Update on BK virus entry and intracellular trafficking

BK virus (BKV) is a small, non-enveloped, double-stranded DNA virus and a member of the Polyomaviridae family. As the recently recognized etiologic agent of polyomavirus-associated nephropathy, the events involved in BKV invasion of host cells are an important area of study. Using cell culture models, the mechanism by which BKV infects permissive hosts to gain access to the replication machinery within these cells is beginning to unfold. BKV uses an N-linked glycoprotein containing an alpha(2,3)-linked sialic acid as a receptor. After this initial attachment, BKV enters cells through caveolae-mediated endocytosis. Intracellular trafficking via cellular cytoskeletal components follows this relatively slow and cholesterol-dependent internalization. BKV must reach the nucleus for viral transcription and replication to occur. Elucidating the steps of the early viral lifecycle would provide clues to help explain the infectious spread and pathology of this human pathogen.

The human polyomaviruses

The Polyomavirus family includes two members, BK virus (BKV) and JC virus (JCV), that naturally infect humans. These viruses are widely distributed among the population worldwide. Primary infection occurs in early childhood and remains for life clinically unapparent in immunocompetent individuals. In the context of severe immunosuppression and other predisposing factors BKV and JCV may reactivate and cause serious illnesses known as Polyomavirus-induced nephropathy and progressive multifocal leukoencephalopathy, respectively. Here we briefly examine the biological and physical characteristics and the lifecycle, namely receptor(s) interaction, mode of entry, intracellular trafficking, viral transcription and replication, and progeny assembly of these two human Polyomaviruses. We also provide an overview of the clinical manifestation of Polyomavirus-induced disorders in affected individuals and discuss the potential involvement of BKV and JCV in human cancer.

An N-linked glycoprotein with alpha(2,3)-linked sialic acid is a receptor for BK virus

BK virus (BKV) is a common human polyomavirus infecting >80% of the population worldwide. Infection with BKV is asymptomatic, but reactivation in renal transplant recipients can lead to polyomavirus-associated nephropathy. In this report, we show that enzymatic removal of alpha(2,3)-linked sialic acid from cells inhibited BKV infection. Reconstitution of asialo cells with alpha(2,3)-specific sialyltransferase restored susceptibility to infection. Inhibition of N-linked glycosylation with tunicamycin reduced infection, but inhibition of O-linked glycosylation did not. An O-linked-specific alpha(2,3)-sialyltransferase was unable to restore infection in asialo cells. Taken together, these data indicate that an N-linked glycoprotein containing alpha(2,3)-linked sialic acid is a critical component of the cellular receptor for BKV.

Involvement of cytoskeletal components in BK virus infectious entry

Posttransplant reactivation of BK virus (BKV) in the renal allograft progresses to polyomavirus-associated nephropathy in 1% to 8% of kidney recipients. Graft dysfunction and loss in 30% to 45% of polyomavirus-associated nephropathy-affected patients are secondary to extensive tubular epithelial cell injury induced by the lytic replication of BKV. The early events in productive BKV infection are not thoroughly understood. We have previously shown that BKV enters cells by caveola-mediated endocytosis. In this report we examine the role of microfilaments and microtubules during early viral infection. Our results show that BKV infection of Vero cells is sensitive to nocodazole-induced disassembly of the microtubule network for the initial 8 hours following virus binding. In contrast, suppression of microtubule turnover with the stabilizing agent paclitaxel has no effect on BKV infectivity. Selective disassembly of the actin filaments with latrunculin A does not impede BKV infection, while inhibition of microfilament dynamics with jasplakinolide results in reduced numbers of viral antigen-positive cells. These data demonstrate that BKV, like other polyomaviruses, relies on an intact microtubule network during early infection. BKV, however, does not share the requirement with the closely related JC virus for an intact actin cytoskeleton during intracellular transport.

The human polyomavirus, JCV, uses serotonin receptors to infect cells

The human polyomavirus, JCV, causes the fatal demyelinating disease progressive multifocal leukoencephalopathy in immunocompromised patients. We found that the serotonergic receptor 5HT2AR could act as the cellular receptor for JCV on human glial cells. The 5HT2A receptor antagonists inhibited JCV infection, and monoclonal antibodies directed at 5HT2A receptors blocked infection of glial cells by JCV, but not by SV40. Transfection of 5HT2A receptor-negative HeLa cells with a 5HT2A receptor rescued virus infection, and this infection was blocked by antibody to the 5HT2A receptor. A tagged 5HT2A receptor colocalized with labeled JCV in an endosomal compartment following internalization. Serotonin receptor antagonists may thus be useful in the treatment of progressive multifocal leukoencephalopathy.

