JC virus minor capsid proteins Vp2 and Vp3 are essential for virus propagation

Revisiting JC virus and progressive multifocal leukoencephalopathy

Since its definition 65 years ago, progressive multifocal leukoencephalopathy (PML) has continued to devastate a growing population of immunosuppressed patients despite major advances in our understanding of the causative JC virus (JCV). Unless contained by the immune system, JCV lyses host oligodendrocytes collateral to its life cycle, leading to demyelination, neurodegeneration, and death. Novel treatments have stagnated in the absence of an animal model while current antiviral agents fail to address the now ubiquitous polyomavirus. In this review, we highlight the established pathogenesis by which JCV infection progresses to PML, highlighting major challenges that must be overcome to eliminate the underlying virus and, therefore, the debilitating disease.

JC polyomavirus: a short review of its biology, its association with progressive multifocal leukoencephalopathy, and the diagnostic value of different methods to manifest its activity or presence

INTRODUCTION

JC polyomavirus is the causative agent of progressive multifocal leukoencephalopathy (PML), a demyelinating disease resulting from the lytic infection of oligodendrocytes that may develop in immunosuppressed individuals: HIV1 infected or individuals under immunosuppressive therapies. Understanding the biology of JCPyV is necessary for a proper patient management, the development of diagnostic tests, and risk stratification.

AREAS COVERED

The review covers different areas of expertise including the genomic characterization of JCPyV strains detected in different body compartments (urine, plasma, and cerebrospinal fluid) of PML patients, viral mutations, molecular diagnostics, viral miRNAs, and disease.

EXPERT OPINION

The implementation of molecular biology techniques improved our understanding of JCPyV biology. Deep sequencing analysis of viral genomes revealed the presence of viral quasispecies in the cerebrospinal fluid of PML patients characterized by noncoding control region rearrangements and VP1 mutations. These neurotropic JCPyV variants present enhanced replication and an altered cell tropism that contribute to PML development. Monitoring these variants may be relevant for the identification of patients at risk of PML. Multiplex realtime PCR targeting both the LTAg and the archetype NCCR could be used to identify them. Failure to amplify NCCR should indicate the presence of a JCPyV prototype speeding up the diagnostic process.

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.

Trafficking of JC virus-like particles across the blood-brain barrier

Hollow viral vectors, such as John Cunningham virus-like particles (JC VLPs), provide a unique opportunity to deliver drug cargo into targeted cells and tissue. Current understanding of the entry of JC virus in brain cells has remained insufficient. In particular, interaction of JC VLPs with the blood-brain barrier (BBB) has not been analyzed in detail. Thus, JC VLPs were produced in this study for investigating the trafficking across the BBB. We performed a carotid artery injection procedure for mouse brain to qualitatively study JC VLPs' in vivo binding and distribution and used in vitro approaches to analyze their uptake and export kinetics in brain endothelial cells. Our results show that clathrin-dependent mechanisms contributed to the entry of VLPs into brain endothelial cells, and exocytosis or transcytosis of VLPs across the BBB was observed in vitro. VLPs were found to interact with sialic acid glycans in mouse brain endothelia. The ability of JC VLPs to cross the BBB can be useful in developing a delivery system for transport of genes and small molecule cargoes to the brain.

Human Polyomavirus JCPyV and Its Role in Progressive Multifocal Leukoencephalopathy and Oncogenesis

The human neurotropic virus JCPyV, a member of the Polyomaviridiae family, is the opportunistic infectious agent of Progressive Multifocal Leukoencephalopathy (PML), a fatal disease seen in severe immunosuppressive conditions and, during the last decade, in patients undergoing immunotherapy. JCPyV is a ubiquitous pathogen with up to 85% of the adult population word-wide exhibiting antibodies against it. Early experiments demonstrated that direct inoculation of JCPyV into the brain of different species resulted in the development of brain tumors and other neuroectodermal-derived neoplasias. Later, several reports showed the detection of viral sequences in medulloblastomas and glial tumors, as well as expression of the viral protein T-Antigen. Few oncogenic viruses, however, have caused so much controversy regarding their role in the pathogenesis of brain tumors, but the discovery of new Polyomaviruses that cause Merkel cell carcinomas in humans and brain tumors in racoons, in addition to the role of JCPyV in colon cancer and multiple mechanistic studies have shed much needed light on the role of JCPyV in cancer. The pathways affected by the viral protein T-Antigen include cell cycle regulators, like p53 and pRb, and transcription factors that activate pro-proliferative genes, like c-Myc. In addition, infection with JCPyV causes chromosomal damage and T-Antigen inhibits homologous recombination, and activates anti-apoptotic proteins, such as Survivin. Here we review the different aspects of the biology and physiopathology of JCPyV.

Progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) is a relatively common complication of HIV disease. In this chapter changes to the epidemiology are discussed along with an update in its pathogenesis and treatment. Immune reconstitution inflammatory syndrome is increasingly frequent in PML; accordingly management strategies and prognosis are detailed.

Progressive Multifocal Leukoencephalopathy: Endemic Viruses and Lethal Brain Disease

In 1971, the first human polyomavirus was isolated from the brain of a patient who died from a rapidly progressing demyelinating disease known as progressive multifocal leukoencephalopathy. The virus was named JC virus after the initials of the patient. In that same year a second human polyomavirus was discovered in the urine of a kidney transplant patient and named BK virus. In the intervening years it became clear that both viruses were widespread in the human population but only rarely caused disease. The past decade has witnessed the discovery of eleven new human polyomaviruses, two of which cause unusual and rare cancers. We present an overview of the history of these viruses and the evolution of JC polyomavirus-induced progressive multifocal leukoencephalopathy over three different epochs. We review what is currently known about JC polyomavirus, what is suspected, and what remains to be done to understand the biology of how this mostly harmless endemic virus gives rise to lethal disease.

[How polyomavirus crosses the endoplasmic reticulum membrane to gain entry into the cytosol]

Polyomavirus (Py) is a non-enveloped, double stranded DNA virus that causes a myriad of devastating human diseases for immunocompromised individuals. To cause infection, Py binds to its receptors on the plasma membrane, is endocytosed, and sorts to the endoplasmic reticulum (ER). From here, Py penetrates the ER membrane to reach the cytosol. Ensuing nuclear entry enables the virus to cause infection. How Py penetrates the ER membrane to access the cytosol is a decisive infection step that is enigmatic. In this review, I highlight the mechanisms by which host cell functions facilitate Py translocation across the ER membrane into the cytosol.

The Presumed Polyomavirus Viroporin VP4 of Simian Virus 40 or Human BK Polyomavirus Is Not Required for Viral Progeny Release

The minor capsid protein of human BK polyomavirus (BKPyV), VP2, and its N-terminally truncated form, VP3, are both important for viral entry. The closely related simian virus 40 (SV40) reportedly produces an additional truncated form of VP2/3, denoted VP4, apparently functioning as a viroporin promoting progeny release. The VP4 open reading frame is conserved in some polyomaviruses, including BKPyV. In this study, we investigated the role of VP4 in BKPyV replication. By transfecting viral genomes into primary human renal proximal tubule epithelial cells, we demonstrated that unaltered BKPyV and mutants with start codon substitutions in VP4 (VP2M229I and VP2M229A) abolishing putative VP4 production were released at the same level to supernatants. However, during infection studies, VP2M229I and VP2M229A exhibited 90% and 65% reduced infectivity, respectively, indicating that isoleucine substitution inadvertently disrupted VP2/3 function to the detriment of viral entry, while inhibition of VP4 production during late infection was well tolerated. Unexpectedly, and similarly to BKPyV, wild-type SV40 and the corresponding VP4 start codon mutants (VP2M228I and VP2M228A) transfected into monkey kidney cell lines were also released at equal levels. Upon infection, only the VP2M228I mutant exhibited reduced infectivity, a 43% reduction, which also subsequently led to delayed host cell lysis. Mass spectrometry analysis of nuclear extracts from SV40-infected cells failed to identify VP4. Our results suggest that neither BKPyV nor SV40 require VP4 for progeny release. Moreover, our results reveal an important role in viral entry for the amino acid in VP2/VP3 unavoidably changed by VP4 start codon mutagenesis.

IMPORTANCE

Almost a decade ago, SV40 was reported to produce a late nonstructural protein, VP4, which forms pores in the nuclear membrane, facilitating progeny release. By performing transfection studies with unaltered BKPyV and SV40 and their respective VP4-deficient mutants, we found that VP4 is dispensable for progeny release, contrary to the original findings. However, infection studies demonstrated a counterintuitive reduction of infectivity of certain VP4-deficient mutants. In addition to the isoleucine-substituted SV40 mutant of the original study, we included alanine-substituted VP4-deficient mutants of BKPyV (VP2M229A) and SV40 (VP2M228A). These revealed that the reduction in infectivity was not caused by a lack of VP4 but rather depended on the identity of the single amino acid substituted within VP2/3 for VP4 start codon mutagenesis. Hopefully, our results will correct the longstanding misconception of VP4's role during infection and stimulate continued work on unraveling the mechanism for release of polyomavirus progeny.

