The HCMV chemokine receptor US28 is a potential target in vascular disease

The Viral G-Protein-Coupled Receptor Homologs M33 and US28 Promote Cardiac Dysfunction during Murine Cytomegalovirus Infection

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that infects the majority of the world population and causes lifelong latent infection. HCMV has been shown to exacerbate cardiovascular diseases, including myocarditis, vascular sclerosis, and transplant vasculopathy. Recently, we have shown that murine CMV (MCMV) recapitulates the cardiovascular dysfunction observed in patients with HCMV-induced myocarditis. To understand the viral mechanisms involved in CMV-induced heart dysfunction, we further characterized cardiac function in response to MCMV and examined virally encoded G-protein-coupled receptor homologs (vGPCRs) US28 and M33 as potential factors that promote infection in the heart. We hypothesized that the CMV-encoded vGPCRs could exacerbate cardiovascular damage and dysfunction. Three viruses were used to evaluate the role of vGPCRs in cardiac dysfunction: wild-type MCMV, a M33-deficient virus (∆M33), and a virus with the M33 open reading frame (ORF) replaced with US28, an HCMV vGPCR (i.e., US28+). Our in vivo studies revealed that M33 plays a role in promoting cardiac dysfunction by increasing viral load and heart rate during acute infection. During latency, ΔM33-infected mice demonstrated reduced calcification, altered cellular gene expression, and less cardiac hypertrophy compared with wild-type MCMV-infected mice. Ex vivo viral reactivation from hearts was less efficient in ΔM33-infected animals. HCMV protein US28 expression restored the ability of the M33-deficient virus to reactivate from the heart. US28+ MCMV infection caused damage to the heart comparable with wild-type MCMV infection, suggesting that the US28 protein is sufficient to complement the function of M33 in the heart. Altogether, these data suggest a role for vGPCRs in viral pathogenesis in the heart and thus suggest that vGPCRs promote long-term cardiac damage and dysfunction.

Sequencing Directly from Clinical Specimens Reveals Genetic Variations in HCMV-Encoded Chemokine Receptor US28 That May Influence Antibody Levels and Interactions with Human Chemokines

Human cytomegalovirus (HCMV) is a beta-herpesvirus carried by ∼80% of the world’s population. Acute infections are asymptomatic in healthy individuals but generate diverse syndromes in neonates, solid organ transplant recipients, and HIV-infected individuals. The HCMV gene US28 encodes a homolog of a human chemokine receptor that is able to bind several chemokines and HIV gp120. Deep sequencing technologies were used to sequence US28 directly from 60 clinical samples from Indonesian HIV patients and Australian renal transplant recipients, healthy adults, and neonates. Molecular modeling approaches were used to predict whether nine nonsynonymous mutations in US28 may alter protein binding to a panel of six chemokines and two variants of HIV gp120. Ninety-two percent of samples contained more than one variant of HCMV, as defined by at least one nonsynonymous mutation. Carriage of these variants differed between neonates and adults, Australian and Indonesian samples, and saliva samples and blood leukocytes. Two nonsynonymous mutations (N170D and R267K) were associated with increased levels of immediate early protein 1 (IE-1) and glycoprotein B (gB) HCMV-reactive antibodies, suggesting a higher viral burden. Seven of the nine mutations were predicted to alter binding of at least one ligand. Overall, HCMV variants are common in all populations and have the potential to affect US28 interactions with human chemokines and/or gp120 and alter responses to the virus. The findings relied on deep sequencing technologies applied directly to clinical samples, so the variants exist in vivo.

IMPORTANCE Human cytomegalovirus (HCMV) is a common viral pathogen of solid organ transplant recipients, neonates, and HIV-infected individuals. HCMV encodes homologs of several host genes with the potential to influence viral persistence and/or pathogenesis. Here, we present deep sequencing of an HCMV chemokine receptor homolog, US28, acquired directly from clinical specimens. Carriage of these variants differed between patient groups and was associated with different levels of circulating HCMV-reactive antibodies. These features are consistent with a role for US28 in HCMV persistence and pathogenesis. This was supported by in silico analyses of the variant sequences demonstrating altered ligand-binding profiles. The data delineate a novel approach to understanding the pathogenesis of HCMV and may impact the development of an effective vaccine.

