The betaherpesvirus human cytomegalovirus (HCMV) is a ubiquitous pathogen that infects nearly 80% of the population by 40 years of age. Like all herpesviruses, HCMV infections are life-long, where the virus remains latent within progenitor cells of the host’s hematopoietic compartment. Healthy individuals, for the most part, remain asymptomatic. However, infection is problematic for those with weakened immune systems, where reactivation of HCMV from latency can cause severe morbidity and mortality. In addition to immunocomprimised patients, HCMV reactivation poses an additional threat to those with atherosclerosis and certain cancers (e.g.: glioblastoma). In fact, viral DNA is found in the arterial walls of atherosclerosis patients and in a vast majority of glioblastoma tumors tested to-date. HCMV has the capacity to encode over 200 open reading frames (ORFs), four of which are G-protein-coupled receptor (GPCR) homologs: UL33, UL78, US27, and US28. Cellular GPCRs are signaling proteins that when activated, can induce a variety of downstream signaling events, thereby altering the host cell environment. Although these viral GPCRs are homologous to cellular GPCRs by sequence analysis, only UL33 and US28 have known signaling properties. The four HCMV-encoded GPCRs are expressed during the lytic life cycle of the virus, yet the impact these four proteins have on this phase of the viral life cycle is incomplete. During latency, only a handful of HCMV genes are expressed, including US28. We hypothesize that US28 influences HCMV latency via specific signaling pathways during this phase of infection. Our work aims to elucidate the functions of the HCMV-encoded GPCRs during both lytic and latent infection to better understand how these proteins influence pathogenesis and subsequent disease.
We currently focus on three main areas of research:
1. Defining the contribution of the HCMV GPCRs to lytic replication
We have shown that the HCMV GPCRs have a variety of functions during HCMV infection. We have demonstrated the importance of US27 in extracellular spread of the virus. In parallel, we found that US27 is dispensable for viral growth in epithelial cells, where the mode of viral spread is restricted to cell-to-cell. In contrast, we have shown that UL78 is required for efficient viral infection in epithelial cells, as it functions during entry. We are currently investigating the additive contribution of these proteins towards promoting a successful lytic infection.
2. Understanding the role of US28 during HCMV latency
US28 is expressed during both the lytic and latent cycles of HCMV infection. To elucidate its function during latent infection, we utilized a panel of mutant viruses and our novel in vitro latency model system. We have shown that HCMV US28 is required for viral latency in hematopoietic progenitor cells, as viruses that lack the US28 ORF fail to undergo latent infection and instead favor lytic replication. Further, we have shown that US28’s continued expression is necessary to maintain latency. Our current work is aimed at understanding the underlying mechanisms by which US28 influences the latent state of HCMV.
3. Elucidating the contribution of US28 towards the progression of atherosclerosis
Many US28-induced signaling cascades and the cellular factors that are subsequently altered are also involved arterial plaque formation during the progression to atherosclerosis, leading us to hypothesize that HCMV’s involvement in atherosclerotic progression could be due, at least in part, to US28-mediated events. We, and others, have described signaling properties of US28, which unlike cellular GPCRs, signals through a variety of ligands and couples to many G-proteins. US28, like some cellular GPCRs that are dysregulated during disease, can also induce signaling events in the absence of a ligand (constitutive activation). We have found that the signaling properties of US28 vary by cell type, which may lead to our understanding how HCMV accelerates disease states in various tissues. Lytically replicating HCMV, and more specifically US28, influences almost every stage of atherosclerosis, including the recruitment of immune cells to sites of injury at the vascular wall, aiding in the formation of plaques, and accelerating the mobilization of plaques to cause subsequent thrombosis and stroke. However, the mechanism(s) underlying HCMV’s contribution to these processes remain poorly understood. Our goal is to define the involvement of US28 in the acceleration of atherosclerotic disease progression by: (A) defining the host and viral factors with which US28, and the other viral GPCRs, interact to facilitate cellular processes associated with atherosclerotic progression, and (B) defining the cellular signaling pathways US28 influences during plaque formation.
Taken together, our work is aimed at gaining a better understanding of how the HCMV GPCRs mediate viral infection as well as the host cell milieu towards accelerating disease processes. Over 2/3rds of approved drugs target the GPCR family of proteins, making the HCMV GPCRs ideal therapeutic targets. Defining a role for these viral proteins in pathogenesis will provide avenues for novel therapeutics that may aid in preventing and/or slowing the progress of HCMV-associated disease.
Human cytomegalovirus (HCMV) is a herpesvirus that is prevalent in the population, where it remains latent, or quiet, in the host for life. However, when an individual’s immune system is weakened, the virus can wake-up, or reactivate, to cause severe complications and often death. Our lab focuses on the processes that allow the virus to re-awaken after long periods of dormancy, and how this event leads to viral pathogenesis and disease progression.
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