Replication of ICP0-Null Mutant Herpes Simplex Virus Type 1 Is Restricted by both PML and Sp100

Abstract

Herpes simplex virus type 1 (HSV-1) mutants that fail to express the viral immediate-early protein ICP0 have a pronounced defect in viral gene expression and plaque formation in limited-passage human fibroblasts. ICP0 is a RING finger E3 ubiquitin ligase that induces the degradation of several cellular proteins. PML, the organizer of cellular nuclear substructures known as PML nuclear bodies or ND10, is one of the most notable proteins that is targeted by ICP0. Depletion of PML from human fibroblasts increases ICP0-null mutant HSV-1 gene expression, but not to wild-type levels. In this study, we report that depletion of Sp100, another major ND10 protein, results in a similar increase in ICP0-null mutant gene expression and that simultaneous depletion of both proteins complements the mutant virus to a greater degree. Although chromatin assembly and modification undoubtedly play major roles in the regulation of HSV-1 infection, we found that inhibition of histone deacetylase activity with trichostatin A was unable to complement the defect of ICP0-null mutant HSV-1 in either normal or PML-depleted human fibroblasts. These data lend further weight to the hypothesis that ND10 play an important role in the regulation of HSV-1 gene expression.Herpes simplex virus type 1 (HSV-1) is a common human pathogen that causes recurrent infections through its ability to establish a latent state in sensory ganglia after primary epithelial infections (for a general review, see reference 58). Lytic HSV-1 infection is characterized by abundant transcription from the entire viral genome in a temporal cascade of immediate-early (IE), early, and late gene products. The IE gene products regulate the expression of later classes of viral genes. In contrast, lytic cycle genes are repressed during latency, and only the latency-associated transcripts (LATs; derived from a single locus that lies countersense to the IE gene encoding ICP0) are expressed in readily detectable amounts (8,55,69). The IE protein ICP0 is a RING finger E3 ubiquitin ligase (4) that is required for efficient entry into the lytic cycle and which can induce reactivation of latent or quiescent genomes (reviewed in references 12, 14, 15, 29-31, and 55). ICP0 influences many cellular pathways, and one of its most prominent activities is its ability to localize to and disrupt nuclear substructures known as PML nuclear bodies (also known as ND10) (reviewed in references 10, 14, 16, and 43). This disruption occurs through ICP0-induced degradation of PML (17), the key component of ND10 which is required for assembly of these structures (34, 76). HSV-1 mutants that fail to express ICP0 or that express mutant ICP0 proteins that lack RING finger activity are unable to disrupt ND10 or to degrade PML (4,11,17,44,45). Such mutants have a profound defect in HSV-1 gene expression after infection of limited-passage human fibroblasts (9,21,62,63).The strong correlation between the effects of ICP0 on ND10and its requirement for lytic virus infection prompted the hypothesis that ND10 might ...

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