TDP-43 directly inhibits RNA accumulation in neurites through modulation of RNA stability

The subcellular localization of hundreds of RNAs to neuronal projections allows neurons to efficiently and rapidly react to spatially restricted external cues. However, for the vast majority of these RNAs, the mechanisms that govern their localization are unknown. Here we demonstrate that the ALS-associated RNA binding protein TDP-43 primarily acts to keep RNAs out of neuronal projections. Using subcellular fractionation and single molecule FISH we find that TDP-43 loss results in the increased neurite accumulation of hundreds of RNAs. These RNAs were highly enriched for known TDP-43 binding sites, suggesting that TDP-43 directly binds them to regulate their localization. We then identified precise regions within them that mediate their TDP-43-dependent localization and interaction with TDP-43 using high-throughput functional assays in cells and high-throughput binding assays in vitro. We found that these regions also mediated TDP-43-dependent RNA instability, identifying the mechanism by which TDP-43 regulates RNA localization. ALS-associated mutations in TDP-43 resulted in similar RNA mislocalization phenotypes as did TDP-43 loss in human iPS-derived motor neurons. These findings establish TDP-43 as a direct negative regulator of RNA abundance in neurites and suggest that mislocalization of specific transcripts may occur in ALS patients.