Infection of vero cells by BK virus is dependent on caveolae

Polyomavirus-associated nephropathy occurs in approximately 5% of renal transplant recipients and results in loss of graft function in 50 to 70% of these patients. The disease is caused by reactivation of the common human polyomavirus BK (BKV) in the transplanted kidney. The early events in productive BKV infection are unknown. In this report, we focus on elucidating the mechanisms of BKV internalization in its target cell. Our data reveal that BKV entry into permissive Vero cells is slow, is independent of clathrin-coated-pit assembly, is dependent on an intact caveolin-1 scaffolding domain, is sensitive to tyrosine kinase inhibition, and requires cholesterol. BKV colocalizes with the caveola-mediated endocytic marker cholera toxin subunit B but not with the clathrin-dependent endocytic marker transferrin. In addition, BKV infectious entry is sensitive to elevation in intracellular pH. These findings indicate that BKV entry into Vero cells occurs by caveola-mediated endocytosis involving a pH-dependent step.

Basic and clinical research in polyomavirus nephropathy

Over the last decade, polyomavirus nephropathy (PVN) has emerged as an important cause of renal allograft dysfunction and graft loss. PVN occurs with a prevalence of 1%-8% in renal transplant recipients and is most commonly reported within the first 12 months posttransplant. The human polyomavirus, BK virus, is thought to be the primary etiologic agent of PVN. Risk factors for PVN are not well defined and are most likely a result of a complex interaction between multiple donor and recipient factors. Definitive diagnosis of PVN is made through histological assessment of a renal allograft biopsy. Recent studies have also evaluated noninvasive urine and serum markers for screening of BK virus replication and as adjunct tools in PVN diagnosis and monitoring. The principal treatment for PVN is immunosuppression reduction, but this must be balanced against the risks of rejection. If rejection occurs concurrently with PVN, a brief increase in immunosuppression to treat the rejection episode followed by a subsequent reduction in immunosuppression is recommended. No antiviral treatments for PVN have been approved by the Food and Drug Administration. Although the antiviral drug cidofovir has shown in vitro activity against murine polyomaviruses, and has been effective in some patients, it is associated with significant nephrotoxicity. Small series of patients treated with leflunomide and intravenous immune globulin therapy for PVN have also recently been reported. Retransplantation after graft loss due to PVN is feasible, but experience is limited. Current research is focusing on identifying PVN risk factors, refining screening, diagnostic and monitoring methods, and developing therapy for prophylaxis and treatment of PVN with the goals of decreasing the prevalence of PVN and improving allograft outcomes in renal transplant recipients diagnosed with PVN. This review will present recent advances in basic and clinical research related to PVN and renal transplantation.

Differential distribution of the JC virus receptor-type sialic acid in normal human tissues

JC virus (JCV), a member of the polyomavirus family, causes a demyelinating disease of the central nervous system (CNS) in humans known as progressive multifocal leukoencephalopathy. Although glial cells are the principal target of JCV productive infection in progressive multifocal leukoencephalopathy patients, little is known regarding the site of JCV persistence and the mechanisms by which the virus spreads to the CNS to cause disease. Previous work has demonstrated the presence of replicating JCV DNA in B lymphocytes from peripheral blood, tonsil, and spleen and it has been hypothesized that lymphocytes may be one site of JCV persistence. Detection of viral gene products in renal tubules and excretion of JC virions in the urine suggests JCV persistence in the kidney. A respiratory route of viral transmission has also been hypothesized implicating the lung as another possible site of persistent JCV infection. Earlier studies from our laboratory have shown that terminal alpha 2,6-linked sialic acid is a critical component of the JCV receptor. In this report we examined the tissue distribution of this JCV receptor-type sialic acid in a panel of normal human tissues. Our results demonstrate that in normal brain JCV receptor-type sialic acids are expressed on oligodendrocytes and astrocytes, but not on cortical neurons. The receptor-type sialic acid is also more highly expressed on B lymphocytes than on T lymphocytes in normal human spleen and tonsil. In addition, both the kidney and lung express abundant levels of alpha 2-6-linked sialic acids. Our data show a striking correlation between the expression of the JCV receptor-type sialic acid on cells and their susceptibility to infection by the virus. These findings also support the hypothesis of JCV persistence in lymphoid tissue and B-cell-facilitated viral dissemination to the CNS.