CMV-encoded GPCRs in infection, disease, and pathogenesis

G protein coupled receptors (GPCRs) are seven-transmembrane domain proteins that modulate cellular processes in response to external stimuli. These receptors represent the largest family of membrane proteins, and in mammals, their signaling regulates important physiological functions, such as vision, taste, and olfaction. Many organisms, including yeast, slime molds, and viruses encode GPCRs. Cytomegaloviruses (CMVs) are large, betaherpesviruses, that encode viral GPCRs (vGPCRs). Human CMV (HCMV) encodes four vGPCRs, including UL33, UL78, US27, and US28. Each of these vGPCRs, as well as their rodent and primate orthologues, have been investigated for their contributions to viral infection and disease. Herein, we discuss how the CMV vGPCRs function during lytic and latent infection, as well as our understanding of how they impact viral pathogenesis.

Identification of a novel signaling complex containing host chemokine receptor CXCR4, Interleukin-10 receptor, and human cytomegalovirus US27

Human cytomegalovirus (HCMV) is a widespread herpesvirus that establishes latency in myeloid cells and persists by manipulating immune signaling. Chemokine receptor CXCR4 and its ligand CXCL12 regulate movement of myeloid progenitors into bone marrow and out into peripheral tissues. HCMV amplifies CXCL12-CXCR4 signaling through viral chemokine receptor US27 and cmvIL-10, a viral cytokine that binds the cellular IL-10 receptor (IL-10R), but precisely how these viral proteins influence CXCR4 is unknown. We used the proximity ligation assay (PLA) to examine association of CXCR4, IL-10R, and US27 in both transfected and HCMV-infected cells. CXCR4 and IL-10R colocalized to discrete clusters, and treatment with CXCL12 and cmvIL-10 dramatically increased receptor clustering and calcium flux. US27 was associated with CXCR4 and IL-10R in PLA clusters and further enhanced cluster formation and calcium signaling. These results indicate that CXCR4, IL-10R, and US27 form a novel virus-host signaling complex that enhances CXCL12 signaling during HCMV infection.

The Requirement for US28 During Cytomegalovirus Latency Is Independent of US27 and US29 Gene Expression

The ability to establish a latent infection with periodic reactivation events ensures herpesviruses, like human cytomegalovirus (HCMV), lifelong infection, and serial passage. The host-pathogen relationship throughout HCMV latency is complex, though both cellular and viral factors influence the equilibrium between latent and lytic infection. We and others have shown one of the viral-encoded G protein-coupled receptors, US28, is required for HCMV latency. US28 potentiates signals both constitutively and in response to ligand binding, and we previously showed deletion of the ligand binding domain or mutation of the G protein-coupling domain results in the failure to maintain latency similar to deletion of the entire US28 open reading frame (ORF). Interestingly, a recent publication detailed an altered phenotype from that previously reported, showing US28 is required for viral reactivation rather than latency, suggesting the US28 ORF deletion impacts transcription of the surrounding genes. Here, we show an independently generated US28-stop mutant, like the US28 ORF deletion mutant, fails to maintain latency in hematopoietic cells. Further, we found US27 and US29 transcription in each of these mutants was comparable to their expression during wild type infection, suggesting neither US28 mutant alters mRNA levels of the surrounding genes. Finally, infection with a US28 ORF deletion virus expressed US27 protein comparable to its expression following wild type infection. In sum, our new data strongly support previous findings from our lab and others, detailing a requirement for US28 during HCMV latent infection.

The Human Cytomegalovirus US27 Gene Product Constitutively Activates Antioxidant Response Element-Mediated Transcription through Gβγ, Phosphoinositide 3-Kinase, and Nuclear Respiratory Factor 1

