Human neurotropic JC virus early protein deregulates glial cell cycle pathway and impairs cell differentiation

Progressive multifocal leukoencephalopathy (PML), a human demyelinating disease of the central nervous system (CNS), is induced upon replication of the human neurotropic virus, JCV, in glial cells. Similar to other polyomaviruses, replication of JCV is initiated and orchestrated by the viral early protein, T-antigen, and results in the cytolytic destruction of oligodendrocytes, the subset of glial cells responsible for myelin production, and the appearance of bizarre astrocytic glial cells in affected individuals. Earlier results from studies in transgenic animals have suggested that in the absence of viral replication, expression of JCV T-antigen induces pathology consistent with hypomyelination of the brain. These observations suggest that JCV T-antigen has the ability to deregulate oligodendrocyte and perhaps astrocyte function in the CNS. Here we demonstrate that expression of JCV T-antigen in the bipotential glial cell line, CG-4, severely affects the ability of these cells to differentiate toward oligodendrocyte and astrocyte lineages as evidenced by their distinct morphological changes. Examination of the activity of cell cycle regulatory proteins including cyclins and their associated kinases reveals that in the absence of T-antigen, differentiation of CG-4 cells toward astrocytes and oligodendrocytes is accompanied by a decline in cyclin E, cdk2, cyclin A, and cyclin B activity. In contrast, cdc2 activity increased upon CG-4 differentiation. In T-antigen-producing cells, distinct variations in the activity of several cyclins was observed. For example, while the activity of cdk2 and cyclin E was enhanced in T-antigen expressing astrocytes compared to their levels in control cells, the activity of cdc2 was decreased in this cell type. In oligodendrocytes, expression of T-antigen decreased the activity of several cyclins and cdks including cyclin E and cdc2. On the other hand, the level of expression and activity of cyclin A was increased. Thus, it is evident that JCV T-antigen deregulates several important cell cycle regulators during CG-4 differentiation, and these alterations may contribute to the process of cell growth and differentiation in glial cells. The importance of our findings with regard to the neuropathogenesis of PML is discussed.