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Recently, we found that the cytidine deaminase APOBEC3G (A3G) inhibits measles (MV) replication. Using a microarray, we identified differential regulation of several host genes upon ectopic expression of A3G. One of the up-regulated genes, the endoplasmic reticulum (ER) protein retention receptor KDELR2, reduced MV replication ~5 fold when it was over-expressed individually in Vero and CEM-SS T cells. Silencing of KDELR2 in A3G-expressing Vero cells abrogated the antiviral activity induced by A3G, confirming its role as an A3G-regulated antiviral host factor. Recognition of the KDEL (Lys-Asp-Glu-Leu) motif by KDEL receptors initiates the retrograde transport of soluble proteins that have escaped the ER and play an important role in ER quality control. Although KDELR2 over-expression reduced MV titers in cell cultures, we observed no interaction between KDELR2 and the MV hemagglutinin (H) protein. Instead, KDELR2 retained chaperones in the ER, which are required for the correct folding and transport of the MV envelope glycoproteins H and fusion protein (F) to the cell surface. Our data indicate that KDELR2 competes with MV envelope proteins for binding to calnexin and GRP78/Bip, and that this interaction limits the availability of the chaperones for MV proteins, causing the reduction of virus spread and titers.
Transmission of measles virus (MV) from dendritic to airway epithelial cells is considered as crucial to viral spread late in infection. Therefore, pathways and effectors governing this process are promising targets for intervention. To identify these, we established a 3D respiratory tract model where MV transmission by infected dendritic cells (DCs) relied on the presence of nectin-4 on H358 lung epithelial cells. Access to recipient cells is an important prerequisite for transmission, and we therefore analyzed migration of MV-exposed DC cultures within the model. Surprisingly, enhanced motility toward the epithelial layer was observed for MV-infected DCs as compared to their uninfected siblings. This occurred independently of factors released from H358 cells indicating that MV infection triggered cytoskeletal remodeling associated with DC polarization enforced velocity. Accordingly, the latter was also observed for MV-infected DCs in collagen matrices and was particularly sensitive to ROCK inhibition indicating infected DCs preferentially employed the amoeboid migration mode. This was also implicated by loss of podosomes and reduced filopodial activity both of which were retained in MV-exposed uninfected DCs. Evidently, sphingosine kinase (SphK) and sphingosine-1-phosphate (S1P) as produced in response to virus-infection in DCs contributed to enhanced velocity because this was abrogated upon inhibition of sphingosine kinase activity. These findings indicate that MV infection promotes a push-and-squeeze fast amoeboid migration mode via the SphK/S1P system characterized by loss of filopodia and podosome dissolution. Consequently, this enables rapid trafficking of virus toward epithelial cells during viral exit.