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

Role of the human cytomegalovirus major immediate-early promoter's 19-base-pair-repeat cyclic AMP-response element in acutely infected cells

Prior studies have suggested a role of the five copies of the 19-bp-repeat cyclic AMP (cAMP)-response element (CRE) in major immediate-early (MIE) promoter activation, the rate-limiting step in human cytomegalovirus (HCMV) replication. We used two different HCMV genome modification strategies to test this hypothesis in acutely infected cells. We report the following: (i) the CREs do not govern basal levels of MIE promoter activity at a high or low multiplicity of infection (MOI) in human foreskin fibroblast (HFF)- or NTera2-derived neuronal cells; (ii) serum and virion components markedly increase MIE promoter-dependent transcription at a low multiplicity of infection (MOI), but this increase is not mediated by the CREs; (iii) forskolin stimulation of the cAMP signaling pathway induces a two- to threefold increase in MIE RNA levels in a CRE-specific manner at a low MOI in both HFF- and NTera2-derived neuronal cells; and (iv) the CREs do not regulate basal levels of HCMV DNA replication at a high or low MOI in HFF. Their presence does impart a forskolin-induced increase in viral DNA replication at a low MOI but only when basal levels of MIE promoter activity are experimentally diminished. In conclusion, the 19-bp-repeat CREs add to the robust MIE promoter activity that occurs in the acutely infected stimulated cells, although the CREs' greater role may be in other settings.

An essential role of the enhancer for murine cytomegalovirus in vivo growth and pathogenesis

The transcription of cytomegalovirus (CMV) immediate-early (IE) genes is regulated by a large and complex enhancer containing an array of binding sites for a variety of cellular transcription factors. Previously, using bacterial artificial chromosome recombinants of the virus genome, it was reported that the enhancer region of murine CMV (MCMV) is dispensable but performs a key function for viral multiplication (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal, J. Virol. 72:8502-8509, 1998). In the present study, we defined, through the reconstitution of infectious enhancerless MCMVs, the growth requirement for the enhancer in tissue culture and explored its significance for steering a productive infection in vivo. A comparison of cis and trans complementation systems for infection of enhancerless virus in permissive fibroblasts revealed a multiplicity-dependent growth phenotype that is severely compromised in the rate of infectious-virus multiplication. The in vivo impact of viruses that have an amputated enhancer was investigated in an extremely sensitive model of MCMV infection, the SCID mouse. Histological examination of spleens, livers, lungs, and salivary glands from animals infected with enhancer-deficient MCMV demonstrated an absence of tissue damage associated with CMV infection. The lack of pathogenic lesions correlated with a defect in replication competence. Enhancerless viruses were not detectable in major target organs harvested from SCID mice. The pathogenesis and growth defect reverted upon restoration of the enhancer. Markedly, while SCID mice infected with 5 PFU of parental MCMV died within 50 days postinfection, all mice infected with enhancerless virus survived for the duration of the experiment (1 year) after infection with 5 x 10(5) PFU. Together, these results clarify the importance of the enhancer for MCMV growth in cell culture and underscore the in vivo significance of this region for MCMV virulence and pathogenesis.

The human cytomegalovirus major immediate-early enhancer determines the efficiency of immediate-early gene transcription and viral replication in permissive cells at low multiplicity of infection

To determine the effect of the human cytomegalovirus (CMV) major immediate-early (MIE) enhancer or promoter on the efficiency of viral replication in permissive human cells, we constructed recombinant viruses with their human MIE promoter, enhancer, and promoter plus enhancer replaced with the murine CMV components. After a low multiplicity of infection (MOI) (0.01 PFU/cell), recombinant human CMV with the murine CMV promoter replicated like the wild type but recombinant virus with the murine enhancer replicated less efficiently. Immediate-early (IE) viral protein pIE72 (UL123), early viral protein (UL44), and viral DNA synthesis were significantly decreased. The effect of the human CMV enhancer substitution with the murine CMV enhancer was also demonstrated in different cell types by using recombinant virus with the UL127 promoter, driving the expression of green fluorescent protein (GFP). After an MOI of 1, GFP expression was high with the human CMV enhancer and significantly lower with the murine CMV enhancer. Even though at a high MOI (10 PFU/cell), the murine CMV enhancer was as efficient as the human CMV enhancer for the transcription of IE genes in human foreskin fibroblast cells, at lower MOIs, the murine CMV enhancer was less efficient. Proximal and distal chimeras of the human and murine enhancers also replicated less efficiently at a low MOI and expressed lower levels of GFP from the UL127 promoter. These experiments demonstrate that the entire human CMV enhancer has evolved for the efficient expression of the viral IE and early genes in human cells. Possible functions of the human CMV enhancer and promoter at a low MOI are discussed.

