Functional interaction between the pp71 protein of human cytomegalovirus and the PML-interacting protein human Daxx

The tegument protein pp71 (UL82) of human cytomegalovirus (HCMV) has previously been shown to transactivate the major immediate-early enhancer-promoter of HCMV. Furthermore, this protein is able to enhance the infectivity of viral DNA and to accelerate the infection cycle, suggesting an important regulatory function during viral replication. To gain insight into the underlying mechanisms that are used by pp71 to exert these pleiotropic effects, we sought for cellular factors interacting with pp71 in a yeast two-hybrid screen. Here, we report the isolation of the human Daxx (hDaxx) protein as a specific interaction partner of HCMV pp71. hDaxx, which was initially described as an adapter protein involved in apoptosis regulation, has recently been identified as a nuclear protein that interacts and colocalizes with PML in the nuclear domain ND10. In order to assess whether pp71 can also be detected in ND10 structures, a vector expressing pp71 in fusion with the green fluorescent protein was used for transfection of human fibroblasts. This revealed a colocalization of pp71 with the ND10 proteins PML and Sp100. In addition, cotransfection of a hDaxx expression vector resulted in an enhanced recruitment of pp71 to ND10. Targeting of pp71 to nuclear dots could also be observed in infected human fibroblasts in the absence of de novo viral protein synthesis. Moreover, cotransfection experiments revealed that pp71-mediated transactivation of the major immediate-early enhancer-promoter was synergistically enhanced in the presence of hDaxx. These results suggest an important role of hDaxx for pp71 protein function.     

Interaction between the human cytomegalovirus UL82 gene product (pp71) and hDaxx regulates immediate-early gene expression and viral replication

The human cytomegalovirus UL82-encoded pp71 protein is required for efficient virus replication and immediate-early gene expression when cells are infected at a low multiplicity. Functions attributed to pp71 include the ability to enhance the infectivity of viral DNA, bind to and target hypophosphorylated Rb family member proteins for degradation, drive quiescent cells into the cell cycle, and bind to the cellular protein hDaxx. Using UL82 mutant viruses, we demonstrate that the LXCXD motif within pp71 is not necessary for efficient virus replication in fibroblasts, suggesting that pp71's ability to degrade hypophosphorylated Rb family members and induce quiescent cells into the cell cycle is not responsible for the growth defect associated with a UL82 deletion mutant. However, UL82 mutants that cannot bind to hDaxx are unable to induce immediate-early gene expression and are severely attenuated for viral replication. These results indicate that the interaction between the human cytomegalovirus UL82 gene product (pp71) and hDaxx regulates immediate-early gene expression and viral replication.     

The human cytomegalovirus major immediate-early distal enhancer region is required for efficient viral replication and immediate-early gene expression

The human cytomegalovirus (HCMV) major immediate-early (MIE) genes, encoding IE1 p72 and IE2 p86, are activated by a complex enhancer region (base positions -65 to -550) that operates in a cell type- and differentiation-dependent manner. The expression of MIE genes is required for HCMV replication. Previous studies analyzing functions of MIE promoter-enhancer segments suggest that the distal enhancer region variably modifies MIE promoter activity, depending on cell type, stimuli, or state of differentiation. To further understand the mechanism by which the MIE promoter is regulated, we constructed and analyzed several different recombinant HCMVs that lack the distal enhancer region (-300 to -582, -640, or -1108). In human fibroblasts, the HCMVs without the distal enhancer replicate normally at high multiplicity of infection (MOI) but replicate poorly at low MOI in comparison to wild-type virus (WT) or HCMVs that lack the neighboring upstream unique region and modulator (-582 or -640 to -1108). The growth aberrancy was normalized after restoring the distal enhancer in a virus lacking this region. For HCMVs without a distal enhancer, the impairment in replication at low MOI corresponds to a deficiency in production of MIE RNAs compared to WT or virus lacking the unique region and modulator. An underproduction of viral US3 RNA was also evident at low MOI. Whether lower production of IE1 p72 and IE2 p86 causes a reduction in expression of the immediate-early (IE) class US3 gene remains to be determined. We conclude that the MIE distal enhancer region possesses a mechanism for augmenting viral IE gene expression and genome replication at low MOI, but this regulatory function is unnecessary at high MOI.     

