The Herpes Simplex Virus Neurovirulence Factor γ34.5: Revealing Virus–Host Interactions

Abstract

Herpes simplex virus (HSV) is a ubiquitous human pathogen that causes a wide spectrum of disease, ranging from asymptomatic viral shedding to lethal encephalitis and disseminated disease [1,2]. These viruses belong to the neurotropic subfamily of α-herpesviruses, and after initial replication in epithelial cells, HSV enters sensory neurons to establish latency in neural ganglia. HSV can also progress to active lytic replication in the central nervous system, resulting in devastating encephalitis. To successfully replicate in the host nervous system, HSV encodes several viral proteins to counter the host innate response to infection. Among these, the multifunctional viral protein γ34.5 is central to countering several effector pathways in the host type I interferon (IFN) response. HSV γ34.5 is present in two copies in the repeated regions of the viral genome, and although initially described as a late gene, its expression is actually “leaky late,” with γ34.5 functioning to counter the host response after late viral DNA synthesis but also in the first hours of infection. Within γ34.5 are domains that specifically target host shutoff of protein synthesis [3], type I IFN induction through TANK-binding kinase (TBK1) [4], and inhibition of autophagy through Beclin 1 binding (Fig 1) [5]. HSV γ34.5 is required for full virulence in the murine brain [6,7]; however, recent evidence suggests that γ34.5 may function differently in newborn models of HSV disease compared to the adult [8]. Furthermore, some functions of γ34.5 are required for pathogenesis in non-nervous system tissue [9]. Here, we provide a brief overview of the multiple host responses modulated by γ34.5 for successful HSV replication in the nervous system and also discuss recent evidence that expands the role of γ34.5 to promote pathogenesis in several different tissue-types and across different developmental ages of the host. Fig 1 The HSV-1 major neurovirulence factor γ34.5 targets multiple different host pathways. HSV-1 γ34.5 Mediates Reversal of Host Shutoff of Total Protein Synthesis One of the earliest responses to infection is the type I IFN response and the innate pathways modulated by the IFN-inducible, double-stranded RNA–dependent protein kinase R (PKR) system. An important function of activated PKR during HSV infection is phosphorylation of the translation initiation factor eIF2α, resulting in translational arrest and reduction in the global synthesis of viral and cellular proteins [10]. However, HSV has evolved an effective strategy through γ34.5 to reverse the eIF2α kinase-mediated translational arrest to allow for successful viral replication. The carboxyl terminus of HSV-1 γ34.5 binds and retargets the host phosphatase PP1α to eIF2α, thus targeting eIF2α for dephosphorylation and reversing the shutoff of protein synthesis (Fig 2) [11]. Mutant viruses engineered to specifically disrupt the interaction between γ34.5 and the host phosphatase PP1α demonstrate the requirement of HSV-1–mediated retargeting of PP1α for pathogenesis in several different models of disease, including HSV keratitis [12], encephalitis, and disseminated disease in the neonate [9]. Interestingly, the carboxyl terminus of HSV-1 γ34.5 shares sequence homology with the host protein GADD34 (growth arrest and DNA damage-inducible gene 34) [13], which acts as PP1α regulatory subunit to target PP1α to eIF2α during periods of endoplasmic reticulum (ER) stress and the unfolded protein response. Earlier studies have shown that this host sequence and γ34.5 are interchangeable in the HSV-1 genome to preclude the premature shutoff of total protein synthesis, suggesting that during herpesvirus evolution, the virus acquired the GADD34 host sequence to improve viral replication and fitness [14]. Fig 2 Reversal of the host shutoff of protein synthesis mediated by HSV γ34.5. γ34.5 Binds TBK1 to Prevent Activation of the Type I IFN Response Prior to the initiation of the type I IFN response, HSV is detected in the host cell through several different pattern recognition receptors. For example, Toll-like receptor 3 (TLR3) detects HSV dsRNA in endosomes to stimulate IFN expression. In the cytoplasm, intracellular RNA and DNA sensors, such as retinoic acid-inducible gene I (RIG-I), melanoma differentiation-associated gene 5 (MDA5), interferon γ-inducible protein 16 (IFI16), and cyclic GMP-AMP synthase (cGAS), also detect HSV in the host cell [15–17]. Although these receptors detect different pathogen-associated molecular patterns, downstream signals are relayed through TBK1, which in turn phosphorylates and activates the interferon regulatory factor 3/7 (IRF3/7) for production of type I IFNs. HSV-1 γ34.5 counters this induction of the type I IFN response through binding of TBK1 with its amino terminus (Fig 1) [4]. Targeting of TBK1 by γ34.5 competes for IRF3 binding and ultimately inhibits IRF3 phosphorylation by TBK1, preventing IRF3 nuclear localization for type I IFN expression. A mutant virus deleted for the amino terminus of γ34.5 to demolish TBK1 binding demonstrates significantly increased IFN-β and interferon-stimulated gene (ISG) production in the first three to six hours of infection. In an ocular model of HSV disease, a virus deleted for TBK1 binding replicated poorly in the corneal epithelium and trigeminal ganglion and was effectively controlled by the host response before it reached the brain [18]. These findings reveal an additional role for γ34.5 in inhibiting the host response prior to transcription of type I IFNs and PKR up-regulation and demonstrate a role for early expression of this “leaky-late” gene.