A diverse virome in kidney transplant patients contains multiple viral subtypes with distinct polymorphisms

Recent studies have established that the human urine contains a complex microbiome, including a virome about which little is known. Following immunosuppression in kidney transplant patients, BK polyomavirus (BKV) has been shown to induce nephropathy (BKVN), decreasing graft survival. In this study we investigated the urine virome profile of BKV+ and BKV- kidney transplant recipients. Virus-like particles were stained to confirm the presence of VLP in the urine samples. Metagenomic DNA was purified, and the virome profile was analyzed using metagenomic shotgun sequencing. While the BK virus was predominant in the BKV+ group, it was also found in the BKV- group patients. Additional viruses were also detected in all patients, notably including JC virus (JCV) and Torque teno virus (TTV) and interestingly, we detected multiple subtypes of the BKV, JCV and TTV. Analysis of the BKV subtypes showed that nucleotide polymorphisms were detected in the VP1, VP2 and Large T Antigen proteins, suggesting potential functional effects for enhanced pathogenicity. Our results demonstrate a complex urinary virome in kidney transplant patients with multiple viruses with several distinct subtypes warranting further analysis of virus subtypes in immunosuppressed hosts.

Modulation of a pore in the capsid of JC polyomavirus reduces infectivity and prevents exposure of the minor capsid proteins

JC polyomavirus (JCPyV) infection of immunocompromised individuals results in the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). The viral capsid of JCPyV is composed primarily of the major capsid protein virus protein 1 (VP1), and pentameric arrangement of VP1 monomers results in the formation of a pore at the 5-fold axis of symmetry. While the presence of this pore is conserved among polyomaviruses, its functional role in infection or assembly is unknown. Here, we investigate the role of the 5-fold pore in assembly and infection of JCPyV by generating a panel of mutant viruses containing amino acid substitutions of the residues lining this pore. Multicycle growth assays demonstrated that the fitness of all mutants was reduced compared to that of the wild-type virus. Bacterial expression of VP1 pentamers containing substitutions to residues lining the 5-fold pore did not affect pentamer assembly or prevent association with the VP2 minor capsid protein. The X-ray crystal structures of selected pore mutants contained subtle changes to the 5-fold pore, and no other changes to VP1 were observed. Pore mutant pseudoviruses were not deficient in assembly, packaging of the minor capsid proteins, or binding to cells or in transport to the host cell endoplasmic reticulum. Instead, these mutant viruses were unable to expose VP2 upon arrival to the endoplasmic reticulum, a step that is critical for infection. This study demonstrated that the 5-fold pore is an important structural feature of JCPyV and that minor modifications to this structure have significant impacts on infectious entry.

IMPORTANCE

JCPyV is an important human pathogen that causes a severe neurological disease in immunocompromised individuals. While the high-resolution X-ray structure of the major capsid protein of JCPyV has been solved, the importance of a major structural feature of the capsid, the 5-fold pore, remains poorly understood. This pore is conserved across polyomaviruses and suggests either that these viruses have limited structural plasticity in this region or that this pore is important in infection or assembly. Using a structure-guided mutational approach, we showed that modulation of this pore severely inhibits JCPyV infection. These mutants do not appear deficient in assembly or early steps in infectious entry and are instead reduced in their ability to expose a minor capsid protein in the host cell endoplasmic reticulum. Our work demonstrates that the 5-fold pore is an important structural feature for JCPyV.

Antibodies reacting with JCPyV_VP2 _167-15mer as a novel serological marker for JC polyomavirus infection

Background

JC polyomavirus (JCPyV) is a widespread human polyomavirus that usually resides latently in its host, but can be reactivated under immune-compromised conditions potentially causing Progressive Multifocal Leukoencephalopathy (PML). Detection of antibodies against the major capsid protein VP1 currently is the main marker for assessment of infection with JCPyV.

Methods

Based on a peptide microarray, peptide JCPyV_VP2_167-15mer was selected and a peptide ELISA was developed for detection of antibodies directed against this peptide. Epitope mapping and computational modelling was performed to further characterize this peptide. In a cohort of 204 healthy subjects it was investigated whether antibodies against JCPyV_VP2_167-15mer were correlated with VP1 serology or urinary viral load.