Novel Strategies to Combat CMV-Related Cardiovascular Disease

Cytomegalovirus (CMV), a ubiquitous human pathogen that is never cleared from the host, has long been thought to be relatively innocuous in immunocompetent adults, but causes severe complications including blindness, end-organ disease, and death in newborns and in immuno-compromised individuals, such as organ transplant recipients and those suffering from AIDS. Yet even in persons with intact immunity, CMV infection is associated with profound stimulation of immune and inflammatory pathways. Carriers of CMV infection also have an elevated risk of developing cardiovascular complications. In this review, we define the proposed mechanisms of how CMV contributes to cardiovascular disease (CVD), describe current approaches to target CMV, and discuss how these strategies may or may not alleviate cardiovascular complications in those with CMV infection. In addition, we discuss the special situation of CMV coinfection in people with HIV infection receiving antiretroviral therapy, and describe how these 2 viral infections may interact to potentiate CVD in this especially vulnerable population.

Expansions of Cytotoxic CD4+CD28- T Cells Drive Excess Cardiovascular Mortality in Rheumatoid Arthritis and Other Chronic Inflammatory Conditions and Are Triggered by CMV Infection

A large proportion of cardiovascular (CV) pathology results from immune-mediated damage, including systemic inflammation and cellular proliferation, which cause a narrowing of the blood vessels. Expansions of cytotoxic CD4⁺ T cells characterized by loss of CD28 (“CD4⁺CD28⁻ T cells” or “CD4⁺CD28^null cells”) are closely associated with cardiovascular disease (CVD), in particular coronary artery damage. Direct involvement of these cells in damaging the vasculature has been demonstrated repeatedly. Moreover, CD4⁺CD28⁻ T cells are significantly increased in rheumatoid arthritis (RA) and other autoimmune conditions. It is striking that expansions of this subset beyond 1–2% occur exclusively in CMV-infected people. CMV infection itself is known to increase the severity of autoimmune diseases, in particular RA and has also been linked to increased vascular pathology. A review of the recent literature on immunological changes in CVD, RA, and CMV infection provides strong evidence that expansions of cytotoxic CD4⁺CD28⁻ T cells in RA and other chronic inflammatory conditions are limited to CMV-infected patients and driven by CMV infection. They are likely to be responsible for the excess CV mortality observed in these situations. The CD4⁺CD28⁻ phenotype convincingly links CMV infection to CV mortality based on a direct cellular-pathological mechanism rather than epidemiological association.

Structure-based discovery of novel US28 small molecule ligands with different modes of action

The human cytomegalovirus-encoded G protein-coupled receptor US28 is a constitutively active receptor, which can recognize various chemokines. Despite the recent determination of its 2.9 Å crystal structure, potent and US28-specific tool compounds are still scarce. Here, we used structural information from a refined US28:VUF2274 complex for virtual screening of >12 million commercially available small molecule compounds. Using a combined receptor- and ligand-based approach, we tested 98 of the top 0.1% ranked compounds, revealing novel chemotypes as compared to the ~1.45 million known ligands in the ChEMBL database. Two compounds were confirmed as agonist and inverse agonist, respectively, in both IP accumulation and Ca²⁺ mobilization assays. The screening setup presented in this work is computationally inexpensive and therefore particularly useful in an academic setting as it enables simultaneous testing in binding as well as in different functional assays and/or species without actual chemical synthesis.

