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Lytic cycle switches of oncogenic human gammaherpesviruses.
The seminal experiments of George and Eva Klein helped to define the two life cycles of Epstein-Barr Virus (EBV), namely latency and lytic or productive infection. Their laboratories described latent nuclear antigens expressed during latency and discovered several chemicals that activated the viral lytic cycle. The mechanism of the switch between latency and the lytic cycle of EBV and Kaposi's sarcoma-associated herpesvirus (KSHV) can be studied in cultured B cell lines. Lytic cycle activation of EBV is controlled by two viral transcription factors, ZEBRA and Rta. The homologue of Rta encoded in ORF50 is the lytic cycle activator of KSHV. Control of the lytic cycle can be divided into two distinct phases. Upstream events control expression of the virally encoded lytic cycle activator genes. Downstream events represent tasks carried out by the viral proteins in driving expression of lytic cycle genes and lytic viral DNA replication. In this chapter, we report three recent groups of experiments relating to upstream and downstream events. Azacytidine (AzaC) is a DNA methyltransferase inhibitor whose lytic cycle activation capacity was discovered by G. Klein and coworkers. We find that AzaC rapidly activates the EBV lytic cycle but does not detectably alter DNA methylation or histone acetylation on the promoters of the EBV lytic cycle activator genes. AzaC probably acts via a novel, yet to be elucidated, mechanism. The lytic cycle of both EBV and KSHV can be activated by sodium butyrate (NaB), a histone deacetylase inhibitor whose activity in disrupting latency was also discovered by G. Klein and coworkers. Activation of EBV by NaB requires protein synthesis; activation of KSHV is independent of protein synthesis. Thus, NaB works by a different pathway on the two closely related viruses. ZEBRA, the major downstream mediator of EBV lytic cycle activation is both a transcription activator and an essential replication protein. We show that phosphorylation of ZEBRA at its casein kinase 2 (CK2) site separates these two functions. Phosphorylation by CK2 is required for ZEBRA to activate lytic replication but not to induce expression of early lytic cycle genes. We discuss a number of unsolved mysteries about lytic cycle activation which should provide fertile territory for future research.George Miller, Ayman El-Guindy, Jill Countryman, Jianjiang Ye, Lyn Gradoville
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Role of glial glutamate transporters in the facilitatory action of FK960 on hippocampal neurotransmission.
We found previously that N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate (FK960) facilitated hippocampal neurotransmission in the dentate gyrus of rat hippocampal slices. The present study was conducted to understand the mechanism underlying the facilitatory action of FK960. The facilitation was inhibited by H-89, an inhibitor of cAMP-dependent protein kinase (PKA), but it was not affected by cycloheximide, a protein synthesis blocker. In cultured rat hippocampal neurons, the drug had no effect on either spontaneous miniature excitatory postsynaptic currents or whole-cell membrane currents evoked by glutamate, kainate, or NMDA, suggesting that the facilitatory action of FK960 is not caused by increasing presynaptic transmitter release or excitatory postsynaptic conductances. FK960 inhibited responses of the glial glutamate transporter, GLT-1, expressed in Xenopus oocytes, and a similar effect was found with cultured rat astrocytes. The FK960 action was inhibited in the presence of H-89. The results of the present study thus suggest that FK960 facilitates hippocampal neurotransmission by inhibiting GLT-1 glial glutamate reuptake via a PKA pathway, thereby increasing synaptic glutamate concentrations.H Tozaki, T Kanno, T Nomura, T Kondoh, N Kodama, N Saito, H Aihara, T Nagata, S Matsumoto, K Ohta, K Nagai, Y Yajima, T Nishizaki