Tn7-mediated introduction of DNA sequences into bacmid-cloned cytomegalovirus genomes for rapid recombinant virus construction

Our basic understanding of how viruses infect, replicate, and cause disease has been largely derived from genetic approaches in which viral sequences have been mutated and the consequences of those mutations determined. In the herpesvirus field, deletions or insertions that preclude expression of specific viral proteins or remove critical cis elements have been invaluable in identifying their overall functions. We are now ready to move to a new level of detail-mapping functional domains within viral proteins or defining cis elements to the nucleotide level. This level of detail will require mutagenesis on a new scale, with recombinant viruses containing mutations within a given locus perhaps numbering in the hundreds. Mutagenesis on this scale would be greatly facilitated by more rapid methods of recombinant virus construction. In this report, we adapted a technology employing Tn7-mediated site-specific transposition [J. Virol. 67 (1993) 4566] as a rapid and highly reliable method to introduce novel sequences into bacmid-cloned herpesvirus genomes. We show that recombinant viruses can be rapidly created and that a deletion of the human cytomegalovirus (HCMV) essential gene ie2 can be complemented in cis by reintroduction, via transposition, of an ie2 cDNA; detailed mutagenesis of the complementing ie2 gene can now follow. Tn7-mediated transposition should accelerate greatly the pace at which recombinant herpesviruses can be constructed and, thus, facilitate the use of recombinant viruses for detailed mutagenic studies of both cis- and trans-acting genetic elements.

Ets-2 Repressor Factor (ERF) mediates repression of the human cytomegalovirus major immediate-early promoter in undifferentiated non-permissive cells

The repression of human cytomegalovirus immediate-early (IE) lytic gene expression is crucial for the maintenance of the latent viral state. By using conditionally permissive cell lines, which provide a good model for the differentiation state-dependent repression of IE gene expression, we have identified several cellular factors that bind to the major immediate-early promoter (MIEP) and whose expression is down-regulated after differentiation to a permissive phenotype. Here we show that the cellular protein Ets-2 Repressor Factor (ERF) physically interacts with the MIEP and represses MIEP activity in undifferentiated non-permissive T2 embryonal carcinoma cells. This factor binds to the dyad element and the 21 bp repeats within the MIEP - regions known to be important for the negative regulation of MIEP activity. Finally, we show that following differentiation to a permissive phenotype ERF's repressive effects are severely abrogated.

NF-kappaB--a potential therapeutic target for inhibition of human cytomegalovirus (re)activation?

From clinical studies the proinflammatory cytokine TNFalpha was proposed to play a key role in human cytomegalovirus (HCMV) reactivation from latency. In vitro experiments confirmed that TNFalpha stimulates the activity of the HCMV IE1/2 enhancer/promoter, which controls immediate early protein IE1 and IE2 gene expression via activation of the transcription factor NF-kappaB and its binding to putative binding sites in the IE1/2 enhancer. NF-kappaB was also proposed to be involved in IE1-mediated autostimulation of this promoter. The IE1/2 enhancer of HCMV contains four putative NF-kappaB binding sites which differ in their distance to the transcription start site as well as in their sequence. Construction and testing of a series of promoter mutants demonstrated that NF-kappaB is essential for both TNFalpha and IE1 stimulation. Furthermore, we were able to show that although all four NF-kappaB sites bind NF-kappaB with similar affinity in vitro, the contribution to TNFalpha and IE1 stimulation differs in correlation with the distance to the transcription start site and the sequence. Site 1 and 3 play the most dominant role and site 2 an intermediate, while site 4, which is conserved in sequence but far distant from the transcription start site, had no influence on NF-kappaB-mediated regulation of the IE1/2 promoter. Specific inhibition of NF-kappaB signalling by co-expression of a dominant-negative IkappaB variant reduced TNFalpha stimulation of the IE1/2 enhancer/promoter by up to 80%. From this data, inhibitors of NF-kappaB activation are suggested to be an alternative therapeutical strategy to interfere with HCMV (re)activation in undifferentiated monocyte/granulocyte progenitor cells in patients with a high risk of inflammation-related HCMV (re)activation.

