Immune surveillance and response to JC virus infection and PML.

The ubiquitous human polyomavirus JC virus (JCV) is the established etiological agent of the debilitating and often fatal demyelinating disease, progressive multifocal leukoencephalopathy (PML). Most healthy individuals have been infected with JCV and generate an immune response to the virus, yet remain persistently infected at subclinical levels. The onset of PML is rare in the general population, but has become an increasing concern in immunocompromised patients, where reactivation of JCV leads to uncontrolled replication in the CNS. Understanding viral persistence and the normal immune response to JCV provides insight into the circumstances which could lead to viral resurgence. Further, clues on the potential mechanisms of reactivation may be gleaned from the crosstalk among JCV and HIV-1, as well as the impact of monoclonal antibody therapies used for the treatment of autoimmune disorders, including multiple sclerosis, on the development of PML. In this review, we will discuss what is known about viral persistence and the immune response to JCV replication in immunocompromised individuals to elucidate the deficiencies in viral containment that permit viral reactivation and spread.

Neurofibromatosis type 2 protein, NF2: an uncoventional cell cycle regulator.

Neurofibromatosis type 2 protein (NF2) is an underappreciated tumor suppressor involved in a broad range of nervous system tumors. Inactivation of the NF2 gene leads to neurofibromatosis type-2, which is characterized by multiple benign nervous system tumors and mutations in the gene have been demonstrated in many other tumor types as well. All tumors, regardless of location or grade, lack a fundamental control over cell cycle progression. Historically, NF2 is an unconventional tumor suppressor protein in that it does not directly influence the cell cycle. NF2 links receptors at the plasma membrane to their cytoplasmic kinases to facilitate contact inhibition. However, NF2 can also interact with an array of cytoplasmic and nuclear proteins that affect cell cycle progression. Furthermore, through some of these pathways, NF2 may reverse the functional inhibition of conventional tumor suppressor pathways. Here we review mechanisms utilized by NF2 to regain control of the cell cycle.

Neurofibromatosis type 2 tumor suppressor protein, NF2, induces proteasome-mediated degradation of JC virus T-antigen in human glioblastoma.

Neurofibromatosis type 2 protein (NF2) has been shown to act as tumor suppressor primarily through its functions as a cytoskeletal scaffold. However, NF2 can also be found in the nucleus, where its role is less clear. Previously, our group has identified JC virus (JCV) tumor antigen (T-antigen) as a nuclear binding partner for NF2 in tumors derived from JCV T-antigen transgenic mice. The association of NF2 with T-antigen in neuronal origin tumors suggests a potential role for NF2 in regulating the expression of the JCV T-antigen. Here, we report that NF2 suppresses T-antigen protein expression in U-87 MG human glioblastoma cells, which subsequently reduces T-antigen-mediated regulation of the JCV promoter. When T-antigen mRNA was quantified, it was determined that increasing expression of NF2 correlated with an accumulation of T-antigen mRNA; however, a decrease in T-antigen at the protein level was observed. NF2 was found to promote degradation of ubiquitin bound T-antigen protein via a proteasome dependent pathway concomitant with the accumulation of the JCV early mRNA encoding T-antigen. The interaction between T-antigen and NF2 maps to the FERM domain of NF2, which has been shown previously to be responsible for its tumor suppressor activity. Co-immunoprecipitation assays revealed a ternary complex among NF2, T-antigen, and the tumor suppressor protein, p53 within a glioblastoma cell line. Further, these proteins were detected in various degrees in patient tumor tissue, suggesting that these associations may occur in vivo. Collectively, these results demonstrate that NF2 negatively regulates JCV T-antigen expression by proteasome-mediated degradation, and suggest a novel role for NF2 as a suppressor of JCV T-antigen-induced cell cycle regulation.

Extinction of Tumor Antigen Expression by SF2/ASF in JCV-Transformed Cells.

The human neurotropic polyomavirus JC (JCV) induces a broad range of neural-origin tumors in experimental animals and has been repeatedly detected in several human cancers, most notably neural crest-origin tumors including medulloblastomas and glioblastomas. The oncogenic activity of JCV is attributed to the viral early gene products, large T and small t antigens, as evident by results from in vitro cell culture and in vivo animal studies. Recently, we have shown that alternative splicing factor, SF2/ASF, has the capacity to exert a negative effect on transcription and splicing of JCV genes in glial cells through direct association with a specific DNA motif within the viral promoter region. Here, we demonstrate that SF2/ASF suppresses large T antigen expression in JCV-transformed tumor cell lines, and the expression of SF2/ASF in such tumor cells thereby inhibits the transforming capacity of the viral tumor antigens. Moreover, down-regulation of SF2/ASF in viral-transformed tumor cell lines induces growth and proliferation of the tumor cells. Mapping analysis of the minimal peptide domain of SF2/ASF responsible for JCV promoter silencing and tumor suppressor activity suggests that amino acid residues 76 to 100 of SF2/ASF are functionally sufficient to suppress the growth of the tumor cells. These observations demonstrate a role for SF2/ASF in JCV-mediated cellular transformation and provide a new avenue of research to pathogenic mechanisms of JCV-induced tumors.