The HCMV US28 vGPCR induces potent Gαq/PLC-β signaling in monocytes leading to increased adhesion to endothelial cells

US28 transcripts have been detected in primary monocytes and in THP-1 monocytes infected with HCMV but US28 protein expression has not yet been demonstrated in these cell types. Moreover, the mechanism(s) by which US28 signals and contributes to viral pathogenesis in monocytes remains unclear. Here, we show that US28 protein is robustly expressed in HCMV infected THP-1 monocytes and that US28 can trigger Gαq dependent signaling in THP-1 cells infected with HCMV and in THP-1 cells stably expressing US28. US28 signaling in these cells is dependent on G-protein coupling, but independent of chemokine binding. Importantly, we demonstrate that this US28 signaling is functionally important as it stimulates the adhesion of monocytes to an endothelial monolayer. Our studies, which demonstrate that US28-driven Gαq signaling has profound effects on monocyte biology, suggest that US28 driven phenotypic changes in HCMV infected monocytes may play important roles in HCMV dissemination and/or pathogenesis.

Infections and autoimmunity: role of human cytomegalovirus in autoimmune endothelial cell damage

Molecular mimicry between infectious agents and normal human host cell proteins represents one of the possible mechanisms responsible for autoimmunity. Among infectious agents, human cytomegalovirus (HCMV) is an ideal candidate for involvement in autoimmune disorders because of its lifelong persistence through periods of reactivation and latency and because of the extensive manipulation of innate and adaptive immunity. HCMV has been implicated in the pathogenesis of vascular damage in systemic sclerosis (SSc) and atherosclerosis. Based on our data, which demonstrate a cause-and-effect relationship between HCMV and endothelial cell aggression in SSc and atherosclerosis, we propose that immune responses to particular HCMV proteins may result in autoaggression through a mechanism of molecular mimicry of normally expressed endothelial cell surface molecules.

Allosteric modulation of the G protein-coupled US28 receptor of human cytomegalovirus: are the small-weight inverse agonist of US28 'camouflaged' agonists?

The highly constitutively active G protein-coupled receptor US28 of human cytomegalovirus (HCMV) is thought to camouflage agonism by mediating constitutive endocytosis. With the use of the US28Δ300 mutant, which is largely devoid of constitutive internalization, I have demonstrated that the coupling of the receptor to its downstream signaling partners is responsible for the inverse agonism to agonism efficacy switch in some small-weight ligands of US28.

HCMV pUS28 initiates pro-migratory signaling via activation of Pyk2 kinase

Background

Human Cytomegalovirus (HCMV) has been implicated in the acceleration of vascular disease and chronic allograft rejection. Recently, the virus has been associated with glioblastoma and other tumors. We have previously shown that the HCMV-encoded chemokine receptor pUS28 mediates smooth muscle cell (SMC) and macrophage motility and this activity has been implicated in the acceleration of vascular disease. pUS28 induced SMC migration involves the activation of the protein tyrosine kinases (PTKs) Src and Focal adhesion kinase as well as the small GTPase RhoA. The PTK Pyk2 has been shown to play a role in cellular migration and formation of cancer, especially glioblastoma. The role of Pyk2 in pUS28 signaling and migration are unknown.

Methods

In the current study, we examined the involvement of the PTK Pyk2 in pUS28-induced cellular motility. We utilized in vitro migration of SMC to determine the requirements for Pyk2 in pUS28 pro-migratory signaling. We performed biochemical analysis of Pyk2 signaling in response to pUS28 activation to determine the mechanisms involved in pUS28 migration. We performed mass spectrometric analysis of Pyk2 complexes to identify novel Pyk2 binding partners.

Results

Expression of a mutant form of Pyk2 lacking the autophosphorylation site (Tyr-402) blocks pUS28-mediated SMC migration in response to CCL5, while the kinase-inactive Pyk2 mutant failed to elicit the same negative effect on migration. pUS28 stimulation with CCL5 results in ligand-dependent and calcium-dependent phosphorylation of Pyk2 Tyr-402 and induced the formation of an active Pyk2 kinase complex containing several novel Pyk2 binding proteins. Expression of the autophosphorylation null mutant Pyk2 F402Y did not abrogate the formation of an active Pyk2 kinase complex, but instead prevented pUS28-mediated activation of RhoA. Additionally, pUS28 activated RhoA via Pyk2 in the U373 glioblastoma cells. Interestingly, the Pyk2 kinase complex in U373 contained several proteins known to participate in glioma tumorigenesis.