Requirement of multiple cis-acting elements in the human cytomegalovirus major immediate-early distal enhancer for viral gene expression and replication

We have shown previously that the human cytomegalovirus (HCMV) major immediate-early (MIE) distal enhancer is needed for MIE promoter-dependent transcription and viral replication at low multiplicities of infection (MOI). To understand how this region works, we constructed and analyzed a series of HCMVs with various distal enhancer mutations. We show that the distal enhancer is composed of at least two parts that function independently to coordinately activate MIE promoter-dependent transcription and viral replication. One such part is contained in a 47-bp segment that has consensus binding sites for CREB/ATF, SP1, and YY1. At low MOI, these working parts likely function in cis to directly activate MIE gene expression, thus allowing viral replication to ensue. Three findings support the view that these working parts are likely cis-acting elements. (i) Deletion of either part of a bisegmented distal enhancer only slightly alters MIE gene transcription and viral replication. (ii) Reversing the distal enhancer's orientation largely preserves MIE gene transcription and viral replication. (iii) Placement of stop codons at -300 or -345 in all reading frames does not impair MIE gene transcription and viral replication. Lastly, we show that these working parts are dispensable at high MOI, partly because of compensatory stimulation of MIE promoter activity and viral replication that is induced by a virion-associated component(s) present at a high viral particle/cell ratio. We conclude that the distal enhancer is a complex multicomponent cis-acting region that is required to augment both MIE promoter-dependent transcription and HCMV replication.

Identification of TNF-alpha-sensitive sites in HCMVie1 promoter

Viral vectors using the human cytomegalovirus immediate-early promoter (HCMVie1 promoter) are potentially efficient tools for gene delivery in vivo to diverse cell types. We previously demonstrated that two cytokines, tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (INF-gamma), inhibited transgene expression from this promoter in skeletal and cardiac myocytes. In this study, electrophoretic mobility shift assays (EMSAs) were performed to identify the TNF-alpha response elements from the HCMVie1 promoter. The results show that TNF-alpha enhances the interaction of nuclear proteins from the C2C12 myocyte line with a single restricted segment of the HCMVie1 promoter. In vitro DNase I footprinting defined precisely the sites of interaction to two elements: nucleotides -1 to 0 and +24 to +36 relative to a transcription initiation cap homologous in the HCMVie1 promoter. These sites contain homologous sequences for cap initiation site (82%) and NFkappaB (62%) sites, respectively. Specificity was further ascertained by competitive EMSAs with wild-type and mutant oligonucleotide probes. Southwestern blotting showed that three proteins (45, 30, and 20 kDa) bound to this TNF-alpha-sensitive element, separately. However, EMSAs failed to prove a role for Yin Yang-1 (YY-1), NFkappaB (p65), or NFkappaB (p50) in binding to these sites. Our results provide evidence for two novel sites in the HCMVie1 promoter that are targets for TNF-alpha enhanced binding of transcription factors.

