E2F proteins are posttranslationally modified concomitantly with a reduction in nuclear binding activity in cells infected with herpes simplex virus 1

E2F4DN Transgenic Mice: A Tool for the Evaluation of E2F4 as a Therapeutic Target in Neuropathology and Brain Aging

E2F4 was initially described as a transcription factor with a key function in the regulation of cell quiescence. Nevertheless, a number of recent studies have established that E2F4 can also play a relevant role in cell and tissue homeostasis, as well as tissue regeneration. For these non-canonical functions, E2F4 can also act in the cytoplasm, where it is able to interact with many homeostatic and synaptic regulators. Since E2F4 is expressed in the nervous system, it may fulfill a crucial role in brain function and homeostasis, being a promising multifactorial target for neurodegenerative diseases and brain aging. The regulation of E2F4 is complex, as it can be chemically modified through acetylation, from which we present evidence in the brain, as well as methylation, and phosphorylation. The phosphorylation of E2F4 within a conserved threonine motif induces cell cycle re-entry in neurons, while a dominant negative form of E2F4 (E2F4DN), in which the conserved threonines have been substituted by alanines, has been shown to act as a multifactorial therapeutic agent for Alzheimer’s disease (AD). We generated transgenic mice neuronally expressing E2F4DN. We have recently shown using this mouse strain that expression of E2F4DN in 5xFAD mice, a known murine model of AD, improved cognitive function, reduced neuronal tetraploidization, and induced a transcriptional program consistent with modulation of amyloid-β (Aβ) peptide proteostasis and brain homeostasis recovery. 5xFAD/E2F4DN mice also showed reduced microgliosis and astrogliosis in both the cerebral cortex and hippocampus at 3-6 months of age. Here, we analyzed the immune response in 1 year-old 5xFAD/E2F4DN mice, concluding that reduced microgliosis and astrogliosis is maintained at this late stage. In addition, the expression of E2F4DN also reduced age-associated microgliosis in wild-type mice, thus stressing its role as a brain homeostatic agent. We conclude that E2F4DN transgenic mice represent a promising tool for the evaluation of E2F4 as a therapeutic target in neuropathology and brain aging.

A critical role of E2F transcription factor 2 in proinflammatory cytokines-dependent proliferation and invasiveness of fibroblast-like synoviocytes in rheumatoid Arthritis

As a transcription factor, E2F2 participates in regulation of numerous genes. To investigate the role and mechnism of E2F2 in RA, expression of E2F2 in synovial tissue was detected. Proliferation, invasion, and secretion of inflammatory cytokines were measured after E2F2 was knocked-down in RASFs by siRNA transfection. Induction of TNF-α, IL-6, and LPS on expression and nuclear translocation of E2F2, and signal pathways involved in the process were tested. ChIP was used to investigate direct binding of NF-кB to the promoter of E2F2, and E2F2 to the promoter of IL-6. The correlation between mRNA levels of E2F2 and IL-6 or TNF-α in secreted in supernatant of RASFs were also investigated. As a result, silencing E2F2 could inhibit the proliferation and invasion of RASFs. LPS, IL-6 can stimulate the expression of E2F2 in RASFs both via the NF-кB pathway, while TNF-α via the ERK pathway. TNF-α can facilitate the nuclear translocation of E2F2 and TNF-α can bind to promoter of E2F2, and then E2F2 can bind to the promoter of IL-6 directly. Significant correlations was found between levels of E2F2 and IL-6/TNF-α in synoviocytes of RA patients. Our findings indicate that E2F2 may play an important role in pathogenesis of RA.

Timing Is Everything

N. Drayman et al. in their recent article (mBio 8:e01612-17, 2017, https://doi.org/10.1128/mBio.01612-17) have used dynamic proteomics and machine learning to show that the cell cycle state of any individual cell affects the outcome of a productive herpes simplex virus 1 (HSV-1) infection. Cells infected from early G₁ through S were most permissive for expression of genes from the HSV-1 genome, whereas cells infected in late G₂ to mitosis were much less so. Most of the infected cells that underwent mitosis became permanently nonpermissive for HSV-1 gene expression afterward. The cell cycle stage accounted for 60% of the success of infection, and cell density and motility accounted for most of the rest. To successfully reactivate, HSV-1 must express its genes in neurons and cells of the spinosum and granulosum epidermis strata. These cells are permanently in the cell cycle stages most permissive for HSV-1 gene expression, and none reenters mitosis, thus maximizing the efficiency of a successful HSV-1 reactivation before the adaptive immunity can control it.

