Phosphorylation of human polyomavirus BK agnoprotein at Ser-11 is mediated by PKC and has an important regulative function

Impact of BK Polyomavirus NCCR variations in post kidney transplant outcomes

The human BK Polyomavirus (BKPyV) is a DNA virus that is prevalent in 80 % of the population. Infection with this virus may begin in childhood, followed by asymptomatic persistence in the urinary tract. However, in immunocompromised individuals, especially kidney transplant recipients (KTRs), heightened replication of BKPyV can lead to severe complications. The genome of this virus is divided into three parts; the early and late region, and the non-coding control region (NCCR). Mutations in the NCCR can change the archetype strain to the rearranged strain, and NCCR rearrangements play a significant in virus pathogenesis. Interestingly, diverse types of NCCR block rearrangement result in significant differences in conversion potential and host cell viability in the infected cells. A correlation has been detected between increased viral replication potential and pathogenesis in BKPyV-infected KTRs with specific NCCR rearrangements. The objective of this review study was to examine the disease-causing and clinical consequences of variations in the NCCR in BKPyV-infected KTRs such as virus-associated nephropathy (BKPyVAN).

Evidence of the Mechanism by Which Polyomaviruses Exploit the Extracellular Vesicle Delivery System during Infection

Increasing evidence suggests that human viruses can hijack extracellular vesicles (EVs) to deliver proteins, mRNAs, microRNAs (miRNAs) and whole viral particles during viral persistence in the host. Human polyomavirus (PyV) miRNAs, which downregulate large T-antigen expression and target host factors, help the virus escape immune elimination and may have roles in the success of viral persistence/replication and the development of diseases. In this context, several investigations have detected PyV miRNAs in EVs obtained from cell culture supernatants after viral infection, demonstrating the ability of these vesicles to deliver miRNAs to uninfected cells, potentially counteracting new viral infection. Additionally, PyV miRNAs have been identified in EVs derived from the biological fluids of clinical samples obtained from patients with or at risk of severe PyV-associated diseases and from asymptomatic control healthy subjects. Interestingly, PyV miRNAs were found to be circulating in blood, urine, cerebrospinal fluid, and saliva samples from patients despite their PyV DNA status. Recently, the association between EVs and PyV viral particles was reported, demonstrating the ability of PyV viral particles to enter the cell without natural receptor-mediated entry and evade antibody-mediated neutralization or to be neutralized at a step different from that of the neutralization of naked whole viral particles. All these data point toward a potential role of the association between PyVs with EVs in viral persistence, suggesting that further work to define the implication of this interaction in viral reactivation is warranted.

BK polyomavirus diversity-Why viral variation matters

BK polyomavirus (BKPyV or BKV) is a non-enveloped, circular double-stranded DNA virus that may exceed 80% seroprevalence in adults. BKV infection typically occurs during childhood, and the majority of adults are latently infected. While BKV infection is rarely associated with clinical disease in most individuals, in immunosuppressed individuals, reactivation may cause kidney (BK-associated nephropathy) or bladder (hemorrhagic cystitis and ureteral stenosis) injury. No antiviral therapies have been approved for the treatment of BKV infection. Reducing immunosuppression is the most effective therapy, although this is not feasible in many patients. Thus, a robust understanding of viral pathogenesis and viral diversity remains important for the development of future therapeutic strategies. Studies of BKV diversity are quite sparse compared to other common viral infections; thus, much of our understanding of BVK variability and evolution relies heavily analogous studies of other viruses such as HIV or viral hepatitis. We provide a comprehensive review of BKV diversity at the population and individual level with careful consideration of how viral variability may impact viral replication, pathogenesis, tropism, and protein function. We also discuss a number of outstanding questions related to BK virus diversity that should be explored rigorously in future studies.

BK polyomavirus in solid organ transplantation-Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice

The present AST-IDCOP guidelines update information on BK polyomavirus (BKPyV) infection, replication, and disease, which impact kidney transplantation (KT), but rarely non-kidney solid organ transplantation (SOT). As pretransplant risk factors in KT donors and recipients presently do not translate into clinically validated measures regarding organ allocation, antiviral prophylaxis, or screening, all KT recipients should be screened for BKPyV-DNAemia monthly until month 9, and then every 3 months until 2 years posttransplant. Extended screening after 2 years may be considered in pediatric KT. Stepwise immunosuppression reduction is recommended for KT patients with plasma BKPyV-DNAemia of >1000 copies/mL sustained for 3 weeks or increasing to >10 000 copies/mL reflecting probable and presumptive BKPyV-associated nephropathy, respectively. Reducing immunosuppression is also the primary intervention for biopsy-proven BKPyV-associated nephropathy. Hence, allograft biopsy is not required for treating BKPyV-DNAemic patients with baseline renal function. Despite virological rationales, proper randomized clinical trials are lacking to generally recommend treatment by switching from tacrolimus to cyclosporine-A, from mycophenolate to mTOR inhibitors or leflunomide or by the adjunct use of intravenous immunoglobulins, leflunomide, or cidofovir. Fluoroquinolones are not recommended for prophylaxis or therapy. Retransplantation after allograft loss due to BKPyV nephropathy can be successful if BKPyV-DNAemia is definitively cleared, independent of failed allograft nephrectomy.

Expression of novel proteins by polyomaviruses and recent advances in the structural and functional features of agnoprotein of JC virus, BK virus, and simian virus 40

Polyomavirus family consists of a highly diverse group of small DNA viruses. The founding family member (MPyV) was first discovered in the newborn mouse in the late 1950s, which induces solid tumors in a wide variety of tissue types that are the epithelial and mesenchymal origin. Later, other family members were also isolated from a number of mammalian, avian and fish species. Some of these viruses significantly contributed to our current understanding of the fundamentals of modern biology such as transcription, replication, splicing, RNA editing, and cell transformation. After the discovery of first two human polyomaviruses (JC virus [JCV] and BK virus [BKV]) in the early 1970s, there has been a rapid expansion in the number of human polyomaviruses in recent years due to the availability of the new technologies and brought the present number to 14. Some of the human polyomaviruses cause considerably serious human diseases, including progressive multifocal leukoencephalopathy, polyomavirus-associated nephropathy, Merkel cell carcinoma, and trichodysplasia spinulosa. Emerging evidence suggests that the expression of the polyomavirus genome is more complex than previously thought. In addition to encoding universally expressed regulatory and structural proteins (LT-Ag, Sm t-Ag, VP1, VP2, and VP3), some polyomaviruses express additional virus-specific regulatory proteins and microRNAs. This review summarizes the recent advances in polyomavirus genome expression with respect to the new viral proteins and microRNAs other than the universally expressed ones. In addition, a special emphasis is devoted to the recent structural and functional discoveries in the field of polyomavirus agnoprotein which is expressed only by JCV, BKV, and simian virus 40 genomes.

Agnoprotein Is an Essential Egress Factor during BK Polyomavirus Infection

BK polyomavirus (BKPyV; hereafter referred to as BK) causes a lifelong chronic infection and is associated with debilitating disease in kidney transplant recipients. Despite its importance, aspects of the virus life cycle remain poorly understood. In addition to the structural proteins, the late region of the BK genome encodes for an auxiliary protein called agnoprotein. Studies on other polyomavirus agnoproteins have suggested that the protein may contribute to virion infectivity. Here, we demonstrate an essential role for agnoprotein in BK virus release. Viruses lacking agnoprotein fail to release from host cells and do not propagate to wild-type levels. Despite this, agnoprotein is not essential for virion infectivity or morphogenesis. Instead, agnoprotein expression correlates with nuclear egress of BK virions. We demonstrate that the agnoprotein binding partner α-soluble N-ethylmaleimide sensitive fusion (NSF) attachment protein (α-SNAP) is necessary for BK virion release, and siRNA knockdown of α-SNAP prevents nuclear release of wild-type BK virions. These data highlight a novel role for agnoprotein and begin to reveal the mechanism by which polyomaviruses leave an infected cell.

Biology of the BKPyV: An Update

The BK virus (BKPyV) is a member of the Polyomaviridae family first isolated in 1971. BKPyV causes frequent infections during childhood and establishes persistent infections with minimal clinical implications within renal tubular cells and the urothelium. However, reactivation of BKPyV in immunocompromised individuals may cause serious complications. In particular, with the implementation of more potent immunosuppressive drugs in the last decade, BKPyV has become an emerging pathogen in kidney and bone marrow transplant recipients where it often causes associated nephropathy and haemorrhagic cystitis, respectively. Unfortunately, no specific antiviral against BKPyV has been approved yet and the only therapeutic option is a modulation of the immunosuppressive drug regimen to improve immune control though it may increase the risk of rejection. A better understanding of the BKPyV life cycle is thus needed to develop efficient treatment against this virus. In this review, we provide an update on recent advances in understanding the biology of BKPyV.

