Large T antigens of polyomaviruses: amazing molecular machines

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.

Removal of a small C-terminal region of JCV and SV40 large T antigens has differential effects on transformation

The large T antigen (LT) protein of JCV and SV40 polyomaviruses is required to induce tumors in rodents and transform cells in culture. When both LTs are compared side-by-side in cell culture assays, SV40 shows a more robust transformation phenotype even though the LT sequences are highly conserved. A complete understanding of SV40׳s enhanced transforming capabilities relative to JCV is lacking. When the least conserved region of the LT proteins, the variable linker and host range region (VHR), was removed, changes in T antigen expression and cellular p53 post-translational modifications occurred, but interaction with the pRB pathway was unaffected. Transformation assessed by growth in low serum was reduced after VHR truncation of the SV40, but not the JCV, T antigen. Conversely, anchorage independent transformation was enhanced only by truncation of the JCV VHR. This is the first report to link the SV40 or JCV VHR region to transformation potential.

The human fetal glial cell line SVG p12 contains infectious BK polyomavirus

The human fetal glial cell line SVG was generated in 1985 by transfecting primary fetal brain cells with a plasmid containing an origin-defective mutant of simian virus 40 (SV40). The cells, which express SV40 large T-antigen, support the replication of human JC polyomavirus (JCPyV) and have been used for JCPyV studies but also for other studies in which cells of neural origin were desirable. We intended to use the SVG p12 cells from ATCC for antiviral drug studies with JCPyV. However, during initial experiments, immunofluorescence microscopy controls unexpectedly revealed cells expressing the late viral proteins VP1, VP2/VP3, and agno. This was confirmed by Western blotting. Since our agnoprotein antiserum is specific for BKPyV agnoprotein, infection with BKPyV was suspected. Indeed, specific BKPyV PCR of SVG p12 supernatants revealed a viral load of >1 × 10(10) genomic equivalents/ml. Negative-staining electron microscopy showed characteristic polyomavirus virions, and infectious BKPyV was transmitted from SVG p12 supernatant to other cells. Long-range PCR covering the viral genome, followed by DNA sequencing, identified BKPyV strain UT as well as deletion derivatives. This was confirmed by next-generation sequencing. JCPyV (MAD-4) was found to infect apparently uninfected and BKPyV-infected SVG p12 cells. In total, 4 vials from 2 different ATCC lots of SVG p12 cells dating back to 2006 contained BKPyV, whereas the subclone SVG-A was negative. In conclusion, SVG p12 cells from ATCC contain infectious BKPyV. This may have affected results and interpretations of previous studies, and caution should be taken in future experiments.

IMPORTANCE

This work reveals that one of the most frequently used cell lines for JC polyomavirus (JCPyV) research, the SV40-immortalized human fetal glial cell line SVG p12 obtained directly from ATCC, contains infectious BK polyomavirus (BKPyV) of strain UT and a spectrum of defective mutants. Strain UT has been previously found in urine and in tumors of different patients but is also frequently used for research. It is therefore not clear if BKPyV was present in the brain tissue used to generate the cell line or if this is a contamination. Although productive JCPyV infection of SVG cells was not dependent on prior BKPyV infection, the unnoticed presence of BKPyV may have influenced the results of studies using these cells. The interpretation of past results should therefore be reconsidered and cells tested for BKPyV before new studies are initiated. The frequently used subclone SVG-A did not contain BKPyV and could be a useful substitute.

Insights into the initiation of JC virus DNA replication derived from the crystal structure of the T-antigen origin binding domain

JC virus is a member of the Polyomavirus family of DNA tumor viruses and the causative agent of progressive multifocal leukoencephalopathy (PML). PML is a disease that occurs primarily in people who are immunocompromised and is usually fatal. As with other Polyomavirus family members, the replication of JC virus (JCV) DNA is dependent upon the virally encoded protein T-antigen. To further our understanding of JCV replication, we have determined the crystal structure of the origin-binding domain (OBD) of JCV T-antigen. This structure provides the first molecular understanding of JCV T-ag replication functions; for example, it suggests how the JCV T-ag OBD site-specifically binds to the major groove of GAGGC sequences in the origin. Furthermore, these studies suggest how the JCV OBDs interact during subsequent oligomerization events. We also report that the OBD contains a novel "pocket"; which sequesters the A1 & B2 loops of neighboring molecules. Mutagenesis of a residue in the pocket associated with the JCV T-ag OBD interfered with viral replication. Finally, we report that relative to the SV40 OBD, the surface of the JCV OBD contains one hemisphere that is highly conserved and one that is highly variable.