Role of activating transcription factor 3 in the synthesis of latency-associated transcript and maintenance of herpes simplex virus 1 in latent state in ganglia

A key property of herpes simplex viruses (HSVs) is their ability to establish latent infection in sensory or autonomic ganglia and to reactivate on physical, hormonal, or emotional stress. In latently infected ganglia, HSVs express a long noncoding RNA, a latency-associated transcript (LAT), which plays a key role in maintaining latently infected neurons, but not viral proteins. To investigate the events leading to reactivation, we examined the use of ganglionic organ cultures that enable rapid reactivation in medium containing antibody to nerve growth factor (NGF) or delayed reactivation in medium containing NGF and epidermal growth factor (EGF). Here we report the discovery that activating transcription factor 3 (ATF3), a stress response protein, profoundly affects the interaction of HSV with its host. Specifically, (i) ATF3 is induced by stress, such as inhibition of protein synthesis or infection; (ii) in infected cells, ATF3 enhances the accumulation of LAT by acting on the response elements in the promoter of the LAT precursor RNA; (iii) ATF3 is induced nearly 100-fold in ganglionic organ cultures; and (iv) ATF3 plays a key role in the maintenance of the latent state, inasmuch as expression of ATF3 bereft of the C-terminal activation domain acts as a dominant negative factor, inducing HSV gene expression in ganglionic organ cultures harboring latent virus and incubated in medium containing NGF and EGF. Thus, ATF3 is a component of a cluster of cellular proteins that together with LAT maintain the integrity of the neurons harboring latent virus.

A sequence within the varicella-zoster virus (VZV) OriS is a negative regulator of DNA replication and is bound by a protein complex containing the VZV ORF29 protein

The architecture of the varicella-zoster virus (VZV) origin of DNA replication (OriS) differs significantly from that of the herpes simplex virus (HSV) DNA replication origin. Novel aspects of the VZV OriS include a GA-rich region, three binding sites for the VZV origin-binding protein (OBP) all on the same strand and oriented in the same direction, and a partial OBP binding site of unknown function. We have designated this partial binding site Box D and have investigated the role it plays in DNA replication and flanking gene expression. This has been done with a model system using a replication-competent plasmid containing OriS and a replication- and transcription-competent dual-luciferase reporter plasmid containing both the OriS and the intergenic region between VZV open reading frames (ORFs) 62 and 63. We have found that (i) Box D is a negative regulator of DNA replication independent of flanking gene expression, (ii) the mutation of Box D results in a decrease in flanking gene expression, thus a sequence within the VZV OriS affects transcription, which is in contrast to results reported for HSV-1, (iii) there is a specific Box D complex formed with infected cell extracts in electrophoretic mobility shift assay experiments, (iv) supershift assays show that this complex contains the VZV ORF29 single-strand DNA-binding protein, and (v) the formation of this complex is dependent on the presence of CGC motifs in Box D and its downstream flanking region. These findings show that the VZV ORF29 protein, while required for DNA replication, also plays a novel role in the suppression of that process.

Analysis of the cell cycle regulatory protein (E2F1) after infection of cultured cells with bovine herpesvirus 1 (BHV-1) or herpes simplex virus type 1 (HSV-1)

The E2F family of cellular transcription factors controls cell cycle progression and cell death. During cell cycle progression, activated cyclin-dependent kinases phosphorylate the retinoblastoma (Rb) protein, causing the release and activation of E2F family members. Previous studies demonstrated that bovine herpes virus 1 (BHV-1) productive infection increases E2F1 protein levels, the bICP0 early promoter is activated more than 100 fold by E2F1 or E2F2, and silencing E2F1 reduced the efficiency of productive infection. In this study, the effect of herpes simplex virus type 1 (HSV-1) productive infection on E2F protein levels and regulation of E2F dependent transcription was compared to BHV-1 infection in the same permissive cell line, rabbit skin (RS) cells. Silencing E2F1 with a specific siRNA reduced HSV-1 productive infection approximately 10 fold in RS cells, and total E2F1 protein levels increased during productive infection. In contrast to RS cells infected with BHV-1, a fraction of total E2F1 protein was localized to the cytoplasm in HSV-1 infected RS cells. Furthermore, E2F1 did not efficiently trans-activate the HSV-1 ICP0 or ICP4 promoter. When RS cells were transfected with an E2F reporter construct or the cyclin D1 promoter and then infected with BHV-1, promoter activity increased after infection. In contrast, HSV-1 infection of RS cells had little effect on E2F dependent transcription and cyclin D1 promoter activity was reduced. In summary, these studies indicated that silencing E2F1 reduced the efficiency of HSV-1 and BHV-1 productive infection. However, only BHV-1 productive infection induced E2F dependent transcription.

