HCMV stimulation of interferon signaling to promote viral gene expression and latency - Project Summary Human cytomegalovirus (HCMV) is a ubiquitous and medically significant pathogen which causes severe morbidity and mortality among immunocompromised populations. HCMV, a betaherpesvirus, enters lifelong quiescent persistence in the infected host known as latency. Latency is very complex but is known to require the participation of both host and viral factors, and inhibition of either can lead to a loss of latency in ex vivo model systems. HCMV specifically co-opts innate cellular antiviral signaling via the interferon alpha receptor (IFNAR) signaling pathway, which leads to the activation of signal transducer and activator of transcription 1 (STAT1). STAT1 acts as a transcription factor, binding to a conserved DNA motif known as an interferon- stimulated response element (ISRE) and initiating transcription of interferon stimulated genes (ISGs) which are essential for antiviral defense. During normal cellular homeostasis, activated STAT1 (pSTAT1) is rapidly dephosphorylated resulting in cessation of signaling, however, HCMV maintains STAT1 phosphorylation at high levels through 72 hours. Maintenance of pSTAT1 during infection requires the viral protein UL138 in complex with host UAF1 and USP1, and inhibition of these proteins prevents the establishment of latency, suggesting that pSTAT1 signaling is essential for viral persistence. HCMV also appropriates pSTAT1 by recruiting it to the viral genome, where it binds to viral ISRE sequences, suggesting the existence of viral ISGs. In this proposal we seek to determine the mechanisms by which HCMV maintains STAT1 phosphorylation during infection and the impacts of this prolonged signaling on viral replication and latency. We hypothesize that HCMV manipulates the IFNAR1 signaling pathway to phosphorylate STAT1, thereby inducing transcription of viral genes and promoting viral latency. Our goal is to answer two overarching questions. First, how does HCMV maintain STAT1 signaling? We hypothesize that HCMV licenses STAT1 interaction with IFNAR by removing inhibitory ubiquitin moieties, while simultaneously impeding sorting endosome turnover to prolong IFNAR complex stability and signaling. Second, what is the impact of UL138-induced STAT1 signaling? We hypothesize that STAT1 signaling is used by HCMV to regulate transcription of its own genes and to alter the intracellular environment to promote latency. Answering these questions will provide new insight into viral and cellular processes involved in the pathogenesis of this important human pathogen and potentially lead to novel approaches to therapeutic or curative treatments.