Results

Epitope mapping of peptide JCPyV_VP2_167-15mer showed that the minimal epitope consisted of L₁₇₃PALTSQEI₁₈₁ with amino acids P₁₇₄, L₁₇₆ and E₁₈₀ being essential for antibody recognition. Computational analysis was used to predict that this epitope is located at an exposed domain of the VP2 capsid protein, readily accessible for immune recognition upon infection. No correlation could be observed with JCPyV VP1 antibody levels, or urinary viral load.

Conclusion

This work indicates that specific antibodies against JCPyV_VP2_167-15mer might be considered as a novel serological marker for infection with JCPyV.

Electronic supplementary material

The online version of this article (doi:10.1186/1743-422X-11-174) contains supplementary material, which is available to authorized users.

JC polyomavirus attachment, entry, and trafficking: unlocking the keys to a fatal infection

The human JC polyomavirus (JCPyV) causes a lifelong persistent infection in the reno-urinary tract in the majority of the adult population worldwide. In healthy individuals, infection is asymptomatic, while in immunocompromised individuals, the virus can spread to the central nervous system and cause a fatal demyelinating disease known as progressive multifocal leukoencephalopathy (PML). There are currently very few treatment options for this rapidly progressing and devastating disease. Understanding the basic biology of JCPyV-host cell interactions is critical for the development of therapeutic strategies to prevent or treat PML. Research in our laboratory has focused on gaining a detailed mechanistic understanding of the initial steps in the JCPyV life cycle in order to define how JCPyV selectively targets cells in the kidney and brain. JCPyV requires sialic acids to attach to host cells and initiate infection, and JCPyV demonstrates specificity for the oligosaccharide lactoseries tetrasaccharide c (LSTc) with an α2,6-linked sialic acid. Following viral attachment, JCPyV entry is facilitated by the 5-hydroxytryptamine (5-HT)2 family of serotonin receptors via clathrin-dependent endocytosis. JCPyV then undergoes retrograde transport to the endoplasmic reticulum (ER) where viral disassembly begins. A novel retrograde transport inhibitor termed Retro-2(cycl) prevents trafficking of JCPyV to the ER and inhibits both initial virus infection and infectious spread in cell culture. Understanding the molecular mechanisms by which JCPyV establishes infection will open up new avenues for the prevention or treatment of virus-induced disease.

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.

The human alpha defensin HD5 neutralizes JC polyomavirus infection by reducing endoplasmic reticulum traffic and stabilizing the viral capsid

Progressive multifocal leukoencephalopathy (PML) is a fatal disease with limited treatment options, both clinically and in the research pipeline. Potential therapies would target and neutralize its etiologic agent, JC polyomavirus (JCPyV). The innate immune response to JCPyV infection has not been studied, and little is known about the initial host response to polyomavirus infection. This study examined the ability of a human alpha defensin, HD5, to neutralize JCPyV infection in human fetal glial cells. We show that HD5, by binding to the virion, blocks infection. The JCPyV-HD5 complexes bind to and enter host cells but are reduced in their ability to reach the endoplasmic reticulum (ER), where virions are normally uncoated. Furthermore, HD5 binding to the virion stabilizes the capsid and prevents genome release. Our results show that HD5 neutralizes JCPyV infection at an early postentry step in the viral life cycle by stabilizing the viral capsid and disrupting JCPyV trafficking. This study provides a naturally occurring platform for developing antivirals to treat PML and also expands on the known capabilities of human defensins.

Cysteine residues in the major capsid protein, Vp1, of the JC virus are important for protein stability and oligomer formation

The capsid of the human polyomavirus JC virus (JCV) consists of 72 pentameric capsomeres of a major structural protein, Vp1. The cysteine residues of the related Vp1 of SV40 are known to contribute to Vp1 folding, pentamer formation, pentamer-pentamer contacts, and capsid stabilization. In light of the presence of a slight structural difference between JCV Vp1 and SV40 counterpart, the way the former folds could be either different from or similar to the latter. We found a difference: an important contribution of Vp1 cysteines to the formation of infectious virions, unique in JCV and absent in SV40. Having introduced amino acid substitution at each of six cysteines (C42, C80, C97, C200, C247, and C260) in JCV Vp1, we found that, when expressed in HeLa cells, the Vp1 level was decreased in C80A and C247A mutants, and remained normal in the other mutants. Additionally, the C80A and C247A Vp1-expressing cell extracts did not show the hemagglutination activity characteristic of JCV particles. The C80A and C247A mutant Vp1s were found to be less stable than the wild-type Vp1 in HeLa cells. When produced in a reconstituted in vitro protein translation system, these two mutant proteins were stable, suggesting that some cellular factors were responsible for their degradation. As determined by their sucrose gradient sedimentation profiles, in vitro translated C247A Vp1 formed pentamers, but in vitro translated C80A Vp1 was entirely monomeric. When individually incorporated into the JCV genome, the C80A and C247A mutants, but not the other Vp1 cysteine residues mutants, interfered with JCV infectivity. Furthermore, the C80A, but not the C247A, mutation prevented the nuclear localization of Vp1 in JCV genome transfected cells. These findings suggest that C80 of JCV Vp1 is required for Vp1 stability and pentamer formation, and C247 is involved in capsid assembly in the nucleus.

