Rapid UPF1 depletion illuminates the temporal dynamics of the NMD-regulated human transcriptome

The RNA helicase UPF1 shapes the transcriptome as the core factor of nonsense-mediated mRNA decay (NMD). The essential role of UPF1 in human cells has impeded efforts to delineate its directly regulated transcripts and molecular function. To investigate the effects of rapid UPF1 depletion, we engineered human cell lines with endogenous UPF1 fused to conditional degron tags. Temporal-resolution transcriptomic analyses identified direct target mRNAs, consisting predominantly of NMD substrates that are mostly stabilized within hours of UPF1 depletion. By integrating long-read sequencing and ribosome profiling data, we defined the consolidated NMD-regulated human transcriptome (NMDRHT), uncovering previously unannotated transcripts and establishing alternative splicing as a major contributor of NMD-targeted mRNAs. Additionally, we identified non-canonical NMD events that lack indication of being driven by other UPF1-dependent degradation routes. Our work refines the role of the post-transcriptional regulator UPF1 and introduces an experimentally validated NMD-regulated transcriptome as a navigable resource at https://nmdrht.uni-koeln.de.