Development and application of a phosphoproteomic method using electrostatic repulsion-hydrophilic interaction chromatography (ERLIC), IMAC, and LC-MS/MS analysis to study Marek's Disease Virus infection

Marek's Disease (MD) is an avian neoplastic disease caused by Marek's Disease Virus (MDV). The mechanism of virus transition between the lytic and latent cycle is still being investigated; however, post-translational modifications, especially phosphorylation, have been thought to play an important role. Previously, our group has used strong cation exchange chromatography in conjunction with reversed-phase liquid chromatography-tandem mass spectrometry (LC-MS/MS) to study the changes in global proteomic expression upon MDV infection (Ramaroson , M. F.; Ruby, J.; Goshe, M. B.; Liu , H.-C. S. J. Proteome Res. 2008, 7, 4346-4358). Here, we extend our study by developing an effective separation and enrichment approach to investigate the changes occurring in the phosphoproteome using electrostatic repulsion-hydrophilic interaction chromatography (ERLIC) to fractionate peptides from chicken embryo fibroblast (CEF) digests and incorporating a subsequent IMAC enrichment step to selectively target phosphorylated peptides for LC-MS/MS analysis. To monitor the multidimensional separation between mock- and MDV-infected CEF samples, a casein phosphopeptide mixture was used as an internal standard. With LC-MS/MS analysis alone, no CEF phosphopeptides were detected, while with ERLIC fractionation only 1.2% of all identified peptides were phosphorylated. However, the incorporation of IMAC enrichment with ERLIC fractionation provided a 50-fold increase in the percentage of identified phosphopeptides. Overall, a total of 581 unique phosphopeptides were identified (p < 0.05) with those of the MDV-infected CEF sample containing nearly twice as many as the mock-infected control of which 11% were unique to MDV proteins. The changes in the phosphoproteome are discussed including the role that microtubule-associated proteins may play in MDV infection mechanisms.

Productive infection and bICP0 early promoter activity of bovine herpesvirus 1 are stimulated by E2F1

Bovine herpesvirus 1 (BoHV-1) is an important viral pathogen of cattle. Like other members of the subfamily Alphaherpesvirinae, BoHV-1 establishes latency in sensory neurons and has the potential to reactivate from latency. Dexamethasone (DEX) treatment of latently infected calves or rabbits consistently leads to reactivation from latency. The BoHV-1 transcript encoding the infected cell protein 0 (bICP0) is consistently detected during reactivation from latency, in part because the bICP0 early promoter is activated by DEX. During DEX-induced reactivation from latency, cyclin expression is stimulated in infected sensory neurons. Cyclin-dependent kinase activity phosphorylates Rb (retinoblastoma tumor suppressor gene product) family proteins and consequently releases the E2F family of transcription factors, suggesting that E2F family members stimulate productive infection and/or reactivation from latency. In this study, we provide evidence that repression of E2F1 by a specific small interfering RNA (siRNA) reduced productive infection approximately 5-fold. E2F1 or E2F2 stimulated bICP0 early promoter activity at least 100-fold in transient transfection assays. Two E2F-responsive regions (ERR) were identified within the early promoter, with one adjacent to the TATA box (ERR1) and one approximately 600 bp upstream from the TATA box (ERR2). Mobility shift assays suggested that E2F interacts with ERR1 and ERR2. E2F1 protein levels were increased at late times after infection, which correlated with enhanced binding to a consensus E2F binding site, ERR1, or ERR2. Collectively, these studies suggest that E2F1 stimulates productive infection and bICP0 early promoter activity, in part because E2F family members interact with ERR1 and ERR2.

Binding of herpes simplex virus type-1 virions leads to the induction of intracellular signalling in the absence of virus entry

The envelope of HSV-1 contains a number of glycoproteins, four of which are essential for virus entry. Virus particles lacking gB, gD, gH or gL are entry-defective, although these viruses retain the ability to bind to the plasma membrane via the remaining glycoproteins. Soluble forms of gD have been shown to trigger the nuclear translocation of the NF-κB transcriptional complex in addition to stimulating the production of Type I interferon. By taking advantage of the entry-defective phenotype of glycoprotein-deficient HSV-1 virus particles, the results presented here show that binding of virions to cellular receptors on the plasma membrane is sufficient to stimulate a change in cellular gene expression. Preliminary microarray studies, validated by quantitative real-time PCR, identified the differential expression of cellular genes associated with the NF-κB, PI3K/Akt, Jak/Stat and related Jak/Src pathways by virions lacking gB or gH but not gD. Gene induction occurred at a few particles per cell, corresponding to physiological conditions during primary infection. Reporter assay studies determined that NF-κB transcriptional activity is stimulated within an hour of HSV-1 binding, peaks between two and three hours post-binding and declines to background levels by five hours after induction. The immediate, transient nature of these signalling events suggests that HSV-1 glycoproteins, particularly gD, may alter the cellular environment pre-entry so as to condition the cell for viral replication.

