Tyrosine phosphorylation of measles virus P-phosphoprotein in persistently infected neuroblastoma cells

Host factors and measles virus replication

This review takes a general approach to describing host cell factors that facilitate measles virus (MeV) infection and replication. It relates our current understanding of MeV entry receptors, with emphasis on how these host cell surface proteins contribute to pathogenesis within its host. The roles of SLAM/CD150 lymphocyte receptor and the newly discovered epithelial receptor PVRL4/nectin-4 are highlighted. Host cell factors such as HSP72, Prdx1, tubulin, casein kinase, and actin, which are known to impact viral RNA synthesis and virion assembly, are also discussed. Finally the review describes strategies used by measles virus to circumvent innate immunity and confound the effects of interferon within the host cell. Proteomic studies and genome wide RNAi screens will undoubtedly advance our knowledge in the future.

Phosphorylation events during viral infections provide potential therapeutic targets

For many medically relevant viruses, there is now considerable evidence that both viral and cellular kinases play important roles in viral infection. Ultimately, these kinases, and the cellular signaling pathways that they exploit, may serve as therapeutic targets for treating patients. Currently, small molecule inhibitors of kinases are under investigation as therapy for herpes viral infections. Additionally, a number of cellular or host-directed tyrosine kinase inhibitors that have been previously FDA approved for cancer treatment are under study in animal models and clinical trials, as they have shown promise for the treatment of various viral infections as well. This review will highlight the wide range of viral proteins phosphorylated by viral and cellular kinases, and the potential for variability of kinase recognition sites within viral substrates to impact phosphorylation and kinase prediction. Research studying kinase-targeting prophylactic and therapeutic treatments for a number of viral infections will also be discussed.

Measles virus interaction with host cells and impact on innate immunity

Because viruses are obligate parasites, numerous partnerships between measles virus and cellular molecules can be expected. At the entry level, measles virus uses at least two cellular receptors, CD150 and a yet to be identified epithelial receptor to which the virus H protein binds. This dual receptor strategy illuminates the natural infection and inter-human propagation of this lymphotropic virus. The attenuated vaccine strains use CD46 as an additional receptor, which results in a tropism alteration. Surprisingly, the intracellular viral and cellular protein partnership leading to optimal virus life cycle remains mostly a black box, while the interactions between viral proteins that sustain the RNA-dependant RNA polymerase activity (i.e., transcription and replication), the particle assembly and the polarised virus budding are documented. Hsp72 is the only cellular protein that is known to regulate the virus transcription and replication through its interaction with the viral N protein. The viral P protein is phosphorylated by the casein kinase II with undetermined functional consequences. The cellular partnership that controls the intracellular trafficking of viral components, the assembly and/or the budding of measles virus, remains unknown. The virus to cell innate immunity war is better documented. The 5' triphosphate-ended virus leader transcript is recognised by RIG-I, a cellular helicase, and induces the interferon response. Measles virus V protein binds to the MDAS helicase and prevents the MDA5-mediated activation of interferon. By interacting with STAT1 and Jak1, the viral P and V proteins prevent the type I interferon receptor (IFNAR) signalling. The virus N protein interacts with eIF3-p40 to inhibit the translation of cellular mRNA. The H protein binds to TLR2, which then transduces an activation signal and CD150 expression in monocytes. The P protein activates the expression of the ubiquitin modifier A20, thus blocking the TLR4-mediated signalling. Few other partnerships between measles virus components and cellular proteins have been postulated or demonstrated, and they need further investigations to understand their physiopathological outcome.

Host cell protein kinases in nonsegmented negative-strand virus (mononegavirales) infection

Phosphorylation of one or more viral proteins is probably an essential step in the life cycle of every member of the nonsegmented negative-strand RNA virus (mononegavirales [MNV]) group. Since no virally encoded protein kinases have been discovered in this group, phosphorylation is effected entirely by host cell kinases. The virally encoded P proteins of the MNV are the only ones consistently phosphorylated with a stoichiometry > or =1. The P protein of vesicular stomatitis virus (VSV), and perhaps also of respiratory syncytial virus, are the only ones for which a function of phosphorylation has been established. Phosphorylation by casein kinase 2 at one or more identified sites in the VSV P protein activates transcriptional activity by promoting formation of a homotrimer, which is then capable of binding the RNA polymerase and attaching it to the N protein-RNA template. A second phosphorylation of VSV P protein by a different kinase also occurs, dependent upon prior modification by casein kinase 2, but its function is not definitely known. Phosphorylation of the other MNV P proteins may serve a different purpose. No evidence has been obtained yet for any function for phosphorylation of any other MNV protein.

Persistent measles virus infection of murine neuroblastoma cells differentially affects the expression of PKC individual isoenzymes

Measles virus (MV) is among the infectious agents displaying a propensity for establishing persistent infections of the CNS. It is assumed that continuous presence of MV defective particles or viral genome in persistently infected cells may influence host cellular processes and perturb biochemical signal transduction pathways operating in linkage to various cell surface receptors. PKC expression in a MV persistently infected neuroblastoma cell line (NS20Y/MS) was investigated. The relative levels of PKC isoenzymes were determined by Western blot analysis. We found that protein levels of PKCalpha, epsilon and zeta, but not PKCdelta, were increased in NS20Y/MS cells. PKCbeta, gamma and eta were undetectable. Treatment of NS20Y/MS cells with anti-MV Abs, which downregulated MV protein synthesis, also reduced PKCalpha expression to the basal level observed in the uninfected NS20Y cells. Our results suggest that a persistent MV infection has specific effects on the expression of certain PKC isoenzymes. We postulate that the MV-associated neurologic changes may reflect virus induced changes in biochemical signaling pathways and that these effects are likely to be regulated by the host's anti-viral humoral immune response.