Human cytomegalovirus gene expression is silenced by Daxx-mediated intrinsic immune defense in model latent infections established in vitro

In addition to productive lytic infections, herpesviruses such as human cytomegalovirus (HCMV) establish a reservoir of latently infected cells that permit lifelong colonization of the host. When latency is established, the viral immediate-early (IE) genes that initiate the lytic replication cycle are not expressed. HCMV IE gene expression at the start of a lytic infection is facilitated by the viral pp71 protein, which is delivered to cells by infectious viral particles. pp71 neutralizes the Daxx-mediated cellular intrinsic immune defense that silences IE gene expression by generating a repressive chromatin structure on the viral major IE promoter (MIEP). In naturally latently infected cells and in cells latently infected in vitro, the MIEP also adopts a similar silenced chromatin structure. Here we analyze the role of Daxx in quiescent HCMV infections in vitro that mimic some, but not all, of the characteristics of natural latency. We show that in these "latent-like" infections, the Daxx-mediated defense that represses viral gene expression is not disabled because pp71 and Daxx localize to different cellular compartments. We demonstrate that Daxx is required to establish quiescent HCMV infections in vitro because in cells that would normally foster the establishment of these latent-like infections, the loss of Daxx causes the lytic replication cycle to be initiated. Importantly, the lytic cycle is inefficiently completed, which results in an abortive infection. Our work demonstrates that, in certain cell types, HCMV must silence its own gene expression to establish quiescence and prevent abortive infection and that the virus usurps a Daxx-mediated cellular intrinsic immune defense mechanism to do so. This identifies Daxx as one of the likely multiple viral and cellular determinants in the pathway of HCMV quiescence in vitro, and perhaps in natural latent infections as well.     

Exon 3 of the human cytomegalovirus major immediate-early region is required for efficient viral gene expression and for cellular cyclin modulation

The human cytomegalovirus (HCMV) major immediate-early (IE) proteins share an 85-amino-acid N-terminal domain specified by exons 2 and 3 of the major IE region, UL122-123. We have constructed IE Delta30-77, a recombinant virus that lacks the majority of IE exon 3 and consequently expresses smaller forms of both IE1 72- and IE2 86-kDa proteins. The mutant virus is viable but growth impaired at both high and low multiplicities of infection and exhibits a kinetic defect that is not rescued by growth in fibroblasts expressing IE1 72-kDa protein. The kinetics of mutant IE2 protein accumulation in IE Delta30-77 virus-infected cells are approximately normal compared to wild-type virus-infected cells, but the IE Delta30-77 virus is delayed in expression of early viral genes, including UL112-113 and UL44, and does not sustain expression of mutant IE1 protein as the infection progresses. Additionally, cells infected with IE Delta30-77 exhibit altered expression of cellular proteins compared to wild-type HCMV-infected cells. PML is not dispersed but is retained at ND10 sites following infection with IE Delta30-77 mutant virus. While the deletion mutant retains the ability to mediate the stabilization of cyclin B1, cdc6, and geminin in infected cells, its capacity to upregulate the expression of cyclin E has been reduced. These data indicate that the activity of one or both of the HCMV major IE proteins is required in vivo for the modulation of cell cycle proteins observed in cells infected with wild-type HCMV.     

Dense bodies of human cytomegalovirus induce both humoral and cellular immune responses in the absence of viral gene expression