Cilostazol reduces proliferation through c-Myc down-regulation in MDCK cells

Cilostazol, a drug commonly used in the treatment of intermittent claudication is a selective phosphodiesterase III inhibitor. It affects cell proliferation, increases cAMP levels, activates the cyclic AMP-dependent protein kinase and inhibits E2F in vascular cells. Polycystic kidney disease, a common genetic disorder, is characterized by increased cell proliferation, basement membrane abnormalities and fluid secretion. An established in vitro model of this disease is the canine Madin-Darby cell line (MDCK). In this communication, we investigated the effects of cilostazol exposure in MDCK cells. A reduced cell proliferation rate with an arrest in the G1 phase of the cell cycle was detected. Accordingly, several transcription factors associated with cell cycle control were affected by cilostazol, particularly c-myc. c-Myc DNA binding as well as its transcriptional activity was severely impaired in cilostazol-treated cells. Pharmacological tools demonstrated that besides the involvement of the cyclic AMP-dependent protein kinase, the extracellular signal-regulated kinases I/II participate in the response. These results suggest that cilostazol inhibits cell proliferation through c-myc transcriptional control, also pave the way to our better understanding of molecular transactions triggered by this drug and strengthen its potential use in other malignancies.

Regulation of the retinoblastoma proteins by the human herpesviruses

Viruses are obligate intracellular parasites that alter the environment of infected cells in order to replicate more efficiently. One way viruses achieve this is by modulating cell cycle progression. The main regulators of progression out of G0, through G1, and into S phase are the members of the retinoblastoma (Rb) family of tumor suppressors. Rb proteins repress the transcription of genes controlled by the E2F transcription factors. Because the expression of E2F-responsive genes is required for cell cycle progression into the S phase, Rb arrests the cell cycle in G0/G1. A number of viral proteins directly target Rb family members for inactivation, presumably to create an environment more hospitable for viral replication. Such viral proteins include the extensively studied oncoproteins E7 (from human papillomavirus), E1A (from adenovirus), and the large T (tumor) antigen (from simian virus 40). Elucidating how these three viral proteins target and inactivate Rb has proven to be an invaluable approach to augment our understanding of both normal cell cycle progression and carcinogenesis. In addition to these proteins, a number of other virally-encoded inactivators of the Rb family have subsequently been identified including a surprising number encoded by human herpesviruses. Here we review how the human herpesviruses modulate Rb function during infection, introduce the individual viral proteins that directly or indirectly target Rb, and speculate about what roles Rb modulation by these proteins may play in viral replication, pathogenesis, and oncogenesis.

Herpes simplex virus 1 induces cytoplasmic accumulation of TIA-1/TIAR and both synthesis and cytoplasmic accumulation of tristetraprolin, two cellular proteins that bind and destabilize AU-rich RNAs

Herpes simplex virus 1 causes a shutoff of cellular protein synthesis through the degradation of RNA that is mediated by the virion host shutoff (Vhs) protein encoded by the U(L)41 gene. We reported elsewhere that the Vhs-dependent degradation of RNA is selective, and we identified RNAs containing AU-rich elements (AREs) that were upregulated after infection but degraded by deadenylation and progressive 3'-to-5' degradation. We also identified upregulated RNAs that were not subject to Vhs-dependent degradation (A. Esclatine, B. Taddeo, L. Evans, and B. Roizman, Proc. Natl. Acad. Sci. USA 101:3603-3608, 2004). Among the latter was the RNA encoding tristetraprolin, a protein that binds AREs and is known to be associated with the degradation of RNAs containing AREs. Prompted by this observation, we examined the status of the ARE binding proteins tristetraprolin and TIA-1/TIAR in infected cells. We report that tristetraprolin was made and accumulated in the cytoplasm of wild-type virus-infected human foreskin fibroblasts as early as 2 h and in HEp-2 cells as early as 6 h after infection. The amounts of tristetraprolin that accumulated in the cytoplasm of cells infected with a mutant virus lacking U(L)41 were significantly lower than those in wild-type virus-infected cells. The localization of tristetraprolin was not modified in cells infected with a mutant lacking the gene encoding infected cell protein 4 (ICP4). TIA-1 and TIAR are two other proteins that are associated with the regulation of ARE-containing RNAs and that normally reside in nuclei. In infected cells, they started to accumulate in the cytoplasm after 6 h of infection. In cells infected with the mutant virus lacking U(L)41, TIA-1/TIAR accumulated in the cytoplasm in granular structures reminiscent of stress granules in a significant percentage of the cells. In addition, an antibody to tristetraprolin coprecipitated the Vhs protein from lysates of cells late in infection. The results indicate that the Vhs-dependent degradation of ARE-containing RNAs correlates with the transactivation, cytoplasmic accumulation, and persistence of tristetraprolin in infected cells.