Structure-based release analysis of the JC virus agnoprotein regions: A role for the hydrophilic surface of the major alpha helix domain in release

Agnoprotein (Agno) is an important regulatory protein of JC virus (JCV), BK virus (BKV) and simian virus 40 (SV40) and these viruses are unable to replicate efficiently in the absence of this protein. Recent 3D-NMR structural data revealed that Agno contains two alpha-helices (a minor and a major) while the rest of the protein adopts an unstructured conformation (Coric et al., 2017, J Cell Biochem). Previously, release of the JCV Agno from the Agno-positive cells was reported. Here, we have further mapped the regions of Agno responsible for its release by a structure-based systematic mutagenesis approach. Results revealed that amino acid residues (Lys22, Lys23, Phe31, Glu34, and Asp38) located either on or adjacent to the hydrophilic surface of the major alpha-helix domain of Agno play critical roles in release. Additionally, Agno was shown to strongly interact with unidentified components of the cell surface when cells are treated with Agno, suggesting additional novel roles for Agno during the viral infection cycle.

Nuclear Magnetic Resonance Structure of the Human Polyoma JC Virus Agnoprotein

Agnoprotein is an important regulatory protein of the human polyoma JC virus (JCV) and plays critical roles during the viral replication cycle. It forms highly stable dimers and oligomers through its Leu/Ile/Phe-rich domain, which is important for the stability and function of the protein. We recently resolved the partial 3D structure of this protein by NMR using a synthetic peptide encompassing amino acids Thr17 to Gln52, where the Leu/Ile/Phe- rich region was found to adopt a major alpha-helix conformation spanning amino acids 23-39. Here, we report the resolution of the 3D structure of full-length JCV agnoprotein by NMR, which not only confirmed the existence of the previously reported major α-helix domain at the same position but also revealed the presence of an additional minor α-helix region spanning amino acid residues Leu6 to lys13. The remaining regions of the protein adopt an intrinsically unstructured conformation. J. Cell. Biochem. 118: 3268-3280, 2017. © 2017 Wiley Periodicals, Inc.

BK nephropathy in the native kidneys of patients with organ transplants: Clinical spectrum of BK infection

Nephropathy secondary to BK virus, a member of the Papoviridae family of viruses, has been recognized for some time as an important cause of allograft dysfunction in renal transplant recipients. In recent times, BK nephropathy (BKN) of the native kidneys has being increasingly recognized as a cause of chronic kidney disease in patients with solid organ transplants, bone marrow transplants and in patients with other clinical entities associated with immunosuppression. In such patients renal dysfunction is often attributed to other factors including nephrotoxicity of medications used to prevent rejection of the transplanted organs. Renal biopsy is required for the diagnosis of BKN. Quantitation of the BK viral load in blood and urine are surrogate diagnostic methods. The treatment of BKN is based on reduction of the immunosuppressive medications. Several compounds have shown antiviral activity, but have not consistently shown to have beneficial effects in BKN. In addition to BKN, BK viral infection can cause severe urinary bladder cystitis, ureteritis and urinary tract obstruction as well as manifestations in other organ systems including the central nervous system, the respiratory system, the gastrointestinal system and the hematopoietic system. BK viral infection has also been implicated in tumorigenesis. The spectrum of clinical manifestations from BK infection and infection from other members of the Papoviridae family is widening. Prevention and treatment of BK infection and infections from other Papovaviruses are subjects of intense research.

Emerging From the Unknown: Structural and Functional Features of Agnoprotein of Polyomaviruses

Agnoprotein is an important regulatory protein of polyomaviruses, including JCV, BKV, and SV40. In the absence of its expression, these viruses are unable to sustain their productive life cycle. It is a highly basic phosphoprotein that localizes mostly to the perinuclear area of infected cells, although a small amount of the protein is also found in nucleus. Much has been learned about the structure and function of this important regulatory protein in recent years. It forms highly stable dimers/oligomers in vitro and in vivo through its Leu/Ile/Phe-rich domain. Structural NMR studies revealed that this domain adopts an alpha-helix conformation and plays a critical role in the stability of the protein. It associates with cellular proteins, including YB-1, p53, Ku70, FEZ1, HP1α, PP2A, AP-3, PCNA, and α-SNAP; and viral proteins, including small t antigen, large T antigen, HIV-1 Tat, and JCV VP1; and significantly contributes the viral transcription and replication. This review summarizes the recent advances in the structural and functional properties of this important regulatory protein. J. Cell. Physiol. 231: 2115-2127, 2016. © 2016 Wiley Periodicals, Inc.

