Stability and function of JC virus large T antigen and T' proteins are altered by mutation of their phosphorylated threonine 125 residues

Phosphorylation of Human Polyomavirus Large and Small T Antigens: An Ignored Research Field

Protein phosphorylation and dephosphorylation are the most common post-translational modifications mediated by protein kinases and protein phosphatases, respectively. These reversible processes can modulate the function of the target protein, such as its activity, subcellular localization, stability, and interaction with other proteins. Phosphorylation of viral proteins plays an important role in the life cycle of a virus. In this review, we highlight biological implications of the phosphorylation of the monkey polyomavirus SV40 large T and small t antigens, summarize our current knowledge of the phosphorylation of these proteins of human polyomaviruses, and conclude with gaps in the knowledge and a proposal for future research directions.

Traces of JC polyomavirus in papillary thyroid cancer: a comprehensive study in Iran

Background

JC polyomavirus (JCPyV) is known to induce solid tumors such as astrocytomas, glioblastomas, and neuroblastomas in experimental animals, and recent studies have shown that the virus may be correlated with carcinogenesis. This study aimed to evaluate the impact of JCPyV on the progression of papillary thyroid cancer (PTC).

Methods

A total of 1057 samples, including 645 paraffin-embedded PTC biopsy samples (PEBS) and 412 fresh biopsy samples (FBS), and 1057 adjacent non-cancerous samples were evaluated for the presence of JCPyV DNA and RNA.

Results

We observed that 10.8% (114/1057) samples, including 17.5% (72/412) FBS and 6.5% (42/645) PEBS were positive for the JCPyV DNA. Among the JCPyV-positive samples, the mean JCPyV copy number was lower in patients with PEBS (0.3 × 10^–4 ± 0.1 × 10^–4 copies/cell) compared to FBS (1.8 × 10^–1 ± 0.4 × 10^–1 copies/cell) and non-PTC normal samples (0.2 × 10^–5 ± 0.01 × 10^–5 copies/cell), with a statistically significant difference (P < 0.001). The LT-Ag RNA expression was lower in PEBS than in FBS, while no VP1 gene transcript expression was found.

Conclusions

Although our results confirmed the presence of JCPyV in some Iranian patients with PTC, more research is needed to verify these results.

The oncogenic roles of JC polyomavirus in cancer

JC polyomavirus (JCPyV) belongs to the human polyomavirus family. Based on alternative splicing, the early region encodes the large and small T antigens, while the late region encodes the capsid structural proteins (VP1, VP2, and VP3) and the agnoprotein. The regulatory transcription factors for JCPyV include Sp1, TCF-4, DDX1, YB-1, LCP-1, Purα, GF-1, and NF-1. JCPyV enters tonsillar tissue through the intake of raw sewage, inhalation of air droplets, or parent-to-child transmission. It persists quiescently in lymphoid and renal tissues during latency. Both TGF-β1 and TNF-α stimulates JCPyV multiplication, while interferon-γ suppresses the process. The distinct distribution of caspid receptors (α-2, 6-linked sialic acid, non-sialylated glycosaminoglycans, and serotonin) determines the infection capabilities of JCPyV virions, and JCPyV entry is mediated by clathrin-mediated endocytosis. In permissive cells, JCPyV undergoes lytic proliferation and causes progressive multifocal leukoencephalopathy, while its DNA is inserted into genomic DNA and leads to carcinogenesis in non-permissive cells. T antigen targets p53, β-catenin, IRS, Rb, TGF-β1, PI3K/Akt and AMPK signal pathways in cancer cells. Intracranial injection of T antigen into animals results in neural tumors, and transgenic mice develop neural tumors, lens tumor, breast cancer, gastric, Vater’s, colorectal and pancreatic cancers, insulinoma, and hepatocellular carcinoma. Additionally, JCPyV DNA and its encoded products can be detected in the brain tissues of PML patients and brain, oral, esophageal, gastric, colorectal, breast, cervical, pancreatic, and hepatocellular cancer tissues. Therefore, JCPyV might represent an etiological risk factor for carcinogenesis and should be evaluated for early prevention, diagnosis, and treatment of cancers.