The Merkel cell polyomavirus minor capsid protein

The surface of polyomavirus virions is composed of pentameric knobs of the major capsid protein, VP1. In previously studied polyomavirus species, such as SV40, two interior capsid proteins, VP2 and VP3, emerge from the virion to play important roles during the infectious entry process. Translation of the VP3 protein initiates at a highly conserved Met-Ala-Leu motif within the VP2 open reading frame. Phylogenetic analyses indicate that Merkel cell polyomavirus (MCV or MCPyV) is a member of a divergent clade of polyomaviruses that lack the conserved VP3 N-terminal motif. Consistent with this observation, we show that VP3 is not detectable in MCV-infected cells, VP3 is not found in native MCV virions, and mutation of possible alternative VP3-initiating methionine codons did not significantly affect MCV infectivity in culture. In contrast, VP2 knockout resulted in a >100-fold decrease in native MCV infectivity, despite normal virion assembly, viral DNA packaging, and cell attachment. Although pseudovirus-based experiments confirmed that VP2 plays an essential role for infection of some cell lines, other cell lines were readily transduced by pseudovirions lacking VP2. In cell lines where VP2 was needed for efficient infectious entry, the presence of a conserved myristoyl modification on the N-terminus of VP2 was important for its function. The results show that a single minor capsid protein, VP2, facilitates a post-attachment stage of MCV infectious entry into some, but not all, cell types.

Merkel cell polyomavirus (MCV or MCPyV) is a recently discovered member of the viral family Polyomaviridae. The virus plays a causal role in Merkel cell carcinoma, a highly lethal form of skin cancer. MCV encodes a major capsid protein, VP1, which forms the non-enveloped surface of the virion. Other polyomavirus species encode two minor capsid proteins, VP2 and VP3, which associate with the inner surface of the capsid and facilitate infectious entry. In this report we show that MCV does not have a VP3 minor capsid protein. Sequence analyses suggest that more than a quarter of known polyomavirus species share MCV's lack of a VP3 protein. In contrast to VP3, VP2-knockout MCV mutants displayed dramatically reduced infectivity. Consistent with native virion findings, MCV pseudovirions lacking VP2 or carrying mutations in the VP2 myristoylation motif displayed reduced infectivity on several cell lines. Puzzlingly, MCV pseudoviruses lacking VP2 successfully transduced other cell lines with high efficiency. Taken together, the data show that the lone MCV minor capsid protein, VP2, plays an important role during infectious entry into some cell types, but is dispensable for entry into other cell types.

The human JC polyomavirus (JCPyV): virological background and clinical implications

JC polyomavirus (JCPyV) was the first of now 12 PyVs detected in humans, when in 1964, PyV particles were revealed by electron microscopy in progressive multifocal leukoencephalopathy (PML) tissues. JCPyV infection is common in 35-70% of the general population, and the virus thereafter persists in the renourinary tract. One third of healthy adults asymptomatically shed JCPyV at approximately 50,000 copies/mL urine. PML is rare having an incidence of <0.3 per 100,000 person years in the general population. This increased to 2.4 per 1000 person years in HIV-AIDS patients without combination antiretroviral therapy (cART). Recently, PML emerged in multiple sclerosis patients treated with natalizumab to 2.13 cases per 1000 patients. Natalizumab blocks α4-integrin-dependent lymphocyte homing to the brain suggesting that not the overall cellular immunodeficiency but local failure of brain immune surveillance is a pivotal factor for PML. Recovering JCPyV-specific immune control, e.g., by starting cART or discontinuing natalizumab, significantly improves PML survival, but is challenged by the immune reconstitution inflammatory syndrome. Important steps of PML pathogenesis are undefined, and antiviral therapies are lacking. New clues might come from molecular and functional profiling of JCPyV and PML pathology and comparison with other replicative pathologies such as granule cell neuronopathy and (meningo-)encephalitis, and non-replicative JCPyV pathology possibly contributing to some malignancies. Given the increasing number of immunologically vulnerable patients, a critical reappraisal of JCPyV infection, replication and disease seems warranted.