Herpes simplex virus 1 mutant in which the ICP0 HUL-1 E3 ubiquitin ligase site is disrupted stabilizes cdc34 but degrades D-type cyclins and exhibits diminished neurotoxicity

Herpes simplex virus type 1 (HSV-1) infected cell protein 0 (ICP0) is a multifunctional protein that functions as a promiscuous transactivator and promotes the degradation of multiple cellular proteins. In vitro studies indicated that it encodes two physically separated functional E3 ubiquitin ligase domains. One, designated herpesvirus ubiquitin ligase 1 (HUL-1), maps to a region encoded by exon 3 and is contained between residues 543 and 680. Deletion of amino acids 621 to 625 abolishes this activity. The second, designated HUL-2, maps to the RING finger domain present in ICP0 encoded by exon 2. Earlier studies have shown that ICP0 stabilizes cyclins D1 and D3, and several lines of investigation led to the hypothesis that this function of ICP0 is the consequence of degradation of the E2 enzyme cdc34, known to be involved in the proteasome-dependent degradation of D-type cyclins. Consistent with this hypothesis, we have previously shown that cdc34 physically interacts with ICP0 at or near aspartate 199 and at amino acids 621 to 625 and that the former site is required for effective ubiquitylation and degradation of cdc34. Furthermore, the ICP0 HUL-1 domain promotes the polyubiquitination of cdc34 in vitro. If the mechanism by which D-type cyclins are salvaged in wild-type-infected cells is dependent on polyubiquitination and consequent destruction of cdc34, than the mutant virus R6701, which was constructed for these studies and lacks ICP0 residues 621 to 625, should destabilize the D cyclins and preclude the degradation of cdc34. We report that ICP0 residues 621 to 625 are essential for degradation of cdc34 in infected cells and for the ICP0-mediated stabilization of D-type cyclins, that a mutation that specifically disrupted the ring finger domain of the HUL-2 site had no effect on the degradation of cdc34 in infected cells, and that deletion of ICP0 residues 621 to 625 decreased the replicative capacity of the virus in growth-arrested but not in dividing cells and resulted in diminished pathogenicity on intracerebral inoculation of mice. We conclude that the ICP0 HUL-1 domain acts in infected cells to degrade cdc34 and that this function requires the interaction of cdc34 with sequences in exons 2 and 3 but does not involve the HUL-2 RING finger E3 domain.

Oct-1 is posttranslationally modified and exhibits reduced capacity to bind cognate sites at late times after infection with herpes simplex virus 1

In herpes simplex virus 1-infected cells, a high level of alpha gene expression requires the transactivation of the genes by a complex containing the viral alpha transinducing factor (alphaTIF) and two cellular proteins. The latter two, HCF-1 and octamer binding protein Oct-1, are transcriptional factors regulated in a cell cycle-dependent manner. alphaTIF is a protein made late in infection but packaged with the virion to transactivate viral genes in newly infected cells. In light of the accumulation of large amounts of alphaTIF, the absence of alpha gene expression late in infection suggested the possibility that one or more transcriptional factors required for alpha gene expression is modified late in infection. Here we report that Oct-1 is posttranscriptionally modified late in infection, that the modification is mediated by the virus but does not involve viral protein kinases or cdc2 kinase activated by the virus late in infection, and that the modified Oct-1 has a reduced affinity for its cognate DNA site. These results are consistent with the hypothesis that modification of Oct-1 transcriptional factor could account at least in part for the shutoff of alpha gene expression late in infection.