The Oncogenic Effects, Pathways, and Target Molecules of JC Polyoma Virus T Antigen in Cancer Cells

JC polyoma virus (JCPyV) is a ubiquitous polyoma virus that infects the individual to cause progressive multifocal leukoencephalopathy and malignancies. Here, we found that T-antigen knockdown suppressed proliferation, glycolysis, mitochondrial respiration, migration, and invasion, and induced apoptosis and G₂ arrest. The reverse was true for T-antigen overexpression, with overexpression of Akt, survivin, retinoblastoma protein, β-catenin, β-transducin repeat-containing protein (TRCP), and inhibitor of growth (ING)1, and the underexpression of mammalian target of rapamycin (mTOR), phosphorylated (p)-mTOR, p-p38, Cyclin D1, p21, vascular endothelial growth factor (VEGF), ING2, and ING4 in hepatocellular and pancreatic cancer cells and tissues. In lens tumor cells, T antigen transcriptionally targeted viral carcinogenesis, microRNAs in cancer, focal adhesion, p53, VEGF, phosphoinositide 3 kinase-Akt, and Forkhead box O signaling pathways, fructose and mannose metabolism, ribosome biosynthesis, and choline and pyrimidine metabolism. At a metabolomics level, it targeted protein digestion and absorption, aminoacryl-tRNA biosynthesis, biosynthesis of amino acids, and the AMPK signal pathway. At a proteomic level, it targeted ribosome biogenesis in eukaryotes, citrate cycle, carbon metabolism, protein digestion and absorption, aminoacryl-tRNA biosynthesis, extracellular-matrix-receptor interaction, and biosynthesis of amino acids. In lens tumor cells, T antigen might interact with various keratins, ribosomal proteins, apolipoproteins, G proteins, ubiquitin-related proteins, RPL19, β-catenin, β-TRCP, p53, and CCAAT-enhancer-binding proteins in lens tumor cells. T antigen induced a more aggressive phenotype in mouse and human cancer cells due to oncogene activation, inactivation of tumor suppressors, and disruption of metabolism, cell adhesion, and long noncoding RNA-microRNA-target axes.

Human polyomaviruses and cancer: an overview

The name of the family Polyomaviridae, derives from the early observation that cells infected with murine polyomavirus induced multiple (poly) tumors (omas) in immunocompromised mice. Subsequent studies showed that many members of this family exhibit the capacity of mediating cell transformation and tumorigenesis in different experimental models. The transformation process mediated by these viruses is driven by viral pleiotropic regulatory proteins called T (tumor) antigens. Similar to other viral oncoproteins T antigens target cellular regulatory factors to favor cell proliferation, immune evasion and downregulation of apoptosis. The first two human polyomaviruses were isolated over 45 years ago. However, recent advances in the DNA sequencing technologies led to the rapid identification of additional twelve new polyomaviruses in different human samples. Many of these viruses establish chronic infections and have been associated with conditions in immunosuppressed individuals, particularly in organ transplant recipients. This has been associated to viral reactivation due to the immunosuppressant therapy applied to these patients. Four polyomaviruses namely, Merkel cell polyomavirus (MCPyV), Trichodysplasia spinulosa polyomavirus (TSPyV), John Cunningham Polyomavirus (JCPyV) and BK polyomavirus (BKPyV) have been associated with the development of specific malignant tumors. However, present evidence only supports the role of MCPyV as a carcinogen to humans. In the present review we present a summarized discussion on the current knowledge concerning the role of MCPyV, TSPyV, JCPyV and BKPyV in human cancers.