Viroporins: structure, function and potential as antiviral targets

The channel-forming activity of a family of small, hydrophobic integral membrane proteins termed 'viroporins' is essential to the life cycles of an increasingly diverse range of RNA and DNA viruses, generating significant interest in targeting these proteins for antiviral development. Viroporins vary greatly in terms of their atomic structure and can perform multiple functions during the virus life cycle, including those distinct from their role as oligomeric membrane channels. Recent progress has seen an explosion in both the identification and understanding of many such proteins encoded by highly significant pathogens, yet the prototypic M2 proton channel of influenza A virus remains the only example of a viroporin with provenance as an antiviral drug target. This review attempts to summarize our current understanding of the channel-forming functions for key members of this growing family, including recent progress in structural studies and drug discovery research, as well as novel insights into the life cycles of many viruses revealed by a requirement for viroporin activity. Ultimately, given the successes of drugs targeting ion channels in other areas of medicine, unlocking the therapeutic potential of viroporins represents a valuable goal for many of the most significant viral challenges to human and animal health.

Agnoprotein of polyomavirus BK interacts with proliferating cell nuclear antigen and inhibits DNA replication

Background

The human polyomavirus BK expresses a 66 amino-acid peptide referred to as agnoprotein. Though mutants lacking agnoprotein are severely reduced in producing infectious virions, the exact function of this peptide remains incompletely understood. To elucidate the function of agnoprotein, we searched for novel cellular interaction partners.

Methods

Yeast-two hybrid assay was performed with agnoprotein as bait against human kidney and thymus libraries. The interaction between agnoprotein and putative partners was further examined by GST pull down, co-immunoprecipitation, and fluorescence resonance energy transfer studies. Biochemical and biological studies were performed to examine the functional implication of the interaction of agnoprotein with cellular target proteins.

Results

Proliferating cell nuclear antigen (PCNA), which acts as a processivity factor for DNA polymerase δ, was identified as an interaction partner. The interaction between agnoprotein and PCNA is direct and occurs also in human cells. Agnoprotein exerts an inhibitory effect on PCNA-dependent DNA synthesis in vitro and reduces cell proliferation when ectopically expressed. Overexpression of PCNA restores agnoprotein-mediated inhibition of cell proliferation.

Conclusion

Our data suggest that PCNA is a genuine interaction partner of agnoprotein and the inhibitory effect on PCNA-dependent DNA synthesis by the agnoprotein may play a role in switching off (viral) DNA replication late in the viral replication cycle when assembly of replicated genomes and synthesized viral capsid proteins occurs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-014-0220-1) contains supplementary material, which is available to authorized users.

Essential roles of Leu/Ile/Phe-rich domain of JC virus agnoprotein in dimer/oligomer formation, protein stability and splicing of viral transcripts

Agnoprotein is one of the key regulatory proteins of polyomaviruses, including JCV, BKV and SV40 and is required for a productive viral life cycle. We have recently reported that agnoprotein forms stable dimer/oligomers mediated by a predicted amphipathic α-helix, spanning amino acids (aa), 17 to 42. Deletion of the α-helix renders a replication incompetent virus. Here, we have further characterized this region by a systematic deletion and substitution mutagenesis and demonstrated that a Leu/Ile/Phe-rich domain, (spanning aa 28-39) within α-helix is indispensable for agnoprotein structure and function. Deletion of aa 30-37 severely affects the dimer/oligomer formation and stable expression of the protein. Mutagenesis data also indicate that the residues, 34-36, may be involved in regulation of the splicing events of JCV transcripts. Collectively, these data suggest that the Leu/Ile/Phe-rich domain plays critical roles in agnoprotein function and thus represents a potential target for developing novel therapeutics against JCV infections.

JC virus agnoprotein enhances large T antigen binding to the origin of viral DNA replication: evidence for its involvement in viral DNA replication

Agnoprotein is required for the successful completion of the JC virus (JCV) life cycle and was previously shown to interact with JCV large T-antigen (LT-Ag). Here, we further characterized agnoprotein's involvement in viral DNA replication. Agnoprotein enhances the DNA binding activity of LT-Ag to the viral origin (Ori) without directly interacting with DNA. The predicted amphipathic α-helix of agnoprotein plays a major role in this enhancement. All three phenylalanine (Phe) residues of agnoprotein localize to this α-helix and Phe residues in general are known to play critical roles in protein-protein interaction, protein folding and stability. The functional relevance of all Phe residues was investigated by mutagenesis. When all were mutated to alanine (Ala), the mutant virus (F31AF35AF39A) replicated significantly less efficiently than each individual Phe mutant virus alone, indicating the importance of Phe residues for agnoprotein function. Collectively, these studies indicate a close involvement of agnoprotein in viral DNA replication.

