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

Progressive multifocal leukoencephalopathy (PML), a human demyelinating dis ease of the central nervous system (CNS), is induced upon replication of th e human neurotropic virus, JCV, in glial cells. Similar to other polyomavir uses, replication of JCV is initiated and orchestrated by the viral early p rotein, T-antigen, and results in the cytolytic destruction of oligodendroc ytes, the subset of glial cells responsible for myelin production, and the appearance of bizarre astrocytic glial cells in affected individuals. Earli er results from studies in transgenic animals have suggested that in the ab sence of viral replication, expression of JCV T-antigen induces pathology c onsistent with hypomyelination of the brain. These observations suggest tha t JCV T-antigen has the ability to deregulate oligodendrocyte and perhaps a strocyte function in the CNS. Here we demonstrate that expression of JCV T- antigen in the bipotential glial cell line, CG-4, severely affects the abil ity of these cells to differentiate toward oligodendrocyte and astrocyte li neages 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, differentiati on 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, c dc2 activity increased upon CG-4 differentiation. In T-antigen-producing ce lls, distinct variations in the activity of several cyclins was observed. F or example, while the activity of cdk2 and cyclin E was enhanced in T-antig en expressing astrocytes compared to their levels in control cells, the act ivity of cdc2 was decreased in this cell type. In oligodendrocytes, express ion of T-antigen decreased the activity of several cyclins and cdks includi ng cyclin E and cdc2. On the other hand, the level of expression and activi ty of cyclin A was increased. Thus, it is evident that JCV T-antigen deregu lates several important cell cycle regulators during CG-4 differentiation, and these alterations may contribute to the process of cell growth and diff erentiation in glial cells. The importance of our findings with regard to t he neuropathogenesis of PML is discussed. (C) 1999 Wiley-Liss, Inc.