Conclusions

These findings represent the first demonstration that pUS28 signals through Pyk2 and that this PTK participates in pUS28-mediated cellular motility via activation of RhoA. Furthermore, these results provide a potential mechanistic link between HCMV-pUS28 and glioblastoma cell activation.

Human Cytomegalovirus US28: a functionally selective chemokine binding receptor

Chemokines are small cytokines that are part of a large family of molecules that bind to G-protein coupled receptors, which, as a family, are the most widely targeted group of molecules in the treatment of disease. Chemokines are critical for recruiting and activating the cells of the immune system during inflammation especially during viral infections. However, a number of viruses including the large herpes virus human cytomegalovirus (HCMV) encode mechanisms to impede the effects of chemokines or has gained the ability to use these molecules to its own advantage. The Human Cytomegalovirus (HCMV)-encoded chemokine receptor US28 is the best characterized of the four unique chemokine receptor-like molecules found in the HCMV genome. US28 has been studied as an important virulence factor for HCMV-mediated vascular disease and, more recently, in models of HCMV-associated malignancy. US28 is a rare multi-chemokine family binding receptor with the ability to bind ligands from two distinct chemokine classes. Ligand binding to US28 activates cell-type and ligand-specific signaling pathways leading to cellular migration, which is an important example of receptor functional selectivity. Additionally, US28 has been demonstrated to constitutively activate phospholipase C (PLC) and NF-kB signaling pathways. Understanding the structure/function relationships between US28, its ligands and intracellular signaling molecules will provide essential clues for effective pharmacological targeting of this multifunctional chemokine receptor.

Feeling manipulated: cytomegalovirus immune manipulation

No one likes to feel like they have been manipulated, but in the case of cytomegalovirus (CMV) immune manipulation, we do not really have much choice. Whether you call it CMV immune modulation, manipulation, or evasion, the bottom line is that CMV alters the immune response in such a way to allow the establishment of latency with lifelong shedding. With millions of years of coevolution within their hosts, CMVs, like other herpesviruses, encode numerous proteins that can broadly influence the magnitude and quality of both innate and adaptive immune responses. These viral proteins include both homologues of host proteins, such as MHC class I or chemokine homologues, and proteins with little similarity to any other known proteins, such as the chemokine binding protein. Although a strong immune response is launched against CMV, these virally encoded proteins can interfere with the host's ability to efficiently recognize and clear virus, while others induce or alter specific immune responses to benefit viral replication or spread within the host. Modulation of host immunity allows survival of both the virus and the host. One way of describing it would be a kind of "mutually assured survival" (as opposed to MAD, Mutually Assured Destruction). Evaluation of this relationship provides important insights into the life cycle of CMV as well as a greater understanding of the complexity of the immune response to pathogens in general.

Human cytomegalovirus modulation of signal transduction

An upregulation of cellular signaling pathways is observed in multiple cell types upon human cytomegalovirus (HCMV) infection, suggesting that a global feature of HCMV infection is the activation of the host cell. HCMV initiates and maintains cellular signaling through a multitiered process that is dependent on a series of events: (1) the viral glycoprotein ligand interacts with its cognate receptor, (2) cellular enzymes and viral tegument proteins present in the incoming virion are released and (3) a variety of viral gene products are expressed. Viral-mediated cellular modification has differential outcomes depending on the cell type infected. In permissive cell types, such as diploid fibroblasts, the upregulation of cellular signaling pathways following infection can initiate the viral gene cascade and promote the efficient transcription of multiple viral gene classes. In other cell types, such as endothelial cells and monocytes/macrophages, the upregulation of cellular pathways initiates functional host changes that allow viral spread to multiple organ systems. Together, the modification of signaling processes appears to be part of a thematic strategy deployed by the virus to direct the required functional changes in target cells that ultimately promote viral survival and persistence in the host.

