UC San Diego

Nonsense-mediated RNA decay (NMD) is a highly conserved and selective RNA degradation pathway that was originally discovered by virtue of its ability to serve as a quality control mechanism that degrades aberrant mRNAs. NMD has since been shown to degrade specific subsets of normal RNAs and thereby influence normal biological processes. Work from our laboratory demonstrated that NMD impacts the unfolded protein response (UPR), a stress response pathway that protects cells from misfolded and overexpressed proteins (Karam et al. EMBO Reports 2015). NMD increases the threshold for activating the UPR pathway, both in vitro and in vivo, thereby reducing the likelihood of its inappropriate activation in response to innocuous stress. NMD is also required for the timely termination of the UPR. NMD degrades mRNAs encoding several UPR components, which likely explains how NMD shapes the UPR. Here, I report on my experiments to address the hypothesis that NMD impacts the response to a naturally occurring environmental toxicant—arsenic—which is known to activate the UPR. Inorganic arsenic compounds are found in the water supply—to varying degrees—across the world. Prenatal exposure to arsenic causes many defects, including reduced neurocognitive abilities in rodents and humans. Using RNAseq analysis, I discovered that exposure of neural stem cells to arsenic altered the expression of many RNAs encoding proteins involved cell cycle progression, signaling, apoptosis and development. Disruption of the NMD pathway increased the number of genes exhibiting altered expression in response to arsenic exposure. Genes encoding cellular stress signaling proteins particularly depended on NMD for protection from arsenic-induced shifts in gene expression. Together, these data suggested that NMD protects cells from arsenic-induced transcriptome dysregulation. Follow-up experiments suggested that the ability to protect cells from arsenic did not extend to most arsenic-induced phenotypic defects, including cell viability and cell cycle progression. However, loss of the NMD factor UPF3B altered cell viability and proliferation in the brain despite not changing the sensitivity of the brain to arsenic insults. One possible explanation for this is that arsenic suppresses NMD. Indeed, I obtained several lines of evidence that this was the case.