Infection of fibroblast cell cultures with human cytomegalovirus (HCMV) leads to the production of significant amounts of defective enveloped particles, termed dense bodies (DB). These noninfectious structures contain major antigenic determinants which are responsible for induction of both the humoral and the cellular immune response against HCMV. We tested the hypothesis that, by virtue of their unique antigenic and structural properties, DB could induce a significant immune response in the absence of infectious virus. Mice were immunized with gradient-purified DB, which were either left untreated or subjected to sequential rounds of sonication and freeze-thawing to prevent cellular entry. Titers of neutralizing antibodies induced by DB were in a range comparable to levels present in convalescent human sera. The virus-neutralizing antibody response was surprisingly durable, with neutralizing antibodies detected 12 months following primary immunization. The HCMV-specific major histocompatibility complex class I-restricted cytolytic T-cell (CTL) response was assayed using mice transgenic for the human HLA-A2 molecule. Immunization with DB led to high levels of HCMV-specific CTL in the absence of de novo viral protein synthesis. Maximal total cytolytic activity in mice immunized with DB was nearly as efficient as the cytolytic activity induced by a standard immunization with murine cytomegalovirus. Furthermore, DB induced a typical T-helper 1 (Th1)-dominated immune response in mice, as determined by cytokine and immunoglobulin G isotype analysis. Induction of humoral and cellular immune responses was achieved without the concomitant use of adjuvant. We thus propose that DB can serve as a basis for the future development of a recombinant nonreplicating vaccine against HCMV. Finally, such particles could be engineered for efficient delivery of antigens from other pathogens to the immune system.     

Identification and characterization of the human cytomegalovirus immediate-early region 2 gene that stimulates gene expression from an inducible promoter.

The human cytomegalovirus (HCMV) XbaI E cloned DNA fragment of approximately 20 kilobases can complement an adenovirus mutant (dl312) defective in the E1a viral gene product (D. J. Spector and M. J. Tevethia, Virology 151:329-338, 1986). This viral DNA fragment contains three immediate-early (IE) genes between 0.709 and 0.751 map units (M. F. Stinski, D. R. Thomsen, R. M. Stenberg, and L. C. Goldstein, J. Virol. 46:1-14, 1983). Two of the IE genes, IE1 and IE2, were isolated and tested for a role in regulating viral gene expression. Since HCMV early and late promoters require additional characterization, the chloramphenicol acetyl transferase (cat) gene, driven by the adenovirus E2 promoter, was used as an indicator of gene expression. cat expression from this heterologous viral promoter was shown to be stimulated by HCMV at early times after infection. The IE1 gene product did not function independently in activating this promoter. The IE2 gene products could independently stimulate the expression of a plasmid of a plasmid when the cat gene was placed downstream of the inducible E2 promoter (E2CAT). Five proteins of different sizes have been predicted to originate from IE2, depending on mRNA splicing. The protein products specified by the IE2 gene were characterized with an antibody to a synthetic peptide according to the open reading frame of exon 2. Three of the five proteins are encoded by exon 2. Three viral proteins of 82, 54, and 28 kilodaltons (kDa) were detected. The exons contained in the region designated as IE2a have open reading frames that could code for two of the smaller proteins of 27 and 30 kDa. This region, when driven by the HCMV enhancer, could independently stimulate gene expression from E2CAT to a high level. A plasmid with the HCMV enhancer upstream of exons, that could code for the HCMV IE2 proteins of 48 and 51 kDa, as well as 27- and 30-kDa proteins, also stimulated E2CAT expression but at a lower level. The activity of this plasmid was augmented by the IE1 gene product, despite the fact that the latter gene product alone was inactive. It is proposed that the HCMV IE region 2 gene products are involved in the regulation of viral or host cell promoters either independently or in combination with other HCMV IE proteins.     

Up-regulation of Fas ligand expression by human cytomegalovirus immediate-early gene product 2: a novel mechanism in cytomegalovirus-induced apoptosis in human retina

Human CMV (HCMV) is an important pathogen that causes widespread diseases in immunocompromised individuals. Among the opportunistic HCMV infections, HCMV retinitis is most common in transplant recipients and AIDS patients. It often leads to blindness if left untreated. The question as to how HCMV infection causes retinal pathogenesis remains unresolved. Here, we report that viral immediate-early gene product 2 (IE2), but not IE1, up-regulates the Fas ligand (FasL) expression in HCMV-infected human retinal pigment epithelium cells. Increased secretion of FasL from virally infected cells into cultured medium was observed upon HCMV infection. The capability of such cell-free medium to induce apoptosis of Fas (CD95)-expressing Jurkat cells further implies that Fas-FasL interaction might mediate cell death in the lesion of HCMV retinitis. To support this idea, we observed augmented soluble FasL levels in vitreous from AIDS patients with HCMV retinitis as compared with that from AIDS patients without HCMV infection. In addition, by in situ hybridization and immunohistochemistry, we detected enhanced signals of FasL, the existence of viral IE Ags and apoptotic cells at the same sites in the lesion of HCMV-infected retina. These results strongly suggest that IE2 induction of FasL expression in human retina might be an important event that takes place in the early stage of infection and finally leads to visual loss in individuals affiliated with HCMV retinitis.     