Impact of immune suppressive agents on the BK-Polyomavirus non coding control region

BACKGROUND

Reactivation of the BK-Polyomavirus (BKPyV) can cause a polyomavirus associated nephropathy in approx. 10% of kidney transplant recipients. In these cases, current therapy is based on the reduction of immunosuppression. Since BKPyV-transcription is driven by the Non-Coding-Control-Region (NCCR) we were interested whether NCCR-activity is affected by immunosuppressive agents.

METHODS

Plasma samples from 45 BKPyV-positive patients after renal transplantation were subjected to PCR-analysis. NCCR-amplicons were cloned into a plasmid that allows the quantification of early and late NCCR-activity by tdTomato and eGFP expression, respectively. HEK293T-cells were transfected with the reporter-plasmids, treated with immunosuppressive agents, and subjected to FACS-analysis. In addition, H727-cells were infected with patient derived BKPyV, treated with mTOR-inhibitors, and NCCR activity was analysed using qRT-PCR.

RESULTS

While tacrolimus and cyclosporine-A did not affect NCCR-promoter-activity, treatment with mTOR1-inhibitor rapamycin resulted in the reduction of early, but not late-NCCR-promoter-activity. Treatment with dual mTOR1/2 inhibitors (INK128 or pp242) led to significant inhibition of early, however, concomitantly enhanced late-promoter-activity. In BKPyV infected cells both rapamycin and INK128 reduced early expression, however, INK128 resulted in higher late-mRNA levels when compared to rapamycin treatment.

CONCLUSIONS

Our results demonstrate that mTOR1-inhibitors are able to reduce early-expression of wildtype and rearranged NCCRs, which might contribute to previously described inhibition of BKPyV-replication. Dual mTOR1/2-inhibitors, however, additionally might shift viral early into late-expression promoting synthesis of viral structural proteins and particle production.

T cell therapies for human polyomavirus diseases

Rapid restoration of virus-specific T immunity via adoptive transfer of ex vivo generated T cells has been proven as a powerful therapy for patients with advanced cancers and refractory viral infections such as cytomegalovirus (CMV) and Epstein-Barr virus (EBV). BK virus (BKV), John Cunningham virus (JCV), and Merkel cell carcinoma virus (MCV) are the members of the rapidly growing human polyomavirus (hPyV) family that commonly infects most healthy humans. These viruses have a clearly established potential for causing severe end-organ damage or malignant transformation, especially in individuals with weakened immunity who are unable to mount or regain endogenous T-cell responses as a result of underlying leukemia or iatrogenic immunosuppression in autoimmunity, bone marrow and solid organ transplant settings. Here we will discuss recent advances in using T-cell-based immunotherapies to save patients suffering from PyV-associated diseases including hemorrhagic cystitis, BKV virus-associated nephropathy, and JC-associated progressive multifocal leukoencephalopathy (PML). We will also review progress in the understanding of Merkel cell carcinoma (MCC) as a virally driven tumor that is amenable to immune intervention and can be targeted with adoptively transferred T cells specific for viral oncoproteins.

The biology of JC polyomavirus

JC polyomavirus (JCPyV) is the causative agent of a fatal central nervous system demyelinating disease known as progressive multifocal leukoencephalopathy (PML). PML occurs in people with underlying immunodeficiency or in individuals being treated with potent immunomodulatory therapies. JCPyV is a DNA tumor virus with a double-stranded DNA genome and encodes a well-studied oncogene, large T antigen. Its host range is highly restricted to humans and only a few cell types support lytic infection in vivo or in vitro. Its oncogenic potential in humans has not been firmly established and the international committee on oncogenic viruses lists JCPyV as possibly carcinogenic. Significant progress has been made in understanding the biology of JCPyV and here we present an overview of the field and discuss some important questions that remain unanswered.

