Spatiotemporal Modulation of ERK Activation by GPCRs.

ERK1/2 (extracellular signal-regulated protein kinases) are the nodal proteins that regulate diverse cellular functions primarily in response to activation from receptor tyrosine kinases (RTKs). Not only is ERK activated through a variety of RTKs, but noncanonical signaling through GPCRs also activates them. Such multimodal activation allows appropriate integration of many inputs to critical cell fate decisions such as proliferation and differentiation that MAP kinases typically regulate. MAP kinases also regulate many polar responses such as apoptosis and proliferation, dedifferentiation-differentiation, and the diversity in the outcomes though the same terminal molecule can be explained based on differences in the activation dynamics and rates. However, two processes have now been established as drivers for most of the diversity recorded in the outcomes of MAP kinase signaling. These parameters are cellular compartmentalization, i.e., spatial confinement of the molecules participating in a pathway and changes in the kinetics of the activation-deactivation, i.e., temporal regulation. While phosphorylation is the key to activating responses, specifically for ERK, the terminal MAP kinase, it is the spatiotemporal dynamics that governs the outcome generated by it. This chapter reviews our present understanding of the spatial and temporal regulation of MAP kinase cascade and the ERK activity, specifically through GPCRs.

Japanese encephalitis virus activates autophagy through XBP1 and ATF6 ER stress sensors in neuronal cells.

Endoplasmic reticulum (ER) stress and autophagy are key cellular responses to RNA virus infection. Recent studies have shown that Japanese encephalitis virus (JEV)-induced autophagy negatively influences virus replication in mouse neuronal cells and embryonic fibroblasts, and delays virus-induced cell death. Here, we evaluated the role of ER stress pathways in inducing autophagy during JEV infection. We observed that JEV infection of neuronal cells led to activation of all three sensors of ER stress mediated by eIF2α/PERK, IRE1/XBP1 and ATF6. The kinetics of autophagy induction as monitored by levels of SQSTM1 and LC3-II paralleled activation of ER stress. Inhibition of the eIF2α/PERK pathway by siRNA-mediated depletion of proteins and by the PERK inhibitor had no effect on autophagy and JEV replication. However, depletion of XBP1 and ATF6, alone or in combination, prevented autophagy induction and significantly enhanced JEV-induced cell death. JEV-infected cells depleted of XBP1 or ATF6 showed reduced transcription of ER chaperones, ERAD components and autophagy genes, resulting in reduced protein levels of the crucial autophagy effectors ATG3 and BECLIN-1. Conversely, pharmacological induction of ER stress in JEV-infected cells further enhanced autophagy and reduced virus titres. Our study thus demonstrates that a crucial link exists between the ER stress pathways and autophagy in virus-infected cells, and that these processes are highly regulated during virus infection.