Role of JC virus agnoprotein in virion formation

JC virus (JCV) belongs to the polyomavirus family of double-stranded DNA viruses and causes progressive multifocal leukoencephalopathy in humans. JCV encodes early proteins (large T antigen, small T antigen, and T' antigen) and four late proteins (agnoprotein, and three viral capsid proteins, VP1, VP2, and VP3). In the current study, a novel function for JCV agnoprotein in the morphogenesis of JC virion particles was identified. It was found that mature virions of agnoprotein-negative JCV are irregularly shaped. Sucrose gradient sedimentation and cesium chloride gradient ultracentrifugation analyses revealed that the particles of virus lacking agnoprotein assemble into irregularly sized virions, and that agnoprotein alters the efficiency of formation of VP1 virus-like particles. An in vitro binding assay and immunocytochemistry revealed that agnoprotein binds to glutathione S-transferase fusion proteins of VP1 and that some fractions of agnoprotein colocalize with VP1 in the nucleus. In addition, gel filtration analysis of formation of VP1-pentamers revealed that agnoprotein enhances formation of these pentamers by interacting with VP1. The present findings suggest that JCV agnoprotein plays a role, similar to that of SV40 agnoprotein, in facilitating virion assembly.

Principles of polyoma- and papillomavirus uncoating

Virus particles are vehicles for transmission of the viral genetic information between infected and uninfected cells and organisms. They have evolved to self-assemble, to serve as a protective shell for the viral genome during transfer, and to disassemble when entering a target cell. Disassembly during entry is a complex, multi-step process typically termed uncoating. Uncoating is triggered by multiple host-cell interactions. During cell entry, these interactions occur sequentially in different cellular compartments that the viruses pass through on their way to the site of replication. Here, we highlight the general principles of uncoating for two structurally related virus families, the polyoma- and papillomaviruses. Recent research indicates the use of different compartments and cellular interactions for uncoating despite their structural similarity.

Urinary BK Virus Excretion in Children Newly Diagnosed with Acute Lymphoblastic Leukemia

BACKGROUND

Determining the risk factors in developing or increasing the relapses of acute lymphoblastic leukemia (ALL) may help health and preventive systems to launch new programs. Up to 90% of normal population changes to seropositive for BK virus by the age of 10 years. Whether this oncogenic virus is responsible for evolving ALL is unclear. In this study, we evaluated the excretion of urinary BK virus in newly diagnosed children with ALL compared with normal population.

METHODS

This case-control study was carried out on 62 participants (32 ALL patients and 32 normal subjects), aged 1-18 years, in Saint Al-Zahra and Sayyed-Al-Shohada University Hospitals, Isfahan, Iran. A polymerase chain reaction (PCR) method was used to detect the BK virus in specimens. PCR amplification was performed using specific primers of PEP-1 (5'-AGTCTTTAGGGTCTTCTACC-3') and PEP-2 (5'-GGTGCCAACCTATGGAACAG-3').

RESULTS

Thirty-five out of 62 participants (54.8%) were males and the remaining were females. The mean duration of disease was 9.6 ± 9.69 months. Central nervous system (CNS) relapse was seen in 29% of the patients. Positive PCR for urine BK virus was seen in three children with ALL (9.7%). No positive result for urine BKV was achieved in the control group. However, Fisher's exact test did not show any significant difference between the two groups (P > 0.05). In addition, there was no significant correlation between BKV positivity and frequency of relapses.

CONCLUSION

To demonstrate the role of BK virus in inducing ALL or increasing the number of relapses, prospective studies on larger scale of population and evaluating both serum and urine for BK virus are recommended.

Development of infectious recombinant BK virus

The polyomavirus, BK virus (BKV) infects the majority of humans early in life, establishing persistent asymptomatic infections in immunocompetent individuals. The small size and non-redundant nature of the viral genome presents a challenge in developing recombinant BKV (rBKV). A strategy is described for engineering rBKV by fusing sequences coding for foreign polypeptides via the self-processing 2A peptide in frame to the BKV agnoprotein or VP2 capsid protein genes. This novel approach aims to minimize alterations to native BKV polypeptide sequences and expression, potentially allowing maintenance of viral viability. To test this concept, a panel of rBKV was constructed that express either enhanced green fluorescent protein (EGFP), or different forms of the HIV-1 Gag polypeptide under control of the native BKV late transcriptional unit, and with appropriate self-processing. Although most of these rBKV proved to have stability issues, such approaches may have utility as reporter viruses or as gene delivery vectors.