Isolation of a monoclonal antibody that recognizes the origin binding domain of JCV, but not SV40, large T-antigen

Within immunocompromised populations, the JC polyomavirus is the cause of the often-fatal disease Progressive Multifocal Leukoencephalopathy (PML). JC virus encodes a protein, termed T-antigen (T-ag), which is essential for its replication and pathogenicity. Previous studies of JCV T-ag have, in general, used antibodies raised against SV40 T-ag. Unfortunately, SV40 T-ag is also detected in humans and therefore there have been concerns about cross-reactivity. To address this issue, we have isolated a monoclonal antibody that binds to the JCV, but not the SV40, T-ag origin-binding domain (OBD). Furthermore, the region on the surface of the JCV T-ag OBD that is recognized by the "anti-JCV OBD mAb" has been mapped. We also demonstrate that the "anti-JCV OBD mAb" will be a useful reagent for standard techniques (e.g., Westerns blots and ELISAs). Finally, we note that additional monoclonal Abs that are specific for the T-ags encoded by the other human polyomaviruses could be generated by adopting the approach described herein.

A Comprehensive Analysis of Replicating Merkel Cell Polyomavirus Genomes Delineates the Viral Transcription Program and Suggests a Role for mcv-miR-M1 in Episomal Persistence

Merkel cell polyomavirus (MCPyV) is considered the etiological agent of Merkel cell carcinoma and persists asymptomatically in the majority of its healthy hosts. Largely due to the lack of appropriate model systems, the mechanisms of viral replication and MCPyV persistence remain poorly understood. Using a semi-permissive replication system, we here report a comprehensive analysis of the role of the MCPyV-encoded microRNA (miRNA) mcv-miR-M1 during short and long-term replication of authentic MCPyV episomes. We demonstrate that cells harboring intact episomes express high levels of the viral miRNA, and that expression of mcv-miR-M1 limits DNA replication. Furthermore, we present RACE, RNA-seq and ChIP-seq studies which allow insight in the viral transcription program and mechanisms of miRNA expression. While our data suggest that mcv-miR-M1 can be expressed from canonical late strand transcripts, we also present evidence for the existence of an independent miRNA promoter that is embedded within early strand coding sequences. We also report that MCPyV genomes can establish episomal persistence in a small number of cells for several months, a time period during which viral DNA as well as LT-Ag and viral miRNA expression can be detected via western blotting, FISH, qPCR and southern blot analyses. Strikingly, despite enhanced replication in short term DNA replication assays, a mutant unable to express the viral miRNA was severely limited in its ability to establish long-term persistence. Our data suggest that MCPyV may have evolved strategies to enter a non- or low level vegetative stage of infection which could aid the virus in establishing and maintaining a lifelong persistence.

Activation of c-Myc and Cyclin D1 by JCV T-Antigen and β-catenin in colon cancer

During the last decade, mounting evidence has implicated the human neurotropic virus JC virus in the pathology of colon cancer. However, the mechanisms of JC virus-mediated oncogenesis are still not fully determined. One candidate to mediate these effects is the viral early transcriptional product T-Antigen, which has the ability to inactivate cell cycle regulatory proteins such as p53. In medulloblastomas, T-Antigen has been shown to bind the Wnt signaling pathway protein β-catenin; however, the effects of this interaction on downstream cell cycle regulatory proteins remain unknown. In light of these observations, we investigated the association of T-Antigen and nuclear β-catenin in colon cancer cases and the effects of this complex in the activation of the transcription and cell cycle regulators c-Myc and Cyclin D1 in vitro. Gene amplification demonstrated the presence of viral sequences in 82.4% of cases and we detected expression of T-Antigen in 64.6% of cases by immunohistochemistry. Further, we found that T-Antigen and β-catenin co-localized in the nuclei of tumor cells and we confirmed the physical binding between these two proteins in vitro. The nuclear presence of T-Antigen and β-catenin resulted in the significant enhancement of TCF-dependent promoter activity and activation of the β-catenin downstream targets, c-Myc and Cyclin D1. These observations provide further evidence for a role of JCV T-Antigen in the dysregulation of the Wnt signaling pathway and in the pathogenesis of colon cancer.