Agnoprotein of mammalian polyomaviruses

Polyomaviruses are naked viruses with an icosahedral capsid that surrounds a circular double-stranded DNA molecule of about 5000 base-pairs. Their genome encodes at least five proteins: large and small tumor antigens and the capsid proteins VP1, VP2 and VP3. The tumor antigens are expressed during early stages of the viral life cycle and are implicated in the regulation of viral transcription and DNA replication, while the capsid proteins are produced later during infection. Members of the Polyomaviridae family have been isolated in birds (Avipolyomavirus) and mammals (Orthopolyomavirus and Wukipolyomavirus). Some mammalian polyomaviruses encode an additional protein, referred to as agnoprotein, which is a relatively small polypeptide that exerts multiple functions. This review discusses the structure, post-translational modifications, and functions of agnoprotein, and speculates why not all polyomaviruses express this protein.

BK polyomavirus with archetypal and rearranged non-coding control regions is present in cerebrospinal fluids from patients with neurological complications

BK polyomavirus (BKPyV) has recently been postulated as an emerging opportunistic pathogen of the human central nervous system (CNS), but it is not known whether specific strains are associated with the neurotropic character of BKPyV. The presence of BKPyV large T-antigen DNA was examined in 2406 cerebrospinal fluid (CSF) samples from neurological patients with suspected JC polyomavirus infection. Twenty patients had a large T-antigen DNA-positive specimen. The non-coding control region (NCCR) of the BKPyV strains amplified from CSF from these 20 patients, strains circulating in renal and bone marrow transplant recipients and from healthy pregnant women was sequenced. The archetypal conformation was the most prevalent in all groups and 14 of the neurological patients harboured archetypal strains, while the remaining six patients possessed BKPyV with rearranged NCCR similar to previously reported variants from non-neurological patients. Transfection studies in Vero cells revealed that five of six early and four of six late rearranged promoters of these CSF isolates showed significantly higher activity than the corresponding archetypal promoter. From seven of the neurological patients with BKPyV DNA-positive CSF, paired serum samples were available. Five of them were negative for BKPyV DNA, while serum from the remaining two patients harboured BKPyV strains with archetypal NCCR that differed from those present in their CSF. Our results suggest that NCCR rearrangements are not a hallmark for BKPyV neurotropism and the dissemination of a rearranged NCCR from the blood may not be the origin of BKPyV CNS infection.

Human polyomavirus JC small regulatory agnoprotein forms highly stable dimers and oligomers: implications for their roles in agnoprotein function

JC virus (JCV) encodes a small basic phosphoprotein from the late coding region called agnoprotein, which has been shown to play important regulatory roles in the viral replication cycle. In this study, we report that agnoprotein forms highly stable dimers and higher order oligomer complexes. This was confirmed by immunoblotting and mass spectrometry studies. These complexes are extremely resistant to strong denaturing agents, including urea and SDS. Central portion of the protein, amino acids spanning from 17 to 42 is important for dimer/oligomer formation. Removal of 17 to 42 aa region from the viral background severely affected the efficiency of the JCV replication. Extracts prepared from JCV-infected cells showed a double banding pattern for agnoprotein in vivo. Collectively, these findings suggest that agnoprotein forms functionally active homodimer/oligomer complexes and these may be important for its function during viral propagation and thus for the progression of PML.

BKV agnoprotein interacts with α-soluble N-ethylmaleimide-sensitive fusion attachment protein, and negatively influences transport of VSVG-EGFP

Background

The human polyomavirus BK (BKV) infects humans worldwide and establishes a persistent infection in the kidney. The BK virus genome encodes three regulatory proteins, large and small tumor-antigen and the agnoprotein, as well as the capsid proteins VP1 to VP3. Agnoprotein is conserved among BKV, JC virus (JCV) and SV40, and agnoprotein-deficient mutants reveal reduced viral propagation. Studies with JCV and SV40 indicate that their agnoproteins may be involved in transcription, replication and/or nuclear and cellular release of the virus. However, the exact function(s) of agnoprotein of BK virus remains elusive.