Products of US22 genes M140 and M141 confer efficient replication of murine cytomegalovirus in macrophages and spleen

Efficient replication of murine cytomegalovirus (MCMV) in macrophages is a prerequisite for optimal growth and spread of the virus in its natural host. Simultaneous deletion of US22 gene family members M139, M140, and M141 results in impaired replication of MCMV in macrophages and mice. In this study, we characterized the proteins derived from these three genes and examined the impact of individual gene deletions on viral pathogenesis. The M139, M140, and M141 gene products were identified as early proteins that localize to both the nucleus and cytoplasm in infected cells. Gene M139 encodes two proteins, of 72 and 61 kDa, while M140 and M141 each encode a single protein of 56 (pM140) and 52 (pM141) kDa, respectively. No role for the M139 proteins in MCMV replication in macrophages or mice was determined in these studies. In contrast, deletion of either M140 or M141 resulted in impaired MCMV replication in macrophages and spleen tissue. Replication of the M140 deletion mutant was significantly more impaired than that of the virus lacking M141. Further analyses revealed that the absence of the pM140 adversely affected pM141 levels by rendering the latter protein unstable. Since the replication defect due to deletion of M140 was more profound than could be explained by the reduced half-life of pM141, pM140 must exert an additional, independent function in mediating efficient replication of MCMV in macrophages and spleen tissue. These data indicate that the US22 genes M140 and M141 function both cooperatively and independently to regulate MCMV replication in a cell type-specific manner and, thus, to influence viral pathogenesis.

Rat cytomegalovirus major immediate-early enhancer switching results in altered growth characteristics

It has been hypothesized that the major immediate-early (MIE) enhancer of cytomegalovirus (CMV) is important in determining virus tropism and latency because of its essential role in initiating the cascade of early gene expression necessary for virus replication. Although rat CMV (RCMV) and murine CMV (MCMV) exhibit extreme species specificity in vivo, they differ in their ability to replicate in tissue culture. MCMV can replicate in a rat embryo fibroblast (REF) cell line while RCMV does not grow in murine fibroblasts. The tropism is not due to a block in virus entry into the cell. We have constructed a recombinant RCMV in which the RCMV MIE enhancer has been replaced with that of MCMV. Growth of the recombinant virus in tissue culture remains restricted to rat cells, suggesting that other viral and/or host factors are more important in determining in vitro tropism. Unlike findings using recombinant MCMV in which the human CMV (HCMV) MIE enhancer substitutes for the native one (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal, J. Virol. 72:8502-8509, 1998), infection with our recombinant virus at a low multiplicity of infection resulted in a substantial decrease in virus replication. This occurred despite comparable or increased MIE transcription from the recombinant virus. In vivo experiments showed that the recombinant virus replicates normally in the spleen during acute infection. Notably, the recombinant virus appears to be deficient in spreading to the salivary gland, suggesting a role for the MIE enhancer in tropism for certain tissues involved in virus dissemination. Four months after infection, recombinant virus with the foreign MIE enhancer was reactivated from spleen explants.

Reactivation of the human cytomegalovirus major immediate-early regulatory region and viral replication in embryonal NTera2 cells: role of trichostatin A, retinoic acid, and deletion of the 21-base-pair repeats and modulator

Inactivity of the human cytomegalovirus (HCMV) major immediate-early regulatory region (MIERR), which is composed of promoter, enhancer, unique region, and modulator, is linked to lack of HCMV replication in latently infected cells and in other nonpermissive cell types, including human embryonal NTera2 carcinoma (NT2) cells. I refined the embryonal NT2 cell model to enable characterization of the unknown mechanistic basis for silencing of HCMV MIERR-dependent transcription and viral replication in nonpermissive cells. These infected NT2 cells contain nonreplicating viral genomes with electrophoretic mobility equivalent to a supercoiled, bacterial artificial chromosome of comparable molecular weight. MIERR-dependent transcription is minimal to negligible. Increasing the availability of positive-acting transcription factors by retinoic acid (RA) treatment after infection is largely insufficient in reactivating the MIERR. In contrast, trichostatin A (TSA), a histone deacetylase inhibitor, reactivates MIERR-dependent transcription. Contrary to prior findings produced from transfected MIERR segments, deletion of the 21-bp repeats and modulator from the MIERR in the viral genome does not relieve MIERR silencing. To demonstrate that MIERR silencing likely results from enhancer inactivity, I examined an HCMV with a heterologous MIERR promoter that is enhancer dependent but exempt from IE2 p86-mediated negative autoregulation. This heterologous promoter, like its neighboring native MIERR promoter, exhibits immediate-early transcriptional kinetics in fibroblasts. In embryonal NT2 cells, the heterologous MIERR promoter is transcriptionally inactive. This silence is relieved by TSA but not by RA. Remarkably, TSA-induced reactivation of MIERR-dependent transcription from quiescent viral genomes is followed by release of infectious virus. I conclude that a mechanism of active repression imposes a block to MIERR-dependent transcription and viral replication in embryonal NT2 cells. Because TSA overcomes the block, viral gene silencing may involve histone deacetylase-based modification of viral chromatin, which might account for the covalently closed circular conformation of quiescent HCMV genomes.

