The Human Herpesvirus 6 G Protein-Coupled Receptor Homolog U51 Positively Regulates Virus Replication and Enhances Cell-Cell Fusion In Vitro

Abstract

Human herpesvirus 6 (HHV-6) is a ubiquitous T-lymphotropic betaherpesvirus that encodes two G proteincoupled receptor homologs, U12 and U51. HHV-6A U51 has been reported to bind to CC chemokines including RANTES, but the biological function of U51 remains uncertain. In this report, we stably expressed short interfering RNAs (siRNAs) specific for U51 in human T cells and then infected these cells with HHV-6. Viral DNA replication was reduced 50-fold by the U51 siRNA, and virally induced cytopathic effects were also inhibited. In contrast, viral replication and syncytium formation were unaltered in cells that expressed a scrambled derivative of the siRNA or an irrelevant siRNA and were restored to normal when a human codon-optimized derivative of U51 was introduced into cells containing the U51 siRNA. To examine the mechanism whereby U51 might contribute to viral replication, we explored the signaling characteristics of U51. None of the chemokines and opioids tested was able to induce G protein coupling by U51, and no evidence for opioid ligand binding by U51 was obtained. The effect of U51 on cell-cell fusion was also evaluated; these studies showed that U51 enhanced cell fusion mediated by the G protein of vesicular stomatitis virus. However, a U51-specific antiserum had no virus-neutralizing activity, suggesting that U51 may not be involved in the initial interaction between the virus particle and host cell. Overall, these studies suggest that HHV-6 U51 is a positive regulator of virus replication in vitro, perhaps because it may promote membrane fusion and facilitates cell-cell spread of this highly cell-associated virus.Human herpesvirus 6 (HHV-6) was first isolated in 1986 from patients with lymphoproliferative disorders (43) and later was identified as the causative agent of roseola infantum (56) and of acute febrile illness (41, 58) in young children. Following primary infection, the virus is able to establish a highly successful state of coexistence with the host, resulting in persistent infection with occasional but generally nonsymptomatic reactivation (13,24). However, the virus can cause rare, serious complications in immunocompromised hosts or in the context of stem cell transplantation, including encephalitis, hepatitis, and bone marrow suppression (14,54,57). There are two variants of this virus, 6A and 6B, which have characteristic differences in their cell tropism and biological properties (1, 4, 16, 44) as well as approximately 10% overall sequence divergence at the genomic level (18,23,25).The U51 gene is one of the two 7-transmembrane (7-tm) receptors carried by HHV-6 (23). It has been shown to be most closely related to the UL78 gene family from human cytomegalovirus (CMV), and gene knockout experiments using the rat CMV have revealed that this gene (R78) is necessary for efficient virus replication in vivo, suggesting that R78 (and perhaps U51 as well) may play a role in virus replication and virulence (6). Direct analyses of U51 itself have revealed that HHV-6 U51 can bind certain CC chemok...

Human herpesvirus 6 infections after liver transplantation

“…Both quantitative and qualitative methods have been developed to detect HHV-6 DNA in blood and clinical samples [54][55][56][57] . In addition to blood samples, HHV-6 detection by PCR can be performed on biopsy and tissue specimens [58] .…”

Chemokine-Directed Trafficking of Receptor Stimulus to Different G Proteins: Selective Inducible and Constitutive Signaling by Human Herpesvirus 6-Encoded Chemokine Receptor U51

“…In addition, U51 signals in a stable cell line of hemapoietic origin, but this could be either ligand-inducible or constitutive, because the cell line also secreted chemokine ligand (Milne et al, 2000). Very recently, U51 has been shown to positively regulate HHV-6 replication and to enhance cellcell fusion in vitro (Zhen et al, 2005).…”

Understanding the association between chromosomally integrated human herpesvirus 6 and HIV disease: a cross-sectional study

“…DNA samples were tested by a nested qualitative PCR amplifying the HHV-6 U38 DNA polymerase gene as previously described 15 . In order to verify the presence of ciHHV-6, qualitative PCR was repeated on all of the positive samples followed by quantitative real time PCR for the HHV-6 U4 gene as described by Zhen et al 16 with modifications developed in our laboratory 17 .…”