Stimulation of bovine herpesvirus-1 productive infection by the adenovirus E1A gene and a cell cycle regulatory gene, E2F-4

Identifying cellular genes that promote bovine herpesvirus-1 (BHV-1) productive infection is important, as BHV-1 is a significant bovine pathogen. Previous studies demonstrated that BHV-1 DNA is not very infectious unless cotransfected with a plasmid expressing bICP0, a viral protein that stimulates expression of all classes of viral promoters. Based on these and other studies, we hypothesize that the ability of bICP0 to interact with and modify the function of cellular proteins stimulates virus transcription. If this prediction is correct, cellular proteins that activate virus transcription could, in part, substitute for bICP0 functions. The adenovirus E1A gene and bICP0 encode proteins that are potent activators of viral gene expression, they do not specifically bind DNA and both proteins interact with chromatin-remodelling enzymes. Because of these functional similarities, E1A was tested initially to see if it could stimulate BHV-1 productive infection. E1A consistently stimulates BHV-1 productive infection, but not as efficiently as bICP0. The ability of E1A to bind Rb family members plays a role in stimulating productive infection, suggesting that E2F family members activate productive infection. E2F-4, but not E2F-1, E2F-2 or E2F-5, activates productive infection with similar efficiency as E1A. Next, E2F family members were examined for their ability to activate the BHV-1 immediate-early (IE) transcription unit 1 (IEtu1) promoter, as it regulates IE expression of bICP0 and bICP4. E2F-1 and E2F-2 strongly activate the IEtu1 promoter, but not a BHV-1 IEtu2 promoter or a herpes simplex virus type 1 ICP0 promoter construct. These studies suggest that E2F family members can stimulate BHV-1 productive infection.

Active nuclear import and export pathways regulate E2F-5 subcellular localization

Epidermal keratinocyte differentiation is accompanied by differential regulation of E2F genes, including up-regulation of E2F-5 and its concomitant association with the retinoblastoma family protein p130. This complex appears to play a role in irreversible withdrawal from the cell cycle in differentiating keratinocytes. We now report that keratinocyte differentiation is also accompanied by changes in E2F-5 subcellular localization, from the cytoplasm to the nucleus. To define the molecular determinants of E2F-5 nuclear import, we tested its ability to enter the nucleus in import assays in vitro using digitonin-permeabilized cells. We found that E2F-5 enters the nucleus through mediated transport processes that involve formation of nuclear pore complexes. It has been proposed that E2F-4 and E2F-5, which lack defined nuclear localization signal (NLS) consensus sequences, enter the nucleus in association with NLS-containing DP-2 or pRB family proteins. However, we show that nuclear import of E2F-5 only requires the first N-terminal 56 amino acid residues and is not dependent on interaction with DP or pRB family proteins. Because E2F-5 is predominantly cytoplasmic in undifferentiated keratinocytes and in other intact cells, we also examined whether this protein is subjected to active nuclear export. Indeed, E2F-5 is exported from the nucleus through leptomycin B-sensitive, CRM1-mediated transport, through a region corresponding to amino acid residues 130-154. This region excludes the DNA- and the p130-binding domains. Thus, the subcellular distribution of E2F-5 is tightly regulated in intact cells, through multiple functional domains that direct nucleocytoplasmic shuttling of this protein.

cdc2 cyclin-dependent kinase binds and phosphorylates herpes simplex virus 1 U(L)42 DNA synthesis processivity factor

Earlier studies have shown that cdc2 kinase is activated during herpes simplex virus 1 infection and that its activity is enhanced late in infection even though the levels of cyclin A and B are decreased below levels of detection. Furthermore, activation of cdc2 requires the presence of infected cell protein no. 22 and the U(L)13 protein kinase, the same gene products required for optimal expression of a subset of late genes exemplified by U(S)11, U(L)38, and U(L)41. The possibility that the activation of cdc2 and expression of this subset may be connected emerged from the observation that dominant negative cdc2 specifically blocked the expression of U(S)11 protein in cells infected and expressing dominant negative cdc2. Here we report that in the course of searching for a putative cognate partner for cdc2 that may have replaced cyclins A and B, we noted that the DNA polymerase processivity factor encoded by the U(L)42 gene contains a degenerate cyclin box and has been reported to be structurally related to proliferating cell nuclear antigen, which also binds cdk2. Consistent with this finding, we report that (i) U(L)42 is able to physically interact with cdc2 at both the amino-terminal and carboxyl-terminal domains, (ii) the carboxyl-terminal domain of U(L)42 can be phosphorylated by cdc2, (iii) immunoprecipitates obtained with anti U(L)42 antibody contained a roscovitine-sensitive kinase activity, (iv) kinase activity associated with U(L)42 could be immunodepleted by antibody to cdc2, and (v) U(L)42 transfected into cells associates with a nocodazole-enhanced kinase. We conclude that U(L)42 can associate with cdc2 and that the kinase activity has the characteristic traits of cdc2 kinase.