Micrometastasis expressing insulin arise in lung and spleen at advanced stage of rip1-tag2 transgenic mice

Rip1-Tag2 mice is one overt pancreatic β-cell tumor model, which is widely used for studying pancreas tumor angiogenesis and tumor development. However, tumor metastasis in Rip1-Tag2 mice had rarely been reported, in this present study, we find some micrometastasis in lung and spleen of the Rip1-Tag2 mice at advanced stage, which is important for uncovering metastasis cell characteristics and exploring how to survive in cancer microenvironment. To study the micrometastasis of Rip1-Tag2 mice in advanced pancreatic cancer, we first observed the pathology process of β cell tumor in Rip1-Tag2 mice through HE staining, then we performed immunohistochemistry with insulin antibody, T-antigen antibodies and C-petide antibody on lung and spleen tissues sections from advanced stage, comparing with background wild-type C57BL/6 mice sections. The results indicated that micrometastasis expressing insulin was found in the Rip1-Tag2 mice lung, and spleen. Further evidences demonstrate pathology structure of lung and spleen are damaged. Interestingly and importantly, the expression of T antigen and insulin antibodies are all decreased in advanced stage of primary β cell tumor, which suggest that the at least partly micrometastasis is derived from the early stage or from advanced stage of β cell tumor then return to undifferentiated state like cancer stem cell. The findings contributed to the study of cancer metastasis and cancer stem cell.

Human polyoma JC virus minor capsid proteins, VP2 and VP3, enhance large T antigen binding to the origin of viral DNA replication: evidence for their involvement in regulation of the viral DNA replication

JC virus (JCV) lytically infects the oligodendrocytes in the central nervous system in a subset of immunocompromized patients and causes the demyelinating disease, progressive multifocal leukoencephalopathy. JCV replicates and assembles into infectious virions in the nucleus. However, understanding the molecular mechanisms of its virion biogenesis remains elusive. In this report, we have attempted to shed more light on this process by investigating molecular interactions between large T antigen (LT-Ag), Hsp70 and minor capsid proteins, VP2/VP3. We demonstrated that Hsp70 interacts with VP2/VP3 and LT-Ag; and accumulates heavily in the nucleus of the infected cells. We also showed that VP2/VP3 associates with LT-Ag through their DNA binding domains resulting in enhancement in LT-Ag DNA binding to Ori and induction in viral DNA replication. Altogether, our results suggest that VP2/VP3 and Hsp70 actively participate in JCV DNA replication and may play critical roles in coupling of viral DNA replication to virion encapsidation.

Antiviral effects of artesunate on polyomavirus BK replication in primary human kidney cells

Polyomavirus BK (BKV) causes polyomavirus-associated nephropathy (PyVAN) and hemorrhagic cystitis (PyVHC) in renal and bone marrow transplant patients, respectively. Antiviral drugs with targeted activity against BKV are lacking. Since the antimalarial drug artesunate was recently demonstrated to have antiviral activity, the possible effects of artesunate on BKV replication in human primary renal proximal tubular epithelial cells (RPTECs), the host cells in PyVAN, were explored. At 2 h postinfection (hpi), RPTECs were treated with artesunate at concentrations ranging from 0.3 to 80 μM. After one viral replication cycle (approximately 72 hpi), the loads of extracellular BKV DNA, reflecting viral progeny production, were reduced in a concentration-dependent manner. Artesunate at 10 μM reduced the extracellular BKV load by 65%; early large T antigen mRNA and protein expression by 30% and 75%, respectively; DNA replication by 73%; and late VP1 mRNA and protein expression by 47% and 64%, respectively. Importantly, the proliferation of RPTECs was also inhibited in a concentration-dependent manner. At 72 hpi, artesunate at 10 μM reduced cellular DNA replication by 68% and total metabolic activity by 47%. Cell impedance and lactate dehydrogenase measurements indicated a cytostatic but not a cytotoxic mechanism. Flow cytometry and 5-ethynyl-2'-deoxyuridine incorporation revealed a decreased number of cells in S phase and suggested cell cycle arrest in G0 or G2 phase. Both the antiproliferative and antiviral effects of artesunate at 10 μM were reversible. Thus, artesunate inhibits BKV replication in RPTECs in a concentration-dependent manner by inhibiting BKV gene expression and genome replication. The antiviral mechanism appears to be closely connected to cytostatic effects on the host cell, underscoring the dependence of BKV on host cell proliferative functions.