Analysis of JC virus DNA replication using a quantitative and high-throughput assay

Progressive Multifocal Leukoencephalopathy (PML) is caused by lytic replication of JC virus (JCV) in specific cells of the central nervous system. Like other polyomaviruses, JCV encodes a large T-antigen helicase needed for replication of the viral DNA. Here, we report the development of a luciferase-based, quantitative and high-throughput assay of JCV DNA replication in C33A cells, which, unlike the glial cell lines Hs 683 and U87, accumulate high levels of nuclear T-ag needed for robust replication. Using this assay, we investigated the requirement for different domains of T-ag, and for specific sequences within and flanking the viral origin, in JCV DNA replication. Beyond providing validation of the assay, these studies revealed an important stimulatory role of the transcription factor NF1 in JCV DNA replication. Finally, we show that the assay can be used for inhibitor testing, highlighting its value for the identification of antiviral drugs targeting JCV DNA replication.

Role of infectious agents in the carcinogenesis of brain and head and neck cancers

This review concentrates on tumours that are anatomically localised in head and neck regions. Brain cancers and head and neck cancers together account for more than 873,000 cases annually worldwide, with an increasing incidence each year. With poor survival rates at late stages, brain and head and neck cancers represent serious conditions. Carcinogenesis is a multi-step process and the role of infectious agents in this progression has not been fully identified. A major problem with such research is that the role of many infectious agents may be underestimated due to the lack of or inconsistency in experimental data obtained globally. In the case of brain cancer, no infection has been accepted as directly oncogenic, although a number of viruses and parasites are associated with the malignancy. Our analysis of the literature showed the presence of human cytomegalovirus (HCMV) in distinct types of brain tumour, namely glioblastoma multiforme (GBM) and medulloblastoma. In particular, there are reports of viral protein in up to 100% of GBM specimens. Several epidemiological studies reported associations of brain cancer and toxoplasmosis seropositivity. In head and neck cancers, there is a distinct correlation between Epstein-Barr virus (EBV) and nasopharyngeal carcinoma (NPC). Considering that almost every undifferentiated NPC is EBV-positive, virus titer levels can be measured to screen high-risk populations. In addition there is an apparent association between human papilloma virus (HPV) and head and neck squamous cell carcinoma (HNSCC); specifically, 26% of HNSCCs are positive for HPV. HPV type 16 was the most common type detected in HNSCCs (90%) and its dominance is even greater than that reported in cervical carcinoma. Although there are many studies showing an association of infectious agents with cancer, with various levels of involvement and either a direct or indirect causative effect, there is a scarcity of articles covering the role of infection in carcinogenesis of brain and head and neck cancers. We review recent studies on the infectious origin of these cancers and present our current understanding of carcinogenic mechanisms, thereby providing possible novel approaches to cancer treatment.

JC virus small T antigen binds phosphatase PP2A and Rb family proteins and is required for efficient viral DNA replication activity

BACKGROUND

The human polyomavirus, JC virus (JCV) produces five tumor proteins encoded by transcripts alternatively spliced from one precursor messenger RNA. Significant attention has been given to replication and transforming activities of JCV's large tumor antigen (TAg) and three T' proteins, but little is known about small tumor antigen (tAg) functions. Amino-terminal sequences of tAg overlap with those of the other tumor proteins, but the carboxy half of tAg is unique. These latter sequences are the least conserved among the early coding regions of primate polyomaviruses.

METHODOLOGY AND FINDINGS

We investigated the ability of wild type and mutant forms of JCV tAg to interact with cellular proteins involved in regulating cell proliferation and survival. The JCV P99A tAg is mutated at a conserved proline, which in the SV40 tAg is required for efficient interaction with protein phosphatase 2A (PP2A), and the C157A mutant tAg is altered at one of two newly recognized LxCxE motifs. Relative to wild type and C157A tAgs, P99A tAg interacts inefficiently with PP2A in vivo. Unlike SV40 tAg, JCV tAg binds to the Rb family of tumor suppressor proteins. Viral DNAs expressing mutant t proteins replicated less efficiently than did the intact JCV genome. A JCV construct incapable of expressing tAg was replication-incompetent, a defect not complemented in trans using a tAg-expressing vector.