Pocket protein p130/Rb2 is required for efficient herpes simplex virus type 1 gene expression and viral replication

We have reported previously that herpes simplex virus type 1 (HSV-1) infection disrupts normal progression of the mammalian cell cycle, causing cells to enter a G(1)-like state. Infected cells were characterized by a decline in cyclin-dependent kinase 2 (CDK2) activities, loss of hyperphosphorylated retinoblastoma protein (pRb), accumulation of E2F-pocket protein complexes, and failure to initiate cellular DNA replication. In the present study, we investigated the role of the pocket proteins pRb, p107, and p130 in HSV-1-dependent cell cycle inhibition and cyclin kinase regulation by infecting murine 3T3 cells derived from wild-type (WT) mouse embryos or embryos with deletions of pRb (pRb(-/-)), p107 (p107(-/-)), p130 (p130(-/-)), or both p130 and p107 (p130(-/-)/p107(-/-)). With respect to CDK2 inhibition, viral protein accumulation, viral DNA replication, and progeny virus yield, WT, pRb(-/-), and p107(-/-) cells were essentially identical. In contrast, after infection of p130(-/-) cells, we observed no inhibition of CDK2 activity, a 5- to 6-h delay in accumulation of viral proteins, an impaired ability to form viral DNA replication compartments, and reduced viral DNA synthesis. As a result, progeny virus yield was reduced 2 logs compared to that in WT cells. Notably, p130(-/-)/p107(-/-) double-knockout cells had a virus replication phenotype intermediate between those of the p107(-/-) and p130(-/-) cells. We conclude from these studies that p130 is a key factor in regulating aspects of cell cycle progression, as well as the timely expression of viral genes and replication of viral DNA.

Ca2+ and BMP-6 signaling regulate E2F during epidermal keratinocyte differentiation

The epidermis consists of a squamous epithelium continuously replenished by committed stem cells, which can either self-renew or differentiate. We demonstrated previously that E2F genes are differentially expressed in developing epidermis (Dagnino, L., Fry, C. J., Bartley, S. M., Farnham, P., Gallie, B. L., and Phillips, R. A. (1997) Cell Growth Differ. 8, 553-563). Thus, we hypothesized that various E2F proteins likely play distinct growth regulatory roles in the undifferentiated stem cells and in terminally differentiated keratinocytes. To further understand the function of E2F genes in epidermal morphogenesis, we have examined the expression, regulation, and protein-protein interactions of E2F factors in undifferentiated cultured murine primary keratinocytes or in cells induced to differentiate with Ca(2+) or BMP-6 (bone morphogenetic protein 6). We find similar patterns of E2F regulation with both differentiating agents and demonstrate a switch in expression from E2F-1, -2, and -3 in undifferentiated, proliferating cells to E2F-5 in terminally differentiated keratinocytes. Inhibition of keratinocyte proliferation by transforming growth factor-beta1 did not enhance E2F-5 protein levels, suggesting that this response is specific to differentiation rather than reversible cell cycle withdrawal. E2F-5 up-regulation is also accompanied by formation of heteromeric nuclear complexes containing E2F5, p130, and histone deacetylase (HDAC) 1. Overexpression of E2F5 specifically inhibited DNA synthesis in undifferentiated keratinocytes in an HDAC-dependent manner, suggesting that E2F-5.p130.HDAC1 complexes are likely involved in the permanent withdrawal from the cell cycle of keratinocytes responding to differentiation stimuli.

Regulation of herpes simplex virus gene expression

Expression of the more than 80 individual genes of herpes simplex virus 1 (HSV-1) takes place in a tightly regulated sequential manner that was first described over 20 years ago. Investigations since that time have focused on understanding the mechanisms that regulate this orderly and efficient expression of viral genes. This review examines recent findings that have shed light on how this process is regulated during productive infection of the cell. Although the story is still not complete, several aspects of HSV gene expression are now clearer as a result of these findings. In particular, several new functions have recently been ascribed to some of the known viral regulatory proteins. The results indicate that the viral gene expression is regulated through transcriptional as well as post-transcriptional mechanisms. In addition, it has become increasingly clear that the virus has evolved specific functions to interact with the host cell in order to divert and redirect critical host functions for its own needs. Understanding the interactions of HSV and the host cell during infection will be essential for a complete understanding of how viral gene expression is regulated. Future challenges in the field will be to develop a complete understanding of the mechanisms that temporally regulate virus gene expression, and to identify and characterize the relevant interactions between the virus and the distinctive cell types normally infected by the virus.