Phosphorylation of Ser-80 of VP1 and Ser-254 of VP2 is essential for human BK virus propagation in tissue culture

BK virus (BKV) infection may cause polyomavirus-associated nephropathy in patients with renal transplantation. Recently, the phosphorylated amino acids on the structural proteins VP1, VP2 and VP3 of BKV have been identified by liquid chromatography-tandem mass spectrometry in our laboratory. In this study, we further analysed the biological effects of these phosphorylation events. Phosphorylation of the BKV structural proteins was demonstrated by [(32)P]orthophosphate labelling in vivo. Site-directed mutagenesis was performed to replace all of the phosphorylated amino acids. The mutated BKV genomes were transfected into Vero cells for propagation analysis. The results showed that expression of the early protein LT and of the late protein VP1 by the mutants VP1-S80A, VP1-S80-133A, VP1-S80-327A, VP1-S80-133-327A and VP2-S254A was abolished. However, propagation of other mutants was similar to that of wild-type BKV. The results suggest that phosphorylation of Ser-80 of VP1 and Ser-254 of VP2 is crucial for BKV propagation.

Progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) is a disease of the central nervous system (CNS) with destructive infection of oligodendrocytes by JC virus. PML belongs to the opportunistic infections. It is observed in patients with HIV infection, lymphoid malignancies, after organ- and stem cell transplantations and more recently in the context of modern immune-therapies with monoclonal antibodies (mAb) like natalizumab, rituximab, infliximab and efalizumab. The natural course of PML is fatal within months. More recently, the Immune Reconstitution Inflammatory Syndrome (IRIS) has been observed in patients with HIV infection treated with combination antiretroviral therapy (cART) as well as patients in whom the PML-inducing immune therapy has been terminated. In PML-IRIS the immune system contributes to the elimination of JC virus from the CNS and if PML-IRIS emerges, PML can be survived but can lead as well to catastrophic outcomes with brain herniation and death. Therefore the management of IRIS requires special knowledge in JC virus biology and patient care. JC virus infection is possibly involved in a variety of additional neurological conditions and cancer. Much will be learned within the next years that could change our view on the understanding of JC virus and human disease.

Global analysis of modifications of the human BK virus structural proteins by LC-MS/MS

BK virus, a human polyomavirus, may cause nephritis and urological disorders in patients who have undergone renal transplantation. Little is known about the characteristics of the BK viral proteins. In the current study, BK viral proteins were characterized by immunoblotting and LC-MS/MS. The results revealed that BK virus is composed of three structural proteins, VP1, VP2, and VP3 and four cellular histones, H2A, H2B, H3, and H4. The major structural protein, VP1, can be divided into 16 subspecies by two-dimensional gel electrophoresis. Modifications of VP1, VP2, and VP3 were comprehensively identified by LC-MS/MS. The presence of acetylation, cysteinylation, carboxymethylation, carboxyethylation, formylation, methylation, methylthiolation, oxidation, dioxidation, and phosphorylation could be identified. This is the first report providing an analysis of the global modifications present on polyomavirus structural proteins. The identification of these modifications of VP1, VP2, and VP3 should facilitate an understanding of the physiology of BKV during its life cycle.

The human polyoma JC virus agnoprotein acts as a viroporin

Virus infections can result in a range of cellular injuries and commonly this involves both the plasma and intracellular membranes, resulting in enhanced permeability. Viroporins are a group of proteins that interact with plasma membranes modifying permeability and can promote the release of viral particles. While these proteins are not essential for virus replication, their activity certainly promotes virus growth. Progressive multifocal leukoencephalopathy (PML) is a fatal demyelinating disease resulting from lytic infection of oligodendrocytes by the polyomavirus JC virus (JCV). The genome of JCV encodes six major proteins including a small auxiliary protein known as agnoprotein. Studies on other polyomavirus agnoproteins have suggested that the protein may contribute to viral propagation at various stages in the replication cycle, including transcription, translation, processing of late viral proteins, assembly of virions, and viral propagation. Previous studies from our and other laboratories have indicated that JCV agnoprotein plays an important, although as yet incompletely understood role in the propagation of JCV. Here, we demonstrate that agnoprotein possesses properties commonly associated with viroporins. Our findings demonstrate that: (i) A deletion mutant of agnoprotein is defective in virion release and viral propagation; (ii) Agnoprotein localizes to the ER early in infection, but is also found at the plasma membrane late in infection; (iii) Agnoprotein is an integral membrane protein and forms homo-oligomers; (iv) Agnoprotein enhances permeability of cells to the translation inhibitor hygromycin B; (v) Agnoprotein induces the influx of extracellular Ca(2+); (vi) The basic residues at amino acid positions 8 and 9 of agnoprotein key are determinants of the viroporin activity. The viroporin-like properties of agnoprotein result in increased membrane permeability and alterations in intracellular Ca(2+) homeostasis leading to membrane dysfunction and enhancement of virus release.