CONCLUSIONS

JCV tAg possesses unique properties among the polyomavirus small t proteins. It contributes significantly to viral DNA replication in vivo; a tAg null mutant failed to display detectable DNA replication activity, and a tAg substitution mutant, reduced in PP2A binding, was replication-defective. Our observation that JCV tAg binds Rb proteins, indicates all five JCV tumor proteins have the potential to influence cell cycle progression in infected and transformed cells. It remains unclear how these proteins coordinate their unique and overlapping functions.

Regulation of gene expression in primate polyomaviruses

Polyomaviruses are a growing family of small DNA viruses with a narrow tropism for both the host species and the cell type in which they productively replicate. Species host range may be constrained by requirements for precise molecular interactions between the viral T antigen, host replication proteins, including DNA polymerase, and the viral origin of replication, which are required for viral DNA replication. Cell type specificity involves, at least in part, transcription factors that are necessary for viral gene expression and restricted in their tissue distribution. In the case of the human polyomaviruses, BK virus (BKV) replication occurs in the tubular epithelial cells of the kidney, causing nephropathy in kidney allograft recipients, while JC virus (JCV) replication occurs in the glial cells of the central nervous system, where it causes progressive multifocal leukoencephalopathy. Three new human polyomaviruses have recently been discovered: MCV was found in Merkel cell carcinoma samples, while Karolinska Institute Virus and Washington University Virus were isolated from the respiratory tract. We discuss control mechanisms for gene expression in primate polyomaviruses, including simian vacuolating virus 40, BKV, and JCV. These mechanisms include not only modulation of promoter activities by transcription factor binding but also enhancer rearrangements, restriction of DNA methylation, alternate early mRNA splicing, cis-acting elements in the late mRNA leader sequence, and the production of viral microRNA.

Dephosphorylation of JC virus agnoprotein by protein phosphatase 2A: inhibition by small t antigen

Previous studies have demonstrated that the JC virus (JCV) late regulatory protein agnoprotein is phosphorylated by the serine/threonine-specific protein kinase-C (PKC) and mutants of this protein at the PKC phosphorylation sites exhibit defects in the viral replication cycle. We have now investigated whether agnoprotein phosphorylation is regulated by PP2A, a serine/threonine-specific protein phosphatase and whether JCV small t antigen (Sm t-Ag) is involved in this regulation. Protein-protein interaction studies demonstrated that PP2A associates with agnoprotein and dephosphorylates it at PKC-specific sites. Sm t-Ag was also found to interact with PP2A and this interaction inhibited the dephosphorylation of agnoprotein by PP2A. The interaction domains of Sm t-Ag and agnoprotein with PP2A were mapped, as were the interaction domains of Sm t-Ag with agnoprotein. The middle portion of Sm t-Ag (aa 82-124) was found to be critical for the interaction with both agnoprotein and PP2A and the N-terminal region of agnoprotein for interaction with Sm t-Ag. To further understand the role of Sm t-Ag in JCV regulation, a stop codon was introduced at Ser90 immediately after splice donor site of the JCV early gene and the functional consequences of this mutation were investigated. The ability of this mutant virus to replicate was substantially reduced compared to WT. Next, the functional significance of PP2A in JCV replication was examined by siRNA targeting. Downregulation of PP2A caused a significant reduction in the level of JCV replication. Moreover, the impact of Sm t-Ag on agnoprotein phosphorylation was investigated by creating a double mutant of JCV, where Sm t-Ag stop codon mutant was combined with an agnoprotein triple phosphorylation mutant (Ser7, Ser11 and Thr21 to Ala). Results showed that double mutant behaves much like the triple phosphorylation mutant of agnoprotein during viral replication cycle, which suggests that agnoprotein might be an important target of Sm t-Ag with respect to the regulation of its phosphorylation. Collectively, these results suggest that there is an interplay between agnoprotein, Sm t-Ag and PP2A with respect to the regulation of JCV life cycle and this could be important for the progression of the JCV-induced disease, PML.