Merkel cell polyomavirus large T antigen disrupts host genomic integrity and inhibits cellular proliferation

Clonal integration of Merkel cell polyomavirus (MCV) DNA into the host genome has been observed in at least 80% of Merkel cell carcinoma (MCC). The integrated viral genome typically carries mutations that truncate the C-terminal DNA binding and helicase domains of the MCV large T antigen (LT), suggesting a selective pressure to remove this MCV LT region during tumor development. In this study, we show that MCV infection leads to the activation of host DNA damage responses (DDR). This activity was mapped to the C-terminal helicase-containing region of the MCV LT. The MCV LT-activated DNA damage kinases, in turn, led to enhanced p53 phosphorylation, upregulation of p53 downstream target genes, and cell cycle arrest. Compared to the N-terminal MCV LT fragment that is usually preserved in mutants isolated from MCC tumors, full-length MCV LT shows a decreased potential to support cellular proliferation, focus formation, and anchorage-independent cell growth. These apparently antitumorigenic effects can be reversed by a dominant-negative p53 inhibitor. Our results demonstrate that MCV LT-induced DDR activates p53 pathway, leading to the inhibition of cellular proliferation. This study reveals a key difference between MCV LT and simian vacuolating virus 40 LT, which activates a DDR but inhibits p53 function. This study also explains, in part, why truncation mutations that remove the MCV LT C-terminal region are necessary for the oncogenic progression of MCV-associated cancers.

ATM and ATR activities maintain replication fork integrity during SV40 chromatin replication

Mutation of DNA damage checkpoint signaling kinases ataxia telangiectasia-mutated (ATM) or ATM- and Rad3-related (ATR) results in genomic instability disorders. However, it is not well understood how the instability observed in these syndromes relates to DNA replication/repair defects and failed checkpoint control of cell cycling. As a simple model to address this question, we have studied SV40 chromatin replication in infected cells in the presence of inhibitors of ATM and ATR activities. Two-dimensional gel electrophoresis and southern blotting of SV40 chromatin replication products reveal that ATM activity prevents accumulation of unidirectional replication products, implying that ATM promotes repair of replication-associated double strand breaks. ATR activity alleviates breakage of a functional fork as it converges with a stalled fork. The results suggest that during SV40 chromatin replication, endogenous replication stress activates ATM and ATR signaling, orchestrating the assembly of genome maintenance machinery on viral replication intermediates.

All cells have evolved pathways to maintain the integrity of the genetic information stored in their chromosomes. Endogenous and exogenous agents induce mutations and other damage in DNA, most frequently during DNA replication. Such DNA damage is under surveillance by a complex network of proteins that interact with one another to signal damage, arrest DNA replication, and restore genomic integrity before replication resumes. Many viruses that replicate in the nucleus of mammalian host cells have evolved to disable or evade this surveillance system, but others, e.g. polyomaviruses like SV40, activate it and somehow harness it to facilitate robust replication of viral progeny. We have sought to determine how SV40 induces and deploys host DNA damage signaling in infected cells to promote viral chromosome replication. Here we present evidence that, like host DNA, replicating viral DNA suffers damage that activates surveillance and repair pathways. Unlike host replication, viral DNA replication persists despite damage signaling, allowing defective replication products to accumulate. In the presence of host DNA damage signaling, these defective viral products attract proteins of the host damage surveillance network that correct the defects, thus maximizing viral propagation.