Herpes simplex virus 1 alpha regulatory protein ICP0 functionally interacts with cellular transcription factor BMAL1

The infected cell protein no. 0 (ICP0) of herpes simplex virus 1 (HSV-1) is a promiscuous transactivator shown to enhance the expression of gene introduced into cells by infection or transfection. At the molecular level, ICP0 is a 775-aa ring finger protein localized initially in the nucleus and late in infection in the cytoplasm and mediates the degradation of several proteins and stabilization of others. None of the known functions at the molecular level account for the apparent activity of ICP0 as a transactivator. Here we report that ICP0 functionally interacts with cellular transcription factor BMAL1, a member of the basic helix-loop-helix PER-ARNT-SIM (PAS) super family of transcriptional regulators. Specifically, sequences mapped to the exon II of ICP0 interacted with BMAL1 in the yeast two-hybrid system and in reciprocal pull-down experiments in vitro. Moreover, the enhancement of transcription of a luciferase reporter construct whose promoter contained multiple BMAL1-binding sites by ICP0 and BMAL1 was significantly greater than that observed by ICP0 or BMAL1 alone. Although the level of BMAL1 present in nuclei of infected cells remained unchanged between 3 and 8 h after infection, the level of cytoplasmic BMAL1 was reduced at 8 h after infection. The reduction of cytoplasmic BMAL1 was significantly greater in cells infected with the ICP0-null mutant than in the wild-type virus-infected cells, suggesting that ICP0 mediates partial stabilization of the protein. These results indicate that ICP0 interacts physically and functionally with at least one cellular transcription-regulatory factor.

Herpes simplex virus 1 alpha regulatory protein ICP0 functionally interacts with cellular transcription factor BMAL1

The infected cell protein no. 0 (ICP0) of herpes simplex virus 1 (HSV-1) is a promiscuous transactivator shown to enhance the expression of gene introduced into cells by infection or transfection. At the molecular level, ICP0 is a 775-aa ring finger protein localized initially in the nucleus and late in infection in the cytoplasm and mediates the degradation of several proteins and stabilization of others. None of the known functions at the molecular level account for the apparent activity of ICP0 as a transactivator. Here we report that ICP0 functionally interacts with cellular transcription factor BMAL1, a member of the basic helix-loop-helix PER-ARNT-SIM (PAS) super family of transcriptional regulators. Specifically, sequences mapped to the exon II of ICP0 interacted with BMAL1 in the yeast two-hybrid system and in reciprocal pull-down experiments in vitro. Moreover, the enhancement of transcription of a luciferase reporter construct whose promoter contained multiple BMAL1-binding sites by ICP0 and BMAL1 was significantly greater than that observed by ICP0 or BMAL1 alone. Although the level of BMAL1 present in nuclei of infected cells remained unchanged between 3 and 8 h after infection, the level of cytoplasmic BMAL1 was reduced at 8 h after infection. The reduction of cytoplasmic BMAL1 was significantly greater in cells infected with the ICP0-null mutant than in the wild-type virus-infected cells, suggesting that ICP0 mediates partial stabilization of the protein. These results indicate that ICP0 interacts physically and functionally with at least one cellular transcription-regulatory factor.

The role of cdc2 in the expression of herpes simplex virus genes

Earlier reports have shown that cdc2 kinase is activated in cells infected with herpes simplex virus 1 and that the activation is mediated principally by two viral proteins, the infected cell protein 22 (ICP22) and the protein kinase encoded by U(L)13. The same proteins are required for optimal expression of a subset of late (gamma(2)) genes exemplified by U(S)11. In this study, we used a dominant-negative cdc2 protein to determine the role of cdc2 in viral gene expression. We report the following. (i) The cdc2 dominant-negative protein had no effect in the synthesis and accumulation of at least two alpha-regulatory proteins (ICP4 and ICP0), two beta-proteins (ribonucleotide reductase major subunit and single-stranded DNA-binding protein), and two gamma(1)-proteins (glycoprotein D and viral protease). U(S)11, a gamma(2)-protein, accumulated only in cells in which cdc2 dominant-negative protein could not be detected or was made in very small amounts. (ii) The sequence of amino acids predicted to be phosphorylated by cdc2 is present in at least 27 viral proteins inclusive of the regulatory proteins ICP4, ICP0, and ICP22. In in vitro assays, we demonstrated that cdc2 specifically phosphorylated a polypeptide consisting of the second exon of ICP0 but not a polypeptide containing the sequence of the third exon as would be predicted from the sequence analysis. We conclude that cdc2 is required for optimal expression of a subset of gamma(2)-proteins whose expression is also regulated by the viral proteins (ICP22 and U(L)13) that mediate the activation of cdc2 kinase.