Polyomavirus JC infects human brain microvascular endothelial cells independent of serotonin receptor 2A

Although human polyomavirus JC (JCV) is known to cause progressive multifocal leukoencephalopathy (PML) in immunocompromised individuals, the mechanism by which JCV crosses the blood-brain barrier (BBB) remains unclear. To test our hypothesis that cell-free JCV gains entry into the brain by infecting endothelial cells, we inoculated human brain microvascular endothelial (HBMVE) cells with 50 HAU (1.33+/-0.27 x 10(7) genome copies) of JCV(Mad1) and analyzed the expression of early and late viral genes and proteins by immunocytochemistry, quantitative real-time PCR (qPCR), quantitative real-time reverse transcriptase PCR (qRT-PCR) and immunoprecipitation followed by Western blotting. JCV infected and replicated efficiently in HBMVE cells and produced infectious virions several hundred fold higher than the infecting inoculum. HBMVE cells in vitro did not express serotonin receptor 2A (5HT(2A)R), and 5HT(2A)R blockers did not prevent JCV infection of HBMVE cells. Collectively, our data indicate that the productive in vitro infection of HBMVE cells by JCV is independent of 5HT(2A)R.

JC virus T'135, T'136 and T'165 proteins interact with cellular p107 and p130 in vivo and influence viral transformation potential

The JC virus (JCV) regulatory proteins, large T antigen, small t antigen, T'135, T'136, and T'165, are encoded by five transcripts alternatively spliced from the viral early precursor mRNA. T antigen and the T' proteins share N-terminal amino acid sequences that include the L x CxE and J domains, motifs in SV40 T antigen known to mediate binding to the retinoblastoma (Rb) proteins and Hsc70, respectively. In this study, G418-resistant cell lines were created that express wild-type or mutant JCV T antigen and T' proteins individually or in combination. These cell lines were used to evaluate the ability of each viral protein to bind p107 and p130 in vivo, and to influence cellular growth characteristics. Differences were observed in the abilities of individual T' proteins to bind p107 and p130 and to alter their phosphorylation status. The T' proteins were also found to localize to the cell's nucleus and to be phosphorylated in a cell cycle-dependent manner. JCV T antigen and T' proteins expressed from a cytomegalovirus promoter failed to induce dense focus formation in Rat2 cells, but they did cooperate with a mutant Ras protein to overcome cellular senescence and immortalize rat embryo fibroblasts. These data indicate that, despite their sequence similarities, JCV early proteins exhibit unique activities that, in combination, effect the inactivation of cell cycle regulators, a requirement for polyomavirus-induced transformation.

NFAT4 is required for JC virus infection of glial cells

The human polyomavirus JC virus (JCV) infects 70% of the population worldwide. In immunosuppressed patients, JCV infection can lead to progressive multifocal leukoencephalopathy (PML), a fatal demyelinating disease of the central nervous system (CNS). The majority of PML cases occur in the setting of human immunodeficiency virus (HIV) infection, and it has been suggested that the link between HIV and the development of PML is in part related to the production of numerous cytokines in the CNS during HIV infection. To examine the link between the expression of inflammatory cytokines and JCV infection, we tested an anti-inflammatory compound, cyclosporine A (CsA), for its ability to block JCV infection of glial cells. We found that CsA inhibited JCV infection by preventing the activation of the transcription factor nuclear factor of activated T cells 4 (NFAT4). Luciferase reporter assays and chromatin immunoprecipitation assays revealed that NFAT4 directly bound the JCV promoter during infection and was important for the activation of both early and late transcription. In addition, the expression of the JCV early viral gene products increased NFAT activity to further aid viral transcription. The necessity of NFAT for JCV infection suggests that calcium signaling and the activation of NFAT in glial cells are required for JCV infection of the CNS.