response against viral pathogens, in particular innate immunity mediated by so-called tripartite motif (TRIM) proteins. Most of the ~70 TRIM protein family members found in humans are E3 ubiquitin ligases and regulate a variety of cellular processes including antiviral innate immunity by catalyzing specific ubiquitination reactions. Because of their important role in antiviral responses, we postulated that TRIM proteins may also play a crucial role in restricting infection by oncogenic herpesviruses. Through a targeted RNAi screen, we tested the effect of silencing human TRIM proteins on the ability of KSHV to reactivate from latently-infected cells. This screen revealed that knockdown of several TRIM proteins induced spontaneous KSHV reactivation, suggesting that these TRIMs have the ability to suppress KSHV reactivation/lytic replication. Among the TRIM proteins identified in our screen was TRIM43, which has been a completely uncharacterized protein. We focused our in depth-mechanistic studies on TRIM43 because it was 1.) profoundly upregulated upon herpesvirus infection, and 2.) the only TRIM protein that blocked the lytic replication/reactivation of herpesviruses from the alpha-, beta-, and gamma-herpesvirus subfamilies, but did not affect the replication of other viruses (adenovirus and several RNA viruses) tested in our study. This suggested that TRIM43 is a herpesvirus-specific antiviral protein.
Whereas many TRIM proteins either directly target components of the virus, or alternatively, promote antiviral gene expression to inhibit virus replication, TRIM43 exhibited a very unique antiviral activity. Our studies showed that TRIM43 has a unique subcellular localization, which is at the host cell’s centrosome. A combination of proteomics, cell biological and biochemical analyses showed that TRIM43 binds to the centrosomal protein pericentrin and induces its proteasomal degradation via polyubiquitination. Degradation of pericentrin resulted in integrity loss of centrosomes and subsequent disruption of the nuclear envelope architecture. Interestingly, it had been previously established that herpesviruses need an intact nuclear envelope for effective viral gene expression and virus replication. Our studies indicated that the TRIM43-induced nuclear lamina alterations induce viral chromatin changes that ultimately inhibit virus lytic replication/reactivation. Our results therefore describe a hitherto unknown mechanism of cells to ward off herpesvirus infection, which is through regulation of centrosome and nuclear envelope integrity.
Being virologists, we mostly focused on the antiviral mechanism of TRIM43. Besides our findings on TRIM43’s role in anti-herpesviral defense, we made several other fascinating observations, some of which may also be of interest to fields other than virology. In contrast to many TRIM proteins whose expression is type I IFN-inducible, we found that the upregulation of TRIM43 gene expression during herpesvirus infection was not dependent on type I IFN but instead was mediated by the germline-specific transcription factor DUX4, which is itself transcriptionally upregulated during herpesvirus infection via an unknown mechanism. Usually DUX4 is expressed exclusively during early embryogenesis and in germ cells but not in differentiated adult tissues. Therefore several questions still remain unanswered such as: Why does herpesvirus infection lead to DUX4 expression? and; Do other DUX4-target genes also act antivirally? Moreover, DUX4 is also involved in the pathogenesis of facioscapulohumeral muscular dystrophy (FSHD) and the onset of various tumors. Thus, future studies will be needed to shed more light on the role of DUX4 and its target genes including ones encoding TRIM proteins during herpesvirus infection, FSHD, and DUX4-associated tumors.
Figure legend. Model of herpesvirus restriction by the DUX4-TRIM43-pericentrin-lamin axis. Herpesvirus infection leads to gene expression of the transcription factor DUX4 (1). DUX4 then transcriptionally induces TRIM43 expression (2). TRIM43 protein binds to pericentrin at the centrosome and mediates polyubiquitination of pericentrin (3), inducing its proteasomal degradation (4). Loss of pericentrin induces disruption of the nuclear lamina architecture (5), which induces viral chromatin changes that limit expression of viral lytic genes (6).
The paper in Nature Microbiology is here: go.nature.com/2zNc9Ab