The human cytomegalovirus UL37 immediate-early regulatory protein is an integral membrane N-glycoprotein which traffics through the endoplasmic reticulum and Golgi apparatus.

The human cytomegalovirus (HCMV) UL37 immediate-early gene is predicted to encode a type I membrane-bound glycoprotein, gpUL37. Following expression of the UL37 open reading frame in vitro, its signals for translocation and N-glycosylation were recognized by microsomal enzymes. Its orientation in the microsomes is that of a type I protein. gpUL37 produced in HCMV-infected human cells was selectively immunoprecipitated by rabbit polyvalent antiserum generated against the predicted unique domains of the UL37 open reading frame and migrated as an 83- to 85-kDa protein. Tunicamycin treatment, which inhibits N-glycosylation, increased the rate of migration of the UL37 protein to 68 kDa, verifying its modification by N-glycosylation in HCMV-infected cells. Consistent with this observation, gpUL37 was found to be resistant to digestion with either endoglycosidase F or H but sensitive to peptide N-glycosidase F digestion. These results suggested that gpUL37 is N-glycosylated and processed in both the endoplasmic reticulum (ER) and the Golgi apparatus. Direct demonstration of passage of gpUL37 through the ER and the Golgi was obtained by confocal microscopy. gpUL37 colocalized with protein disulfide isomerase, a protein resident in the ER, and with a Golgi protein. Subcellular fractionation of HCMV-infected cells demonstrated that gpUL37 is an integral membrane protein. Taken together, our results demonstrate that the HCMV gpUL37 immediate-early regulatory protein is a type I integral membrane N-glycoprotein which traffics through the ER and the Golgi network.     

Regulated expression of the human cytomegalovirus pp65 gene: octamer sequence in the promoter is required for activation by viral gene products.

To better understand the regulation of late gene expression in human cytomegalovirus (CMV)-infected cells, we examined expression of the gene that codes for the 65-kilodalton lower-matrix phosphoprotein (pp65). Analysis of RNA isolated at 72 h from cells infected with CMV Towne or ts66, a DNA-negative temperature-sensitive mutant, supported the fact that pp65 is expressed at low levels prior to viral DNA replication but maximally expressed after the initiation of viral DNA replication. To investigate promoter activation in a transient expression assay, the pp65 promoter was cloned into the indicator plasmid containing the gene for chloramphenicol acetyltransferase (CAT). Transfection of the promoter-CAT construct and subsequent superinfection with CMV resulted in activation of the promoter at early times after infection. Cotransfection with plasmids capable of expressing immediate-early (IE) proteins demonstrated that the promoter was activated by IE proteins and that both IE regions 1 and 2 were necessary. Analysis of promoter deletion mutants indicated that the 5' minimal sequence required for activation is -61 from the CAP site (+1) and that an 8-base-pair sequence located at -51 to -58 is necessary for activation of the pp65 promoter. This sequence is repeated once at +93 and is found as an inverted repeat at +67. These studies suggest that interactions between IE proteins and this octamer sequence may be important for the regulation and expression of this CMV gene.     

Requirements for enhanced transgene expression by untranslated sequences from the human cytomegalovirus immediate-early gene.

