The unfolded protein response of the endoplasmic reticulum protectsCaenorhabditis elegansagainst DNA damage caused by stalled replication forks

Abstract

AbstractAll animals must maintain genome and proteome integrity, especially when experiencing endogenous or exogenous stress. To cope, organisms have evolved sophisticated and conserved response systems: unfolded protein responses (UPRs) ensure proteostasis while DNA damage responses (DDRs) maintains genome integrity. Emerging evidence suggests that UPRs and DDRs crosstalk, but this remains poorly understood. Here, we demonstrate that depletion of the DNA primasespri-1orpri-2, which synthesize RNA primers at replication forks and whose inactivation causes DNA damage, activates the UPR of the endoplasmic reticulum (UPR-ER) inCaenorhabditis elegans, with especially strong activation in the germline. We observed activation of both the inositol-requiring-enzyme 1 (ire-1) and the protein kinase RNA-like ER kinase (pek-1) branches of the UPR-ER. Interestingly, activation of the UPR-ER output gene heat shock protein 4 (hsp-4) was partially independent of its canonical activators,ire-1and X-box binding protein (xbp-1), and instead required the third branch of the UPR-ER, activating transcription factor 6 (atf-6), suggesting functional redundancy. We further found that primase depletion specifically induces the UPR-ER, but not the distinct cytosolic or mitochondrial UPRs, suggesting that primase inactivation causes compartment-specific rather than global stress. Functionally, loss ofire-1orpek-1sensitized animals to replication stress caused by hydroxyurea. Finally, transcriptome analysis ofpri-1embryos revealed several deregulated processes that could cause UPR-ER activation, including protein glycosylation, calcium signaling, and fatty acid desaturation. Together, our data show that the UPR-ER, but not other UPRs, responds to replication fork stress and that the UPR-ER is required to alleviate this stress.