The human cytomegalovirus IE86 protein can block cell cycle progression after inducing transition into the S phase of permissive cells

Human cytomegalovirus (HCMV) infection of permissive cells has been reported to induce a cell cycle halt. One or more viral proteins may be involved in halting progression at different stages of the cell cycle. We investigated how HCMV infection, and specifically IE86 protein expression, affects the cell cycles of permissive and nonpermissive cells. We used a recombinant virus that expresses the green fluorescent protein (GFP) to determine the effects of HCMV on the cell cycle of permissive cells. Fluorescence by GFP allowed us to select for only productively infected cells. Replication-defective adenovirus vectors expressing the IE72 or IE86 protein were also used to efficiently transduce 95% or more of the cells. The adenovirus-expressed IE86 protein was determined to be functional by demonstrating negative autoregulation of the major immediate-early promoter and activation of an early viral promoter in the context of the viral genome. To eliminate adenovirus protein effects, plasmids expressing GFP for fluorescent selection of only transfected cells and wild-type IE86 protein or a mutant IE86 protein were tested in permissive and nonpermissive cells. HCMV infection induced the entry of U373 cells into the S phase. All permissive cells infected with HCMV were blocked in cell cycle progression and could not divide. After either transduction or transfection and IE86 protein expression, the number of all permissive or nonpermissive cell types in the S phase increased significantly, but the cells could no longer divide. The IE72 protein did not have a significant effect on the S phase. Since IE86 protein inhibits cell cycle progression, the IE2 gene in a human fibroblast IE86 protein-expressing cell line was sequenced. The IE86 protein in these retrovirus-transduced cells has mutations in a critical region of the viral protein. The locations of the mutations and the function of the IE86 protein in controlling cell cycle progression are discussed.

A strong negative transcriptional regulatory region between the human cytomegalovirus UL127 gene and the major immediate-early enhancer

The region of the human cytomegalovirus (CMV) genome between the UL127 open reading frame and the major immediate-early (MIE) enhancer is referred to as the unique region. DNase I protection analysis with human cell nuclear extracts demonstrated multiple protein binding sites in this region of the viral genome (P. Ghazal, H. Lubon, C. Reynolds-Kohler, L. Hennighausen, and J. A. Nelson, Virology 174:18-25, 1990). However, the function of this region in the context of the viral genome is not known. In wild-type human CMV-infected human fibroblasts, cells permissive for viral replication, there is little to no transcription from UL127. We determined that the unique region prevented transcription from the UL127 promoter but had no effect on the divergent MIE promoter. In transient-transfection assays, the basal level of expression from the UL127 promoter increased significantly when the wild-type unique sequences were mutated. In recombinant viruses with similar mutations in the unique region, expression from the UL127 promoter occurred only after de novo viral protein synthesis, typical of an early viral promoter. A 111-bp deletion-substitution of the unique sequence caused approximately a 20-fold increase in the steady-state level of RNA from the UL127 promoter and a 245-fold increase in the expression of a downstream indicator gene. This viral negative regulatory region was also mutated at approximately 50-bp regions proximal and distal to the UL127 promoter. Although some repressive effects were detected in the distal region, mutations of the region proximal to the UL127 promoter had the most significant effects on transcription. Within the proximal and distal regions, there are potential cis sites for known eucaryotic transcriptional repressor proteins. This region may also bind unknown viral proteins. We propose that the unique region upstream of the UL127 promoter and the MIE enhancer negatively regulates the expression from the UL127 promoter in permissive human fibroblast cells. This region may be a regulatory boundary preventing the effects of the very strong MIE enhancer on this promoter.