Endoplasmic Reticulum (ER) Stress Induced by a Neurovirulent Mouse Retrovirus Is Associated with Prolonged BiP Binding and Retention of a Viral Protein in the ER

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Some murine retroviruses cause a spongiform neurodegenerative disease exhibiting pathology resembling that observed in transmissible spongiform encephalopathies. The neurovirulence of these "spongiogenic retroviruses" is determined by the sequence of their respective envelope proteins, although the mechanisms of neurotoxicity are not understood. We have studied a highly neurovirulent virus called FrCasE that causes a rapidly progressive form of this disease. Recently, transcriptional markers of endoplasmic reticulum (ER) stress were detected during the early preclinical period in the brains of FrCasE-infected mice. In contrast, ER stress was not observed in mice infected with an avirulent virus, F43, which carries a different envelope gene, suggesting a role for ER stress in disease pathogenesis. Here we have examined in NIH 3T3 cells the cause of this cellular stress response. The envelope protein of F43 bound BiP, a major ER chaperone, transiently and was processed normally through the secretory pathway. In contrast, the envelope protein of FrCasE bound to BiP for a prolonged period, was retained in the ER, and was degraded by the proteasome. Furthermore, engagement of the FrCasE envelope protein by ER quality control pathways resulted in decreased steady-state levels of this protein, relative to that of F43, both in NIH 3T3 cells and in the brains of infected mice. Thus, the ER stress induced by FrCasE appears to be initiated by inefficient folding of its viral envelope protein, suggesting that the neurodegenerative disease caused by this virus represents a protein misfolding disorder.