HCMV-infection in a human arterial organ culture model: effects on cell proliferation and neointimal hyperplasia

Background

The impact of infections with the human cytomegalovirus (HCMV) for the development of atherosclerosis and restenosis is still unclear. Both a clear correlation and no correlation at all have been reported in clinical, mostly serological studies. In our study we employed a human non-injury ex vivo organ culture model to investigate the effect of an in vitro permissive HCMV-infection on cell proliferation and neointimal hyperplasia for a period of 56 days.

Results

During routine-nephrectomies parts of renal arteries from 71 patients were obtained and prepared as human organ cultures. Cell free HCMV infection was performed with the fibroblast adapted HCMV strain AD169, the endotheliotropic strain TB40E, and a clinical isolate (AN 365). After 3, 7, 14, 21, 28, 35, and 56 days in culture staining of HCMV-antigens was carried out and reactive cell proliferation and neointimal thickening were analysed. Successful HCMV-infection was accomplished with all three virus strains studied. During the first 21 days in organ culture no cell proliferation or neointimal hyperplasia was detected. At day 35 and day 56 moderate cell proliferation and neointimal hyperplasia was found both in HCMV-infected segments and mock infected controls. Neointimal hyperplasia in productively HCMV-infected segments was lower than in non infected at day 35 and day 56, but relatively higher after infection with the endotheliotropic TB40E in comparison with the two other strains.

Conclusion

The data do not support the hypothesis that HCMV-infection triggers restenosis via a stimulatory effect on cell proliferation and neointimal hyperplasia in comparison to non infected controls. Interestingly however, even after lytic infection, a virus strain specific difference was observed.

HCMV-encoded G-protein-coupled receptors as constitutively active modulators of cellular signaling networks

Several herpesviruses encode G-protein-coupled receptor (vGPCR) proteins that are homologous to human chemokine receptors. In contrast to chemokine receptors, many vGPCRs signal in a ligand-independent (constitutive) manner. Such constitutive signaling is of major significance because various pathologies are associated with activating GPCR mutations. Constitutive activity of the human herpesvirus 8-encoded GPCR (ORF74), for example, is essential for its oncogenic potential to cause angioproliferative Kaposi's sarcoma-like lesions. The human cytomegalovirus (HCMV) encodes four GPCRs, of which US28 and UL33 display constitutive activity in transfected, but also HCMV-infected, cells. In addition, US28 is activated by a broad spectrum of chemokines. Furthermore, both US28 and UL33 show promiscuous G-protein coupling, whereas chemokine receptors activate primarily G(i/o) proteins. Thus, these vGPCRs are versatile signaling devices, reprogramming cellular signaling networks to modulate cellular function after infection. By these means, these HCMV-encoded receptors might contribute to HCMV-related pathologies.

Molecular imaging in transplantation: basic concepts and strategies for potential application

BACKGROUND

The potential applications of molecular imaging in the clinical arena are diverse and expanding rapidly. One such area of application is transplantation. Currently, biopsy is the gold standard for monitoring allograft well-being after transplantation of organs or tissues. However, biopsies are invasive, associated with morbidity if performed on a routine basis and can potentially miss focal rejection.

AIM

It is notable that none of the existing studies in the literature have examined the possible role of molecular imaging in transplantation-related indications. In this direction, this paper aims to discuss imaging strategies that could be of pertinence in monitoring immune events and improving long-term outcomes after solid organ or tissue transplantation.

METHODS

This paper discusses the currently available direct/surrogate imaging techniques/agents that can be used to detect chemokine receptors/ligands, leucocyte endothelial events and ischaemia-reperfusion injury in transplantation.

CONCLUSION

Molecular imaging methods can non-invasively detect, quantify and monitor immune phenomena, such as rejection or graft-versus-host disease, after transplantation. Molecular imaging could help in targeted biopsy and could improve graft survival by allowing for early intervention with tailored immunosuppressive regimens. Given the unprecedented progress in the field, the potential benefits of molecular imaging to the speciality of organ and tissue transplantation cannot be underestimated.