Principal Findings

As a strategy of exploring the functions of BKV agnoprotein, we decided to look for cellular interaction partners for the viral protein. Several partners were identified by yeast two-hybrid assay, among them α-SNAP which is involved in disassembly of vesicles during secretion. BKV agnoprotein and α-SNAP were found to partially co-localize in cells, and a complex consisting of agnoprotein and α-SNAP could be co-immunoprecipitated from cells ectopically expressing the proteins as well as from BKV-transfected cells. The N-terminal part of the agnoprotein was sufficient for the interaction with α-SNAP. Finally, we could show that BKV agnoprotein negatively interferes with secretion of VSVG-EGFP reporter suggesting that agnoprotein may modulate exocytosis.

Conclusions

We have identified the first cellular interaction partner for BKV agnoprotein. The most N-terminal part of BKV agnoprotein is involved in the interaction with α-SNAP. Presence of BKV agnoprotein negatively interferes with secretion of VSVG-EGFP reporter.

Infection by agnoprotein-negative mutants of polyomavirus JC and SV40 results in the release of virions that are mostly deficient in DNA content

BACKGROUND

Human polyomavirus JC (JCV) is the etiologic agent of a brain disease, known as progressive multifocal leukoencephalopathy (PML). The JCV genome encodes a small multifunctional phospho-protein, agnoprotein, from the late coding region of the virus, whose regulatory functions in viral replication cycle remain elusive. In this work, the functional role of JCV and SV40 agnoproteins in virion release was investigated using a point mutant (Pt) of each virus, where the ATG codon of agnoprotein was mutated to abrogate its expression.

RESULTS

Analysis of both viral protein expression and replication using Pt mutant of each virus revealed that both processes were substantially down-regulated in the absence of agnoprotein compared to wild-type (WT) virus. Complementation studies in cells, which are constitutively expressing JCV agnoprotein and transfected with the JCV Pt mutant genome, showed an elevation in the level of viral DNA replication near to that observed for WT. Constitutive expression of large T antigen was found to be not sufficient to compensate the loss of agnoprotein for efficient replication of neither JCV nor SV40 in vivo. Examination of the viral release process for both JCV and SV40 Pt mutants showed that viral particles are efficiently released from the infected cells in the absence of agnoprotein but were found to be mostly deficient in viral DNA content.

CONCLUSIONS

The results of this study provide evidence that agnoprotein plays an important role in the polyomavirus JC and SV40 life cycle. Infection by agnoprotein-negative mutants of both viruses results in the release of virions that are mostly deficient in DNA content.

JC virus-induced Progressive Multifocal Leukoencephalopathy

Progressive multifocal encephalopathy (PML) is a fatal demyelinating disease of the central nervous system (CNS), caused by the lytic infection of oligodendrocytes by a human polyomavirus, JC virus (JCV). PML is rare disease but mostly develops in patients with underlying immunosuppressive conditions, including Hodgkin's lymphoma, lymphoproliferative diseases, in those undergoing antineoplastic therapy and AIDS. However, consistent with the occurrence of PML under immunocompromised conditions, this disease seems to be also steadily increasing among autoimmune disease patients (multiple sclerosis and Crohn's disease), who are treated with antibody-based regimens (natalizumab, efalizumab and rituximab). This unexpected occurrence of the disease among such a patient population reconfirms the existence of a strong link between the underlying immunosuppressive conditions and development of PML. These recent observations have generated a new interest among investigators to further examine the unique biology of JCV.

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.

Generation and characterization of JCV permissive hybrid cell lines

JC virus (JCV) is a human neurotropic polyomavirus whose replication in the central nervous system induces the fatal demyelinating disease, progressive multifocal leukoencephalopathy (PML). JCV particles have been detected primarily in oligodendrocytes and astrocytes of the brains of patients with PML and in the laboratory its propagation is limited to primary cultures of human fetal glial cells. In this short communication, the development of a new cell culture system is described through the fusion of primary human fetal astrocytes with the human glioblastoma cell line, U-87MG. The new hybrid cell line obtained from this fusion has the capacity to support efficiently expression of JCV and replication of viral DNA in vitro up to 16 passages. This cell line can serve as a reliable culture system to study the biology of JCV host-cell interaction, determine the mechanisms involved in cell type specific replication of JCV, and provide a convenient cell culture system for high throughput screening of anti-viral agents.