JC virus: an oncogenic virus in animals and humans?

JC virus (JCV) is a human polyomavirus of the Polyomaviridae family, which also includes BK virus and simian vacuolating virus 40 (SV40). JC virus was first isolated in 1971 from the brain of a patient with Progressive Multifocal Leukoencephalopathy (PML). Like other polyomaviruses, JCV has a restricted host range. The virus infects the majority of the human population with seroconversion occurring during adolescence. JCV has a limited and specific tissue tropism infecting the kidney and oligodendrocytes and astrocytes in the central nervous system (CNS). Initial JCV infection is generally asymptomatic in immunocompetent hosts, and it establishes a persistent infection in the kidney and possibly bone marrow. In immunocompromised individuals JCV can cause a lytic infection in the CNS and lead to development of the fatal, demyelinating disease PML. The name polyoma is derived from the Greek terms: poly, meaning many, and oma, meaning tumors, owing to the capacity of this group of viruses to cause tumors. JCV inoculation of small animal models and non-human primates, which are not permissive to a productive JCV infection, leads to tumor formation. Given the ubiquitous nature of the virus and its strong association with cancer in animal models, it is hypothesized that JCV plays a role in human cancers. However, the role for JCV in human cancers and tumor formation is not clear. Some researchers have reported an association of JCV with human cancers including brain tumors, colorectal cancers, and cancers of the gastrointestinal tract, while other groups report no correlation. Here, we review the role of JCV in cancers in animal models and present the findings on JCV in human cancers.

Structure, attachment and entry of polyoma- and papillomaviruses

Polyoma- (PY) and Papillomavirus (PV) virions have remarkable structural equivalence although no discernable sequence similarities among the capsid proteins can be detected. Their similarities include the overall surface organization, the presence of 72 capsomeres composed of five molecules of the major capsid proteins, VP1 and L1, respectively, the structure of the core segment of capsomeres with classical antiparallel "jelly roll" beta strands as the major feature, and the linkage of neighboring capsomeres by invading C-terminal arms. Differences include the size of surface exposed loops that contain the dominant neutralizing epitopes, the details of the intercapsomeric interactions, and the presence of 2 or 1 minor capsid proteins, respectively. These differences may affect the dramatic differences observed in receptor binding and internalization pathways utilized by these viruses, but as detailed later even structural differences cannot completely explain receptor and pathway usage. In recent years, technical advances aiding the study of entry processes have allowed the identification of novel endocytic compartments and an appreciation of the links between endocytic pathways that were previously thought to be completely separable. This review is intended to highlight recent advances in our understanding of virus receptor interactions and their consequences for endocytosis and intracellular trafficking.

Minor capsid proteins of simian virus 40 are dispensable for nucleocapsid assembly and cell entry but are required for nuclear entry of the viral genome

We investigated the roles of simian virus 40 capsid proteins in the viral life cycle by analyzing point mutants in Vp1 and Vp2/3, as well as a deletion mutant lacking the Vp2/3 coding sequence. The Vp1 mutants (V243E and L245E) and the Vp2/3 mutants (F157E-I158E and P164R-G165E-G166R) were previously shown to be defective in Vp1-Vp2/3 interaction and to be noninfectious or poorly infectious, respectively. Here, we show that all these point mutants form stable particles following DNA transfection into cells. The Vp2/3-mutant particles contained very low levels of Vp2/3, whereas the Vp1 mutant particles contained no detectable Vp2/3. As expected, the deletion mutant also formed particles that were noninfectious. We further characterized the two Vp1 point mutants and the deletion mutant. All three mutant particles comprised Vp1 and histone-associated viral DNA, and all were able to enter cells. However, the mutant complexes failed to associate with host importins (owing to the loss of the Vp2/3 nuclear localization signal), and the mutant viral DNAs prematurely dissociated from the Vp1s, suggesting that the nucleocapsids did not enter the nucleus. Consistently, all three mutant particles failed to express large T antigen. Together, our results demonstrate unequivocally that Vp2/3 is dispensable for the formation of nucleocapsids. Further, the nucleocapsids' ability to enter cells implies that Vp1 contains the major determinants for cell attachment and entry. We propose that the major role of Vp2/3 in infectivity is to mediate the nuclear entry of viral DNA.