US28 actions in HCMV infection: lessons from a versatile hijacker

Mimicking host proteins is a strategy adopted by several herpesviruses to exploit the host cell for their own benefit. In this respect the human cytomegalovirus (HCMV) chemokine receptor homologue US28, has been extensively studied. Molecular pirates such as US28 can teach us about crucial events in HCMV infection and may either offer a potential target for antiviral therapy or provide an alternative strategy to immune suppression. Despite elaborate research into the chemokine binding affinity, signalling properties, intracellular trafficking and expression kinetics of US28, a solid hypothesis about the role of US28 in HCMV infection has not yet been proposed. It appears that US28 may behave as a molecular pirate that employs smart strategies for cell entry, host gene regulation and immune evasion. This review will elaborate on these aspects of US28 biology and discuss possible implications for HCMV infection.

Human cytomegalovirus-encoded G protein-coupled receptor US28 mediates smooth muscle cell migration through Galpha12

Coupling of G proteins to ligand-engaged chemokine receptors is the paramount event in G-protein-coupled receptor signal transduction. Previously, we have demonstrated that the human cytomegalovirus-encoded chemokine receptor US28 mediates human vascular smooth muscle cell (SMC) migration in response to either RANTES or monocyte chemoattractant protein 1. In this report, we identify the G proteins that couple with US28 to promote vascular SMC migration and identify other signaling molecules that play critical roles in this process. US28-mediated cellular migration was enhanced with the expression of the G-protein subunits Galpha12 and Galpha13, suggesting that US28 may functionally couple to these G proteins. In correlation with this observation, US28 was able to activate RhoA, a downstream effector of Galpha12 and Galpha13 in cell types with these G proteins but not in those without them and activation of RhoA was dependent on US28 stimulation with RANTES. In addition, inactivation of RhoA or the RhoA-associated kinase p160ROCK with a dominant-negative mutant of RhoA or the small molecule inhibitor Y27632, respectively, abrogated US28-induced SMC migration. The data presented here suggest that US28 functionally signals through Galpha12 family G proteins and RhoA in a ligand-dependent manner and these signaling molecules are important for the ability of US28 to induce cellular migration.

The role of viruses in cardiac allograft vasculopathy

Considerable evidence suggests a role for viruses in transplant arteriosclerosis (TA), including observational data, experimental models and therapeutic trials implicating human cytomegalovirus (HCMV) in the progression to TA. In pediatric heart transplant patients, adenoviral genome in endomyocardial biopsies (EMB) is an important predictor of TA and graft loss. During CMV viremia, EMBs from adult patients demonstrate endothelialitis and vascular smooth muscle cell proliferation. These changes are predictors of subsequent diffuse TA. HCMV immediate early proteins (IE-1 and IE-2) increase the constitutive expression of intercellular adhesion molecule-1 (ICAM-1) independent of other intracellular cytokines. Likewise, viral chemokines such as US28 have been implicated in vascular disease because of their ability to induce smooth muscle cell migration. Recent data suggests that CMV might accelerate TA through its ability to abrogate the vascular protective effects of the endothelium-derived nitric oxide system (eNOS). Confirmation of causality requires clinical trials demonstrating that antiviral agents such as ganciclovir inhibit TA. Such studies in patients though limited to retrospective analyses, suggest that ganciclovir prophylaxis early after heart transplantation reduces the risk of TA. These observations emphasize the need for randomized controlled clinical trials to confirm a causal role for CMV (and other viruses) in TA.

Chemokines and their receptors as markers of allograft rejection and targets for immunosuppression

Although almost every known chemokine and chemokine receptor is expressed at some stage during development of allograft rejection, mechanistic studies indicate that the actual key effector mechanisms are rather few. Thus, in vivo studies have alleviated concerns regarding possible biological redundancy and the pleiotropic effects of these molecules, and have resulted in a focus on CXCR3, CCR5 and their respective ligands as key mediators of host alloresponses, especially in acute rejection. Data are also accruing regarding the importance of chemokine/chemokine receptor pathways in ischemia/reperfusion, chronic rejection and tolerance induction following co-stimulation blockade, providing new targets for immune monitoring and therapeutic intervention.