Human cytomegalovirus (HCMV) is a widespread pathogen that modulates host chemokine signaling during persistent infection in the host. HCMV encodes four proteins with homology to the chemokine receptor family of G protein-coupled receptors (GPCRs): US27, US28, UL33, and UL78. Each of the four receptors modulates host CXCR4 signaling. US28, UL33, and UL78 impair CXCR4 signaling outcomes, while US27 enhances signaling, as evidenced by increased calcium mobilization and cell migration to CXCL12. To investigate the effects of US27 on CXCR4 during virus infection, fibroblasts were infected with bacterial artificial chromosome-derived clinical strain HCMV TB40/E-mCherry (wild type [WT]), mutants lacking US27 (TB40/E-mCherry-US27Δ [US27Δ]) or all four GPCRs (TB40 E-mCherry-allΔ), or mutants expressing only US27 but not US28, UL33, or UL78 (TB40/E-mCherry-US27wt [US27wt]). CXCR4 gene expression was significantly higher in WT- and US27wt-infected fibroblasts. This effect was evident at 3 h postinfection, suggesting that US27 derived from the parental virion enhanced CXCR4 expression. Reporter gene assays demonstrated that US27 increased transcriptional activity regulated by the antioxidant response element (ARE), and small interfering RNA treatment indicated that this effect was mediated by NRF-1, the primary transcription factor for CXCR4. Increased translocation of NRF-1 into the nucleus of WT-infected cells compared to mock- or US27Δ-infected cells was confirmed by immunofluorescence microscopy. Chemical inhibitors targeting G_βγ and phosphoinositide 3-kinase (PI3K) ablated the increase in ARE-driven transcription, implicating these proteins as mediators of US27-stimulated gene transcription. This work identifies the first signaling pathway activated by HCMV US27 and may reveal a novel regulatory function for this orphan viral receptor in stimulating stress response genes during infection.IMPORTANCE Human cytomegalovirus (HCMV) is the most common congenital infection worldwide, causing deafness, blindness, and other serious birth defects. CXCR4 is a human chemokine receptor that is crucial for both fetal development and immune responses. We found that the HCMV protein US27 stimulates increased expression of CXCR4 through activation of the transcription factor nuclear respiratory factor 1 (NRF-1). NRF-1 regulates stress response genes that contain the antioxidant response element (ARE), and HCMV infection is associated with increased expression of many stress response genes when US27 is present. Our results show that the US27 protein activates the NRF-1/ARE pathway, stimulating higher expression of CXCR4 and other stress response genes, which is likely to be beneficial for virus replication and/or immune evasion.

Human Cytomegalovirus UL111A and US27 Gene Products Enhance the CXCL12/CXCR4 Signaling Axis via Distinct Mechanisms

Human cytomegalovirus (HCMV) is a prevalent pathogen that establishes lifelong infection in the host. Virus persistence is aided by extensive manipulation of the host immune system, particularly cytokine and chemokine signaling pathways. The HCMV UL111A gene encodes cmvIL-10, an ortholog of human interleukin-10 that has many immunomodulatory effects. We found that cmvIL-10 increased signaling outcomes from human CXCR4, a chemokine receptor with essential roles in hematopoiesis and immune cell trafficking, in response to its natural ligand CXCL12. Calcium flux and chemotaxis to CXCL12 were significantly greater in the presence of cmvIL-10 in monocytes, epithelial cells, and fibroblasts that express CXCR4. cmvIL-10 effects on CXCL12/CXCR4 signaling required the IL-10 receptor and Stat3 activation. Heightened signaling occurred both in HCMV-infected cells and in uninfected bystander cells, suggesting that cmvIL-10 may broadly influence chemokine networks by paracrine signaling during infection. Moreover, CXCL12/CXCR4 signaling was amplified in HCMV-infected cells compared to mock-infected cells even in the absence of cmvIL-10. Enhanced CXCL12/CXCR4 outcomes were associated with expression of the virally encoded chemokine receptor US27, and CXCL12/CXCR4 activation was reduced in cells infected with a deletion mutant lacking US27 (TB40/E-mCherry-US27Δ). US27 effects were Stat3 independent but required close proximity to CXCR4 in cell membranes of either HCMV-infected or US27-transfected cells. Thus, HCMV encodes two proteins, cmvIL-10 and US27, that exhibit distinct mechanisms for enhancing CXCR4 signaling. Either individually or in combination, cmvIL-10 and US27 may enable HCMV to exquisitely manipulate CXCR4 signaling to alter host immune responses and modify cell trafficking patterns during infection.IMPORTANCE The human chemokine system plays a central role in host defense, as evidenced by the many strategies devised by viruses for manipulating it. Human cytomegalovirus (HCMV) is widespread in the human population, but infection rarely causes disease except in immunocompromised hosts. We found that two different HCMV proteins, cmvIL-10 and US27, act through distinct mechanisms to upregulate the signaling activity of a cellular chemokine receptor, CXCR4. cmvIL-10 is a secreted viral cytokine that affects CXCR4 signaling in both infected and uninfected cells, while US27 is a component of the virus particle and impacts CXCR4 activity only in infected cells. Both cmvIL-10 and US27 promote increased intracellular calcium signaling and cell migration in response to chemokine CXCL12 binding to CXCR4. Our results demonstrate that HCMV exerts fine control over the CXCL12/CXCR4 pathway, which could lead to enhanced virus dissemination, altered immune cell trafficking, and serious health implications for HCMV patients.