ALS-Associated TDP-43 Dysfunction Compromises UPF1-Dependent mRNA Metabolism Pathways Including Alternative Polyadenylation and 3'UTR Length.

UPF1-mediated decay entails several mRNA surveillance pathways that play a crucial role in cellular homeostasis. However, the precise role of UPF1 in postmitotic neurons remains unresolved, as does its activity in amyotrophic lateral sclerosis (ALS), a devastating neurodegenerative disease characterized by TDP-43 pathology and disrupted mRNA metabolism. Here, we used human iPSC-derived spinal motor neurons (MNs) to identify mRNAs subject to UPF1 degradation by integrating RNA-seq before and after UPF1 knockdown with RIP-seq to identify RNAs that co-immunoprecipitate with the active form of phosphorylated UPF1. We define a stringent set of bona fide UPF1 targets in MNs that are functionally enriched for autophagy and structurally enriched for GC-rich and long 3' UTRs but not for premature termination codon (PTC)-containing transcripts. TDP-43 depletion in iPSC-derived MNs reduces UPF1 phosphorylation and consequently post-transcriptional upregulation of UPF1 targets, suggesting that TDP-43 dysfunction compromises UPF1-mediated mRNA surveillance. Intriguingly, our datasets reveal that UPF1 and TDP-43 regulate alternative polyadenylation and 3'UTR length of mRNAs associated with synaptic and axonal function, a process that we find to be compromised in ALS models in vitro and ALS patient tissue. Our study provides a comprehensive description of UPF1-mediated mRNA decay activity in neurons, reveals overlapping roles between UPF1 and TDP-43 in regulating 3'UTR length, and offers novel insight into the intricate interplay between RNA metabolism and neurodegeneration in ALS.