BPIFB3 facilitates flavivirus infection by controlling RETREG1-dependent reticulophagy

Abstract

The flavivirus genus, which includes dengue virus (DENV) and Zika virus (ZIKV), are significant human pathogens and the prevalence of infected vectors continues to geographically expand. Both DENV and ZIKV rely on expansion of the endoplasmic reticulum (ER) and the induction of autophagy to establish a productive viral infection. However, little is known regarding the interplay between the requirements for autophagy initiation during infection and the mechanisms used by these viruses to avoid clearance through the autophagic pathway. We recently showed that DENV and ZIKV inhibit reticulophagy (specific degradation of the ER through autophagy) by cleaving reticulophagy regulator 1 (RETREG1), an autophagy receptor responsible for targeted ER sheet degradation. These data suggest that DENV and ZIKV require specific autophagic pathways for their replication, while other autophagic pathways are antiviral. We previously identified BPI Fold Containing Family B Member 3 (BPIFB3) as a regulator of autophagy that negatively controls enterovirus replication. Here, we show that in contrast to enteroviruses, BPIFB3 functions as a positive regulator of DENV and ZIKV infection and that its RNAi-mediated silencing drastically inhibits the formation of viral replication organelles. We show that BPIFB3 depletion enhances ER fragmentation, while its overexpression protects against autophagy-induced ER degradation, demonstrating that BPIFB3 serves as a specific regulator of ER turnover. We further show that the antiviral effects of BPIFB3 depletion on flavivirus infection are reversed in RETREG1-depleted cells, and that BPIFB3 associates with RETREG1 within the ER, suggesting that BPIFB3 regulates a RETREG1-specific reticulophagy pathway. Collectively, these studies identify BPIFB3 as a regulator of the reticulophagy pathway and define the requirements for a novel host regulator of flavivirus infection.\n\nAuthor SummaryFlaviviruses and other arthropod transmitted viruses represent a widespread global health problem with limited treatment options currently available. Thus, greater knowledge of the host factors required for replication and transmission is needed to provide a better understanding of the cellular requirements for infection. Here, we show that the endoplasmic reticulum (ER) localized protein, BPIFB3 is required to facilitate flavivirus infection. Depletion of BPIFB3 in cells inhibits dengue virus and Zika virus infection prior to replication of the viral genome. Mechanistically, we show that BPIFB3 inhibits ER degradation in an autophagy-specific manner and that loss of BPIFB3 decreases the availability of ER membranes needed for flavivirus replication. We further show that BPIFB3 specifically regulates the RETREG1 pathway, but not other pathways of ER turnover. Together, our data define a previously uncharacterized method of regulating ER degradation and show that BPIFB3 is an essential host factor for a productive flavivirus infection.