BACKGROUND: The cytomegalovirus immediate early (CMV IE) promoter has been widely used for heterologous expression. Further enhancements of gene expression from this potent promoter may allow for the development of improved gene transfer strategies. We aimed to determine whether inclusion of the first exon (5' untranslated) and first intron of the CMV IE gene would increase heterologous transgene expression in primary target cells and to determine the sequences required for any observed increases. MATERIALS AND METHODS: Comparisons of reporter gene expression were made following transient transfection of vascular smooth muscle cells (VSMCs) with plasmids containing the first exon and intron from the CMV IE gene or deletional mutations. Comparisons were also made using a heterologous promoter (RSV). RESULTS: Gene expression from the CMV IE promoter was increased 5.7-fold in VSMC with the inclusion of the first exon and intron. Similar increases were seen with other target cells and from the heterologous RSV promoter. This increase was associated with an increase in steady-state mRNA. Deletion analyses demonstrated that the enhancement was dependent on the presence of the 5' portion of the first exon while deletion of large segments within the intron was associated with similar levels of expression compared with the parental plasmid. CONCLUSIONS: Inclusion of the first exon and intron from the CMV IE gene increases expression from the CMV IE promoter. This enhancement is seen with the heterologous RSV promoter and is associated with an increase in steady-state mRNA. Deletion analyses suggest that this enhancement is associated with inclusion of sequences within the 5' portion of the first exon and inclusion of an intron.     

The phosphorylation status of the serine-rich region of the human cytomegalovirus 86-kilodalton major immediate-early protein IE2/IEP86 affects temporal viral gene expression

The 86-kDa major immediate-early protein (IE2/IEP86) of human cytomegalovirus (HCMV) contains a serine-rich region (amino acids 258 to 275) with several consensus casein kinase II (CKII) sites. We performed extensive mutational analysis of this region, changing serines to alternating alanines and glycines. Mutation of the serines between amino acids 266 and 275 eliminated in vitro phosphorylation by CKII. In vitro CKII phosphorylation of the serines between amino acids 266 and 269 or between amino acids 271 and 275 inhibited the ability of IE2/IEP86 to bind to TATA-binding protein. Correspondingly, nonphosphorylatable mutants in these regions showed increased activation of specific HCMV gene promoters in transfection studies. Viruses containing mutations of the serines throughout the entire region (amino acids 258 to 275) or the second half (amino acids 266 to 275) of the region showed delayed expression of all viral proteins tested and, correspondingly, delayed growth compared to wild-type HCMV. Mutation of the serines in the first half of the serine-rich region (amino acids 258 to 264) or between amino acids 266 and 269 propagated very slowly and has not been further studied. In contrast, mutation of the serines between amino acids 271 and 275 resulted in accelerated virus growth and accelerated temporal expression of viral proteins. These results suggest that the serine-rich region is structurally complex, possibly affecting multiple functions of IE2/IEP86. The data show that the phosphorylation state of the serine-rich region, particularly between amino acids 271 and 275, modulates the temporal expression of viral genes.     

CD43 gene expression is mediated by a nuclear factor which binds pyrimidine-rich single-stranded DNA

CD43 is a leukocyte-specific surface molecule which plays an important role both in adhesion and signal transduction. We have identified a site spanning nucleotides +18 to +39 within the human CD43 gene promoter which in vitro is hypersensitive to cleavage by nuclease S1. Repeats of this region are sufficient to activate expression of a heterologous promoter in CD43-positive cell lines. Two nuclear factors, PyRo1 and PyRo2, interact with the hypersensitive site. PyRo1 is a single-stranded DNA-binding protein which binds the pyrimidine-rich sense strand. Mutation analysis demonstrates that the motif TCCCCT is critical for PyRo1 interaction. Replacement of this motif with the sequence CATATA abolishes PyRo1 binding and reduces expression of the CD43 promoter by 35% in Jurkat T lymphocytic cells and by 52% in the pre-erythroid/pre-megakaryocytic cell line K562. However, this same replacement failed to affect expression in U937 monocytic cells or in CEM T lymphocytic cells. PyRo1, therefore, exhibits cell-specific differences in its functional activity. Further analysis demonstrated that PyRo1 not only interacts with the CD43 gene promoter but also motifs present within the promoters of the CD11a, CD11b, CD11c and CD11d genes. These genes encode the α subunits of the β2 integrin family of leukocyte adhesion receptors. Deletion of the PyRo1 binding site within the CD11c gene reduced promoter activity in T lymphocytic cells by 47%. However, consistent with our analysis of the CD43 gene, the effect of this same deletion within U937 monocytic cells was less severe. That PyRo1 binds preferentially to single-stranded DNA and sequences within the CD43 and CD11 gene promoters suggests that expression of these genes is influenced by DNA secondary structure.